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Pawpaws: The tropical fruit from our forests

March 9, 2020September 19, 2022Maryland Grows8 Comments

Spring is almost here and now we can start thinking about trees to plant if we did not get to that in the fall. Thinking about trees for my garden, I came back to one I have been considering for a while now — one that gives delicious fruit, is native, makes me think of tropical lands, and is not liked by deer! Today’s post is going to be about a little-known tree that’s native to our region, and whose fruits were apparently one of George Washington’s favorites: pawpaws!

What are pawpaws?

Pawpaws are trees that belong to the same plant family as chirimoyas and custard apples (Annonaceae, the soursoup family; Figure 1). From a botanical perspective, pawpaws are really special because they are the only member of their family adapted to growing outside of the tropics and able to survive our temperate climate.

pawpaw and related fruits — Figure 1 – Pawpaws (left), chirimoyas (top right) and custard apples (bottom right) are all in the same plant family, but pawpaws are the only group adapted to growing in temperate climates.

All pawpaws grow in southeastern North America, but the most common and widespread species is the common pawpaw, Asimina triloba, which is very abundant in our region. The common pawpaw is adapted to growing in well-drained and fertile habitats, such as those found in our forests. I promise you that if you ever walked in a forest in the area, you have seen hundreds of pawpaws growing in groves (Figure 2).

Pawpaw grove — Figure 2 – Pawpaws grow in well-drained fertile soils, and are common in our forests where they often grow in groves. Photo: Katja Schulz.

Why are pawpaws such a “thing”?

Besides being great native trees that grow well in our region, pawpaws have the most delicious fruits. They are considered the largest edible fruits indigenous to the continental United States. The fruits look a bit like a green mango from the outside, but are white/yellow and fleshy in the inside (Figure 3).

pawpaw fruit — Figure 3 – Pawpaw fruits are green on the outside and white and fleshy on the inside. Note the very large seeds. Photo: Elizabeth.

Their flavor is such a delicious one that I always relate it to tropical fruits. People more technical than I am in terms of flavor description say that it is a custard flavor, close to that of bananas, pineapples, and mangos. In any case, believe me when I tell you that these fruits are absolutely delightful and can be eaten fresh, in yogurts, in cakes, as jams, or frozen in ice cream!

Why didn’t I know about this before?

That was my very question the first time I tried them! It turns out that producing pawpaws for selling is not super simple. In fact, the fruits are fragile and thus can’t be transported long distances, which reduces their marketability. This means that pawpaws are usually produced and consumed locally. If you do not happen to know somebody with some trees on their land, you probably never got to try them.

Also, the pawpaw fruit season is relatively short (end of the summer), which means that one has to be in the right place at the right time to eat them. In season, pawpaws can be purchased at local farmers’ markets or on farms. You can also try to find them in the forests of the area, where you will be able to smell the sweet aroma of the fruits while you hike or bike. However, be sure to check property rules; harvesting plant materials from park lands is typically prohibited.

Why are you talking about this now? It’s not pawpaw season yet!

That is correct. However, it is pawpaw planting season now, and soon will be pawpaw pollination season, both needed to actually get the delicious fruits in the summer. So, how to plant and pollinate them?

Pawpaws can be grown from seed, but the simplest way to get one for your land is from a nursery. Several nurseries in the area sell pawpaw trees, and your best choices are those which grow trees that are adapted to your local conditions.

Pawpaws are not hard to grow and can be actually cultivated in your own back or front yard! Further, some counties and cities provide financial support to plant these native trees (see for example, Chesapeake Bay Trust and the Maryland Department of Natural Resources).

Pawpaw trees start producing fruit a couple years after planting. However, fruit production is a bit different from that of other fruits you may be growing. In fact, fruits will form only if there is cross-pollination (see this other post), since a pawpaw is not able to properly self-pollinate. This means that pawpaws need pollinators to produce fruit.

pawpaw flower — Figure 4 – Pawpaw flowers have evolved to attract and trick flies and beetles by looking dark and smelling like ripe fruits, the insects’ preferred food and egg-laying site. With this trick, the plant cross-pollinates their flowers without offering any reward to the pollinators. Photo: Judy Gallagher

Pawpaws are pollinated by flies and sometimes beetles, which the flowers attract with their maroon flowers and their ‘yeasty’ aromas (Figure 4). These scents are known to ‘trick’ the pollinators into visiting the flowers, mimicking the odor of ripe fruits that these insects prefer to feed on or lay their eggs. Flowers then attract these pollen dispersers, who, while visiting the flowers, will cross-pollinate them without their will.

You can imagine by now that having more than one pawpaw on your land or in the surroundings of your house will increase fruit production. It will then be more likely that the fooled pollinators will have visited another plant and thus carry pollen when they visit your tree.

Alternatively, if you would like to be absolutely sure to get a good pawpaw crop, you can cross-pollinate them by hand. To do that, get a small brush, pick pollen from the anthers of one flower (check the drawing here to find them), and transfer it to the stigma of another. That way you will get to live your best pollinator life! 😊

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

Plant Love & Compatibility: A Valentine’s Tribute to Pollination

February 10, 2020September 19, 2022Maryland Grows14 Comments

February is Valentine’s month, but also that of starting to feel impatient about gardening and all the yummy veggies and fruits that will come through the season. Those two things (Valentine’s, and fruits and veggies) are actually really connected, and in today’s post, we will dive into how that is so.

Valentine’s and fruits and veggies are connected? I don’t get it…

Have you asked yourself how those fruits you see are formed? In fact, this has a lot to do with love (well, plant love) and partner compatibility… quite a bit like Valentine’s couples! Like humans, plants have special organs they use to reproduce, which are all in the flowers (Figure 1).

flower parts — *Figure 1. A flower has all the plant’s reproductive organs.*

When plants reproduce, their offspring are their seeds. Until here all is nice and good, however, a plant’s problem is that because it can’t move, if all the seeds it produces were to just to fall right under it, the mother plant would soon be surrounded by her offspring, which would then compete with each other and herself. To deal with this problem, and because plants are cool, they have evolved some super-smart strategies to help their seeds be dispersed: fruits! (Figure 2)

beans and apples — *Figure 2. Fruits allow plants to disperse their seeds. Some fruits can be fleshy like in apples, or dry like in almonds, while some are able to explode and propel the seeds into the air like in beans.*

In wild plants (and many cultivated ones), the fruit is the packet that will carry or disperse the seeds, and among all fruits, the fleshy ones are actually a ‘bait’ for seed dispersers. In fact, these fleshy parts are usually sweet and nutritious, and thus attract animals that eat this ‘seed packet’ (often along with the seeds). However, because an animal’s digestive system is built the way it is, eating the fruit will usually also mean dispersing the seeds in a different place, when those seeds leave the animal’s body. So, you see, plants having offspring and fruits are directly related, like Valentine’s and my veggies and fruits!

Oooh… so, plants can only make fruit if there has been pollination?

Making a fruit requires a lot of energy from the plant (think about all those sugars and colors that go into that delicious tomato!). This means that it usually is the case that fruits will only form if pollination has happened and seeds have formed. So, if one wants a garden to produce fruits and vegetables, it is very likely that pollination and seed formation will need to happen for it to be productive.

Plants produce seeds and are pollinated in many different ways (see this blog post for examples), but it is pretty common that plants will produce more seeds and larger fruits if they receive pollen from a plant individual that is distantly related. In fact, as for many animals, crossing among too closely-related individuals can lead to genetic diseases and poor health. And this is why pollinators are so important; they allow pollen from different individuals to be transferred among plants of the same species and allow for healthy seeds.

Is this why I sometimes need many similar plants to produce fruit?

Yes! Many groups of plants have developed reproductive strategies that advantage fertilization by pollen from plants that are more distantly related over those from themselves or a very closely-related individual. We say that these plants need cross-pollination for producing seeds (and fruit).

On the other hand, there are some plants that are able to make fruits while receiving pollen from themselves or from closely-related individuals. Because these plants are equally exposed to the health problems associated with producing seeds with a closely-related individual, they have developed other strategies that can help them reduce the occurrence of such an event; for example, their seeds can disperse very long distances.

What are the plants that need cross-pollination?

Apples, pears, almonds, pistachio, some cherries, apricots, figs, and paw-paws are all plants that need to receive pollen from another plant of the same species to make fruits. Further, note that for many of these plants it is not just sufficient to have another plant of the same species; this plant has to be of a different variety and has to be present at a relatively short distance.

Unlike those mentioned above, tomatoes, bell peppers, eggplants, grapes, currants, raspberries, and gooseberries can self-pollinate, and thus do not require other surrounding plants of the same species to produce fruits.

Many plants are able to produce fruit by self-pollinating but are far more productive if they are cultivated along with other plants of the same species. This is the case of blueberries, huckleberries, and persimmon.

Finally, some plants, like kiwi, have male and female flowers on different individuals. In these species, although the only fruit-bearing individuals are the female plants, the fruit will only form if a male is present close by.

To help disperse their seeds, some plants have evolved pretty extreme strategies!

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

Please Pass the Cranberry Sauce and Give Thanks for Pollination

November 25, 2019February 7, 2020Maryland Grows4 Comments

Thanksgiving is a time to gather with loved ones and usually involves first preparing and then ingesting a lot of delicious goodies. Each family has recipes and traditions related to Thanksgiving, and even foreigners (like myself) may join in and create new traditions. Independently of who we are and our origin, the meals we prepare include a number of common foods: cranberries, pumpkins, sweet potatoes, pecans, and potatoes.

For this year’s Thanksgiving, I want to take us on a trip to recognize and thank nature and some of our little winged friends, without whom we would not be sharing all that deliciousness with our loved ones. Plus, after reading this you will know some cool fun facts you can share with others during your Thanksgiving meal!

Cranberry Sauce

Cranberries are the fruits of a plant closely related to blueberries and huckleberries, which all are native to North America. Like blueberries and huckleberries, cranberries need pollinators to produce fruit. The reproductive organs (anthers – where pollen is produced, and carpels – where the ovules are hosted) in a cranberry flower mature at different times, which means that a flower can’t self-pollinate and needs a pollen vector to produce fruit.

As you may know, no pollination means no fruits, and no fruits means no cranberry sauce. Luckily, nature provides and pollinators are around! We know today that several different bees visit cranberry flowers, with bumblebees being some of the best pollinators. Some other wild bees (for example, mining bees, Andrena) also contribute to the pollination of this plant, and honeybees can pollinate as well but not as efficiently as bumblebees and other native bees.

Pumpkin Pie

Pumpkins, squashes, and zucchini are all closely related vegetables that also require pollinators to produce fruit. Unlike cranberries, pumpkin plants produce separate and distinct female and male flowers. Because in these plants the female and male reproductive organs are physically in different parts of the plant, pollination (and thus fruit production) requires a pollen vector. Again, we would have no pumpkin pie if our pollinator friends were not around!

So, who pollinates pumpkins? Pumpkins have very specialized pollinators that do the best job at pollinating. In the US, this specialized pollinator is the squash bee Peponapis, which feeds their larvae a strict diet of squash pollen. Furthermore, unlike many other bees, both the males and females of the squash bee pollinate, since mating happens in the flower. Even though the squash bees are by far the best pollinators of pumpkins, other bees (including honeybees) can occasionally visit and pollinate the flowers… but really it’s these little cuties that we need to thank for all the delicious pumpkin!

Pecan Pie

Like pumpkins, pecan plants can’t automatically self-pollinate and need a pollen vector to produce the yummy nuts we eat. This is not only because their female and male flowers are separated spatially on the plant (like they are for pumpkins); they also flower at different times of the year on the same plant.

To be pollinated, the female flowers of a pecan plant need to receive pollen from the male flowers of another plant which is flowering at the same time. For this reason, pecan flowers need a vector of pollination, which here is not an insect but the wind! Pecan flowers are indeed adapted to wind pollination, displaying hanging bunches that shake with the wind, releasing and catching a lot of the pollen in the air.

Sweet Potatoes

The part of the sweet potato plant that we eat during Thanksgiving is the tubers, which are roots. And since what we eat is not a fruit, pollinators have no role to play for this Thanksgiving ingredient… at least not directly. But the sweet potato plant still needs pollinators to produce seed and breed, because their flowers are unable to be successfully pollinated by the same flower’s pollen.

Pollinators are then really important for the successful maintenance of the genetic diversity of this plant. Sweet potatoes belong to the morning glory family, and as its name suggests, flower in the morning hours. They are pollinated by many different types of bees (from large bumblebees and carpenter bees to smaller bees such as sweat bees), which visit the flowers for nectar and pollen.

Mashed Potatoes

As for sweet potatoes, the part of the potato plant we eat is not the fruits but the tubers. Pollinators are not needed for obtaining these tubers, but the plant requires pollinators to be able to breed and maintain genetic diversity. Since they are in the same family (the nightshades), potato flowers look similar to those of tomatoes and eggplants.

Like in those other vegetables, pollinator visits, and more specifically something called ‘buzz pollination,’ needs to occur for successful pollination. In this type of pollination, the insect visits the flower and buzzes loudly, which shakes the flower, releasing the pollen, which they then transfer to a different flower during their next visits. Among these buzzy bees, bumblebees and mining bees (Andrena) are very efficient at pollinating potatoes.

[VIDEO: Buzz pollination of a bumblebee on a potato flower. Note how the pollen is released from the anthers — the four yellow long organs — and sticks to the bee abdomen where the stigma — the female flower organ in the middle of the flower — rubs the bee abdomen where it collects the pollen and gets pollinated. Video by thyreodon.]

Happy Thanksgiving!

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park

Like Figs? Thank a Wasp!

September 13, 2019March 22, 2021Maryland Grows7 Comments

Last month I wrote about wasps as generalist pollinators (The Buzz on Pollination is Not All About Bees: Wasps Are Pollinators Too!). Today, let’s look at a very specialized type of wasp and its role in pollinating fruit.

Do you love dried figs as much as I do? Well then, thanks to tiny wasps are in the order! In fact, through their evolutionary history, fig trees established a pollination system that is among the most fascinating that exist. Even though we eat only a very small number of them, there are hundreds of fig species, and each one is pollinated by one or a few species of tiny wasps from a family called Agaonids.

fresh and dried figs — *Dried and fresh figs. Photo: Pixabay*

These wasps are specialized in pollinating fig flowers, with a type of pollination interaction called ‘nursery pollination’. In this type of pollination, the wasps offer the service of pollination in exchange for a place to raise (‘nurse’) their offspring. But how does this work?

Let’s start by saying that a fig is like a sunflower that has been turned into itself, with all their tiny flowers facing to the inside of the fig. Some of these flowers are only female (those will make seeds), and some are only male (they offer pollen).

When visiting figs, the female wasps enter and find the female flowers. Using their long ovipositor (their organ for laying eggs), they lay one egg in it. After doing so, they take some of the pollen they previously collected on the male flowers of a different fig and they actively push pollen into the flower. While doing so, they make sure that the female flower is pollinated, and that the flower tissues grow and mature, which is what their offspring will eat while they develop inside the flower. When the offspring emerges from what was the flower, they reach to the male flowers and collect pollen, after which they leave the fig and start searching for a different one to start the cycle again.

Now, you may be thinking that this system can’t work. If the offspring eats all the female flowers (where seeds are!), then the plant has none left for their reproduction! You are right!

Well, it turns out that the plants have found a way around. Among all the female flowers available to the wasps for their egg-laying, some are short and sterile, and some are long and fertile. While the short ones are the ones that wasps can develop in, the long ones will not lead to any larval development.

The female wasps can’t know which one is long or short, so they just try to lay eggs and pollinate all the female flowers. Only some of their offspring will survive. By doing this, the plant sacrifices some flower tissue in exchange for the pollination of all the fertile flowers.

If nobody cheats, both the plant and the wasps benefit from the interaction, with the wasps having a safe place for some of their larvae to develop, and the plants having a safe means for pollen transfer and seed production. Isn’t pollination fascinating?

fig wasps — *Fig wasps entering the fig through the fig ‘entrance’ – the ostiole (left). Female fig-wasps laying eggs within the figs (center) and later emerging from the sterile female flowers (right).*

And to end this blog today, let me tell you a story. It turns out that most fig species are tropical and subtropical, and thus there are not many fig species native to the continental US. However, we love figs, and thus farmers from California decided to try to produce figs there since the climate is very appropriate for fig tree growth. These fig trees that we eat are from varieties of the species Ficus carica, originally from the Mediterranean/Middle East.

At the end of the XVIIIth Century, farmers in California were trying to produce figs, but with little to no success. Eventually, researchers realized that those cultivated figs did not produce fruit if the flowers had not been pollinated. Wasp pollinators were needed to accomplish that. By the end of the XIXth century, the wasps associated with Ficus carica in their native Mediterranean range were imported and introduced into the USA. Figs containing larvae of the wasp species were then left in baskets close to the planted fig trees, letting the females emerge from the figs and pollinate the cultivated Californian trees. After doing this, the fig production picked up, and today a similar technique is still used in those varieties of figs that need pollination for fruit set (for example, the Smyrna and Calimyrna varieties). Today, California is the world’s third top fig producer. Talk about the economic value of pollinator wasps!

By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park

Heirloom or Hybrid: What’s in My Seed Packet?

March 28, 2018March 18, 2022Maryland Grows7 Comments

pepper and seeds Selecting new seeds to buy each year is an exciting activity. Whether choosing vegetables or flowers, there is more to our selection process than falling for a pretty picture. Does it matter if the seed is an open-pollinated variety or a hybrid? Can you grow a hybrid variety from saved seed? To understand differences among types of seeds, you need to understand how the mother plant was pollinated. Here are a few things to think about when you purchase seeds.

Open-pollinated plants may be self-pollinated, like snap beans, or pollinated by natural means – insects, butterflies, hummingbirds, wind, etc. – and then produce seed that will grow into plants very similar to the mother plant. If you buy seeds for an open pollinated plant, then you will be able to save seeds from the plants you grow and you won’t need to buy new seeds each year to grow the same great plants.

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