Trees & shrubs for pollinators

For some reason, I feel that every time I think about what to plant for pollinators, the list of plants that comes to me is one full of herbaceous ones… however, it is odd that this is the case, because it’s not like our region lacks larger plants (e.g., trees, large shrubs) that are both fully able to support pollinators while also supporting other biodiversity and even contributing to flood and rain management! And because if we’re interested in going the large(r)-plant path, we need a bit of planning, in today’s post I would like to present some native shrubs and trees that are great resources for our pollinators. This way, you can start planning where to get them for planting in late winter to early spring.

Why consider trees and shrubs for pollinators?

Large perennial plants such as trees and shrubs have many characteristics that make them very attractive to any pollinator-friendly person in our region. Indeed, while there are many of these plants that act as wonderful food resources for many pollinators (both adult and larval stages), these larger plants represent long(er)-term and abundant resources that can serve different aspects of our ecosystem: they provide shelter and food for birds, they can assist in managing stormwater runoff, retain soil, reduce surface temperatures by their shading abilities, and provide structural complexity to our landscapes. Trees especially are a key component of creating climate-resilient landscapes. In fact, one of Maryland’s climate change mitigation goals is to grow 5 million more trees by 2031!

Planting trees is not necessarily expensive

From a financial perspective, although these plants may be costlier to obtain than the smaller herbaceous ones, there is a multitude of incentives, state vouchers, and programs that strongly reduce or sometimes completely cover the costs of obtaining them. In Maryland, for example, the state provides incentives through the Maryland Department of Natural Resources (discounts to be used at nurseries; all details here), the PG County Rain Check program or the TreeMontgomery program, city incentives, and free tree plantings (e.g., see College Park’s here). In all of these programs, a lot of trees native to our region are covered. If you would like to participate in any of these programs, make sure to check the specific tree lists covered by each (also, see this list of recommended native trees for the state of Maryland). Note that these programs I mentioned here are just a few of the many that exist; if you’re interested in this, make sure to check your city, county, and state resources!

What to plant?

I hope by now I have at least made you curious about the idea of choosing trees and shrubs for pollinators. Below, I made a very small selection of a couple of plants that appear in the native lists, and that are great for pollinators. Let’s take a look at them.

Tulip tree (Liriodendron tulipifera)

Tulip trees are a great native plant that can serve as a great pollinator resource. This tree is in the same family as Magnolia trees. It can reach a large size and it displays stunning yellow and orange flowers. This tree grows fast and is large (considered the tallest native tree in the eastern USA, along with sweetgums), so it can be a good choice for large spaces where a canopy is wanted relatively quickly. The flowers produce a lot of nectar, which attracts a massive number of pollinators. This makes it kind of fun to stand under the tree on warmer days during the blooming time: the buzzing coming from the tree is pretty impressive. Here are some more details on the conditions preferred by this tree.

the yellow and orange flower of a tulip tree
Tulip trees are among the tallest trees in the eastern USA and have wonderful resources for pollinators that they carry in their stunning flowers. Photo: W. Cutler CC.

American linden or basswood (Tilia Americana)

This tree can reach relatively large sizes, and when it grows to full size it has a very rounded canopy. I personally love this tree, because of the fact that I feel it’s a “social” tree: one can sit with friends under its shade on hot summer days, and just enjoy the life it hosts and the cool breeze it forms under it. Once the season is coming to an end, this tree’s leaves turn a lovely yellow. The flowers of this tree are small and not very colorful, but they are extremely fragrant and full of nectar, which makes them a great magnet for pollinators. You can learn more about the requirements of this tree at Virginia Tech Dendrology.

white flowers blooming on American linden tree
The American linden has discrete flowers that are very attractive to pollinators. Photo: A. Zharkikh CC.

Hawthorns (Crataegus phaenopyrum and C. viridis)

These are mid-size trees that also sustain a variety of fauna through their flowers, fruits, and bird nest-friendly thorny branches. Their flowers are white, have a typical Rose-family structure (like those of cherry trees), and are attractive to bees, syrphids, butterflies, and hummingbirds. Besides being great for fauna, the two species do well in urban environments, because they both tolerate a wide variety of conditions. Here at and on the Lady Bird Johnson Wildlife Center website you can learn more about each of these species.

White flowers of a hawthorn tree in bloom
Hawthorns have lovely white flower clusters. Photo: F. D. Richards CC.

Fringe tree (Chionanthus virginicus)

This small tree/tall shrub is a great addition to green spaces, and is ideal for hedgerows or just as a stand-alone plant. I am always surprised by the super cool shape of the flowers of this plant, which have very elongated petals that create long white fringes. These flowers attract bees and other pollinators, which come to collect some of the nectar that is produced. This plant is usually dioecious, meaning that one individual plant harbors either male or female flowers, but rarely both. This plant is ideal for areas that receive a lot of sun, because it is under those conditions that it will do best (although it can do fine in less-sunny areas as well). You can learn more about this plant by visiting this website.

white flowers of the fringe tree
The whimsical flowers of fringe trees are not just attractive to us, but also to many pollinators! Photo: 阿橋 HQ CC.

Serviceberry (Amelanchier laevis)

Although all of them can provide very good resources for pollinators, I picked this one to showcase because this species grows relatively fast and does not get too large. This is indeed a larger shrub that has beautiful white flowers, and later on, delicious small berries. Because of all this good stuff, the flowers are visited by many insects, and the fruits are favorites of birds (so you’ll have to win over them if you want to get at the fruits! 😉 ). Here and on the University of Maryland Extension website, you can learn a bit more about this cool plant.

Serviceberries make everybody happy: pollinators in the spring with their flowers, and birds and humans in the summer with their berries! Photo: Henna K. CC.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.

Anahí also writes an Extension Blog in Spanish! Check it out here,, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!

Pollination of Vegetable Crops in a Warming Climate

Pollination is the movement of pollen from male to female flower parts of sexually reproducing plants. It is often accomplished by wind and insects and results in the development of some type of fruit containing seeds for the species’ continuation. Farmers and gardeners in the mid-Atlantic are finding that high day and evening temperatures can cause vegetable plants to drop flowers and small fruits or produce deformed and under-sized fruits. This  problem has been observed in crops like bean, tomato, and pepper (mostly self-fertile; individual flowers can pollinate themselves), and in crops like squash and pumpkin (require cross-pollination between flowers).

How do high temperatures affect pollination?

All fruiting plants have an optimal temperature range for the pollination/fertilization process. High temperatures can reduce pollen production, prevent anthers from releasing pollen, kill pollen outright, and interfere with the pollen tubes that serve as conduits for uniting sperm cells and eggs (fertilization) inside undeveloped seeds (ovules). High temperatures can even injure flowers before they open. Night temperatures are increasing at a faster rate than day temperatures as a result of climate change, and seem to be most responsible for these pollination problems.

Continue reading

Planning your garden to support specialized pollinators

Looking out my window, as the ground is covered with snow and I am getting ready for another snowstorm coming tonight, it seems ironic that I have been spending many hours these days ordering seeds and planning my garden. While I am thankful that the winter brings some rest to the soil in my garden, planning this season brings me happy memories of the scents and buzzes in my yard during the growing days… which reminds me that I should also plan for my little buzzing pollinator friends when I plan what to grow this season. In today’s blog, I want to chat about how we can plan for many types of pollinators, with a special focus on planning for specialists and not just for generalist pollinators.

Specialist pollinators – never heard of them?

As we mentioned in a previous post, pollinators visit plants to feed on nectar and/or to collect pollen to feed themselves or their offspring. However, pollen is not just there for pollinators to feed on; pollen is central to plant reproduction, so plants tend to make it both attractive to pollinators but hard to digest. For this reason, and in order to be able to properly digest the pollen, pollinators are often specialized in their pollen choices. This is because being able to digest the compounds that plants add to their pollen to make them hard to eat requires some level of adaptation, which often involves a trade-off with the ability to eat anything. There are, of course, many levels of specialization, and, while many pollinators feed on many plant families, others are more specialized than that, and feed on only specific plant genera or even species! For us gardeners, this means that if we want to support many different pollinators, we need to make sure that we are also providing for those very specialized pollinators as well!

Luckily for us, the floral choices and pollen specialization is known to some extent for Maryland and Eastern USA bees (see this site to learn more). For this reason, we know that many specialized bees in our region are also rare or uncommon… another reason to try to provide resources for them!

Who are pollen specialists in our region?

Many known pollen specialist bees in our region belong to bee genera Andrena, Colletes, Osmia, and Melissodes, which have many species considered rare or uncommon in Maryland and Mid-Atlantic.

Continue reading

How do pollinators find plants and flowers?

As we know, pollinators help plants spread their pollen among flowers, and many plants do indeed need them to be able to reproduce and set seeds. We also know that by planting flowers and providing nesting habitats, we can help pollinators’ populations and thus assist with plant pollination. However, how do pollinators find plants? In this post, we will talk about that topic, which can help us become even better at helping pollinators and the plants they pollinate.

The big picture – pollinators need to be in the area

In order for pollinators to find plants, pollinators need to be present in the general region. In fact, although the vast majority of pollinators can move and travel from place to place, all of them have limitations on the distance they are able to travel. For example, hummingbirds can travel for miles (in Maryland, they are migratory), while large bees are able to travel relatively large distances for an insect (~500m-1km), and smaller insects will not be able to travel that far. This means that if, say, we lived in the middle of a very developed area with very few pollinator-friendly resources (few flowers, lots of cement, no green areas, etc.), planting a pollinator garden will attract few pollinators at first. This is due to the fact that it is likely that few pollinators are present in that area, and thus it will take a while for certain groups to arrive and establish in our garden.

It is for this reason that many communities tend to try to establish joint pollinator-friendly actions, and encourage many people in the region to participate (e.g., becoming Bee City USA-certified, creating “pollinator highways or corridors”). By increasing the regional number of pollinator-friendly resources, the whole region becomes more pollinator-diverse, and any supplementary action is more likely to improve pollinator support. As we talked about in a previous post, if you are interested in promoting pollinator-friendly habitat on your property, it may be a great idea to talk to your neighbors or your City, and see if others may also want to participate. In terms of pollinator-friendly activities, the saying “the more, the merrier” is very much true!

pollinator habitat sign in a garden
Pollinator-friendly actions are very effective when they are coordinated across regions. Photo: A. Kokai.

The local picture – different pollinators prefer different plants

As we mentioned in other posts, not all pollinators are made equal, and this is also true in terms of what plants will be found by what pollinators. For example, hummingbirds tend to visit tubular and reddish flowers, while syrphids prefer open flowers, and bees tend to visit flowers that they can access with their mouth parts (see this post to learn more). 

These floral preferences are due to the different pollinators’ abilities to see different colors, the presence of specific attractive floral scents in different plant species, and the ability of different pollinators with different body and mouth part shapes to handle and feed on flowers, and the matching of pollinator presence and flowering time. The practical consequence of this is that if we want to help many different pollinators find their preferred plants, it is necessary to grow different types of plants in our green spaces. By doing this, we would always provide resources that will be preferred to at least one pollinator, and by providing different types of resources, we can make sure that many different types of pollinators are supported by our plants. In order to do this, there are different floral mixes that exist that allow us to plant diverse floral resources appropriate for our region, which lets us build a diverse and welcoming floral bed for many pollinators.

Planting diverse floral resources will attract many different types of pollinators. Photo: C. Celley/USFWS.

The super-local picture – pollinators need to see the plant to access it

This will sound silly, but pollinators need to be able to have access to the plant to find it. For example, if a plant is not clearly displayed or hidden by many other plants or structures, it will be hard for pollinators to find it… even if the pollinator is present in the area and the plant in question is a preferred plant. This means that for us to help pollinators, we need to make sure that our plants are findable by the pollinators. Picking appropriate parts of our green spaces to plant our pollinator-friendly plants is thus key! For example, plants that require full sun to grow should be planted in those conditions and not under the shade of other plants or behind structures.

To know what these specific conditions are, there exist several resources (for example, see this useful and simple resource (PDF) published by the City of College Park, MD). These resources allow us to pick the best growing spot for our plants, making them easily findable by their pollinator friends.

Finally, pollinators are more likely to find plants if there are several of them! This is particularly true for smaller herbs, which may not display many flowers. By increasing the number of plants planted in an area we are also making the plant species more easily findable to the pollinators.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí. Anahí also writes an Extension Blog in Spanish! Check it out here,, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!

Plants not behaving as expected: vegetable garden edition

Two of the vegetable crops I grew this year are known for loving the heat: okra and eggplant. I grow eggplant in pots on my deck, to avoid flea beetle infestation, and okra directly in the ground in my community garden plot. Both of them produced adequately over the summer. Now it’s fall; we’re having days in the 70s and nights in the 50s, and there are fewer hours of sunlight in the day. Time to pull the summer crops, right?

Except – boom! Both the okra and the eggplant are going gangbusters. More flowers, more fruits than in the hot summer months, by far.

‘Bride’ eggplant on a cool autumn morning

So why aren’t these plants following the rulebook? Do they not know how to read? Or have the rules changed?

Continue reading

Climate change impacts on pollinators and pollination

Do you agree that the weather has been getting a bit crazy? Summers are getting extreme, some being dry, others overly wet and too hot, the winters too warm, hurricanes and tornadoes becoming more common and stronger. Climate change is here, but what does it mean? Does it even matter? In this post I want to spend some time talking about why these strong changes in our climate do not just affect us, but mostly organisms that help us survive, like our beloved pollinators and the plants with which they interact.

What is climate change?

Climate change is any change in the mean and/or the variability of the climate, which persists for an extended period (decades or longer). This means that when we hear about climate change, people can be referring to any long-term change that has been detected in the records of temperature and precipitation, as well as the variations in their extremes and averages over time. Although we tend to think about the current climate change as increases in temperature, temperature is just one aspect of it. Often, it is also the precipitation patterns that are affected, leading to extreme rainfalls or droughts. Climate change is not just a climatic curiosity; because all the places and environments we live in and obtain our foods and livelihoods from are affected by climate, changes in climate affect our lives and those of other organisms with which we coexist.

How does climate change affect pollinators and pollination?

Like all organisms, plants and pollinators are only able to survive under specific climatic conditions. If the conditions change, their options are adapting to the new conditions, migrating to regions where their optimal conditions are still present, or going extinct.

Evolving and adapting to the new and fast-evolving climatic conditions has not been often observed in the case of pollination interactions. Most species are either observed to display what is called plasticity and use their already-evolved ability to adjust to different conditions. This allows them to tolerate more extreme conditions with possibly a small loss in their ability to obtain food, grow, or mate. In the USA, it was shown that some species of bees can advance their emergence time when the winters are warmer, and match to some extent the advancing in the flowering season associated with warmer winters or early springs.

Orange-legged Furrow Bee
The Orange-legged Furrow Bee (Halictus rubicundus) is a species that has been shown to be advancing its time of emergence with increasing temperatures. Photo: M.L. Legrand.

Most species, however, respond to these changes in the climatic conditions by dispersing to new regions, where their preferred conditions are still present. For example, species from Maryland, may disperse further north under a warming climate. Likewise, species that live on the slopes of mountains may disperse upwards, to higher altitudes. However, this is often impossible, when, for example, northern latitudes do not offer the right habitat (e.g., the preferred plants are not available or there is no more natural habitat left due to human pressure), when it is impossible to disperse further to the poles (e.g., for cold-adapted species), or when it is impossible to move higher on a mountain (e.g., for alpine species). These species see their ranges become smaller and smaller, until their population becomes too small to avoid extinction.

various species of butterflies
Some species, like the Giant Swallowtail and the White-M and Red-banded Hairstreaks, have been expanding their ranges northward (top row, left to right). Others, like the cold-adapted Atlantis Fritillary or the Arctic Skipper are likely to become at least locally extinct (bottom row, left to right). Photos: T. Eagle, S. McCann, G. Lasley, S. Elliott, S. King.

Finally, other species are unable to disperse at all, because they may not be mobile or because they move too slowly for the pace of climate change. Although only through modelling approaches, studies suggest that this may be the fate of many mid-Atlantic bumblebee species like the Perplexing or the Golden Northern bumblebees, which may become slowly doomed to extinction.

Perplexing and Golden Northern Bumblebees have been predicted to become unable to track their preferred climatic conditions under many climate change scenarios. Photos: P. Pieluszyński, Molanic.

Climate change and extreme weather events however, also can affect the survival of pollination interactions, and not only of individual species. For example, species that are specialized on their floral preferences or their pollinators may be unable to survive climate change. If the winter is warmer than usual, certain plants and insects may start emerging and developing earlier than usual. If these advances in their developments do not match between the plant and the pollinator, the partners will not co-exist, which can have drastic consequences such as loss of food, nesting sites, or lack of seed production for lack of cross-pollination. This is one of the reasons why the Baltimore Checkerspot, Maryland’s state insect, is at such high risk from warming winters and is currently endangered in the state.

Baltimore Checkerspot caterpillar and adult butterfly
If there are warm spells during the winter, the Baltimore Checkerspot’s larvae leave their “winter sleep” too early, when their plant food is still unavailable to them, and are at high risk of dying from starvation. Photos: WikiCommons, S. Snyder.

Another way these changes in climate can affect pollination is through changes in the functioning of the interacting species. For instance, it has been recently shown that extreme rainfalls and heat are able to affect the quality and composition of nectar and pollen, the odor compounds present in flowers, and the ability of insects to move and reach the flowers for reward collection. This means that even if species were to still co-occur, these extreme climate events can affect their ability to encounter each other, which again, leads to their survival being harmed!

This is complicated! Can I do anything to help?

It’s true! Understanding all biological interactions and protecting species IS complex, but that doesn’t mean that there’s nothing one can do to help! Some ways to support pollination include providing a lot of resources for both the plants and the pollinators to survive. For example, planting many different native species and providing nesting resources in our green spaces supports pollinators that may be migrating to new areas, or ones that may be just under a lot of climatic stress.

Also, it has been thoroughly demonstrated that increased concentrations of carbon in our atmosphere are leading to the climate change patterns we are observing. The good news (if we react quickly) is that we may still be able to do something to revert it, and this does not have to be anything that affects our ability to survive. Even relatively simple behavioral changes like turning off unused lights at home, bringing the thermostat even 4 degrees higher in the summer, or reducing meat consumption make a huge difference in our carbon footprints.

If you would like to start helping pollinators by reducing your carbon footprint, I strongly recommend visiting this website to receive “personalized” free recommendations to successfully reduce your carbon footprints through simple (and also more complex) and very feasible actions. And if you would like to learn more, I also strongly recommend the webinar series by University of Maryland Professor Dr. Sara Via.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.

Anahí also writes an Extension Blog in Spanish! Check it out here,, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!

Pitcher plants: their pools are cool, but not always refreshing

As I’m writing this, my weather station tells me that it feels like 95F outside, and now all I can do is think of jumping into a pool. And because in the natural world pools are not just for refreshing and drinking water, today I wanted to talk about a special natural wonder of our region, a plant that makes pools to have things jump in them… although it doesn’t always end up as a refreshment. Let’s talk about our local pitcher plants!

Pitcher plants are native to many regions of the USA, and one of these plants, the Northern or Purple Pitcher Plant (Sarracenia purpurea), is native to right here! Pitcher plants receive their name from the fascinating structure they have, which allows them to collect fluids, create a sort of pool, and use it to trap prey (usually small arthropods) that they feed upon. Although we tend to think of carnivorous plants as feeding on insects, pitcher plants have a really interesting relationship with them, since they both feed on them, while also needing them to reproduce through pollination. How does this work?

native pitcher plants in a field
The Northern or Purple Pitcher Plant is native to Maryland. Photo: A. Carlson.

Are pitcher plants really carnivorous?

First things first; let’s talk about plant carnivory. Carnivorous plants are an organism that reminds us that nature is always ready to fascinate us in the most unexpected ways. Plants usually grow by absorbing minerals from the soil and combining them with the sugars they synthesize by using the energy from sunlight and CO2. In some nutrient-poor habitats, however, conditions may be a bit too harsh to obtain sufficient minerals and food to survive. In those habitats, other feeding adaptations to supplement these low nutrient levels have evolved, allowing plants to obtain sufficient food to properly develop. The evolution of carnivory in plants is one of these strategies.

Although pitcher plants are carnivorous, this does not mean that they are a sort of plant-lion waiting there to attack and retain prey. Their ways are more intricate, and in some ways, more fascinating than those of an animal predator. As all plants, pitcher plants are not able to ingest their prey; they have no mouths, no teeth… so how do they do it?

pitcher plant
Pitchers in pitcher plants fill with rainfall and get slowly colonized by a multitude of microbes and other organisms. Photo: A. Carlson.

In pitcher plants, the pitcher (a special modified leaf) fills with a liquid formed by water (often from rainfall) and other compounds that make it really favorable for the establishment of microbial communities. Along with the secretion of some plant digestive compounds into the fluid, it is these microbes that actually digest the insects that fall into the pitcher. Once the prey is trapped in the pitcher and then digested, the plant is able to absorb the released nutrients directly through its tissues, eventually obtaining food from animal tissues, thus becoming a carnivore.

Why do insects fall into the pitcher?

Insects are often attracted to the pitcher by the many lures that the structure has. For instance, the walls of the pitcher display lines and markings that are attractive to insects, which direct them to the lid of the pitcher. At the edges of those lids there are small glands that secrete nectar, which is also mixed with some narcotic substances that make insects lose control of their bodies. Once these insects experience the effects of these substances, they lose grip and fall in the pitcher, where hairs and a slippery and narrow surface prevent them from crawling or flying out.

insects inside a pitcher plant
Many small insects fall into the pitchers, where they are digested by the pitcher microbial communities. Photo: A. Carlson.

But pitcher plants need to be pollinated too! How do they do it, if they eat insects?

Yes, pitcher plants need the service of pollinators to produce seeds and reproduce. And indeed, they also have very pretty flowers (which in Maryland bloom in May-June). So, how do they attract pollinators to their flowers instead of to the pitchers, and then, not have them fall in the pitchers by mistake? 

Studies on this are demonstrating that pitchers and flowers in pitcher plants are not active at the same time. While the plant flowers only for a very restricted time in the year, the pitchers are active most of the growing season. However, their level of activity and attraction are reduced during the flowering time. This means that pitcher plants have evolved to allow pollen transfer to happen without endangering the valuable pollinators.

pitcher plant flowers
The pitcher plant flowers are very pretty, with a special round shape and coloring. In Maryland, pitcher plants bloom in the months of May and June. Photo: B. Wheeler.

Pitcher plants are tiny ecosystems

With more and more studies done on these plants, it is now clear that the fluids in the pitchers behave really like tiny ecosystems. In some cases there are not only microbes that help the plant get their nutrients, but also other organisms that feed on these microbes. There are organisms that use the pitcher’s fluids to develop (the larvae of some mosquito species are specialized in developing in these fluids). The pitchers also are used by other arthropods to trap prey (some spiders build their webs in the pitchers).

mosquito larvae in pitcher plants
Some mosquito species, like this Maryland specimen of genus Wyeomyia, have evolved to be able to develop exclusively in the pitcher fluids of pitcher plants. Photo: K. Schultz.

I love pitcher plants and I want one in my house

It is absolutely great to be interested in carnivorous plants and pitcher plants in particular. Unfortunately, the Northern Pitcher Plant is currently classified as Imperiled in our State. Indeed, many of the habitats they prefer (bogs, peatlands) are endangered, or have been profoundly disturbed by human activity. Another additional pressure that our native pitcher plants experience is collections from the wild for trading. Indeed, the market for carnivorous plant lovers is huge, and it is cheaper for a seller to collect a plant from the wild than to grow it from seed in a nursery. If you are considering purchasing pitcher plants for growth at home, make sure that the plant you buy has not been collected from the wild.

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.

Anahí also writes an Extension Blog in Spanish! Check it out here,, and please share and spread the word to your Spanish-speaking friends and colleagues in Maryland. ¡Bienvenidos a Extensión en Español!