Maryland Grows

Plants for Monarchs: Milkweeds and More

asterQ: I planted seeds of what I thought was a milkweed (Asclepias). The plants look somewhat like milkweed, but they are close to 4 feet tall with no sign of flower buds to confirm their identity. There are leaf buds at the axils, which I don’t see on other milkweeds. What is this plant? I would like to have milkweed plants for Monarch butterflies.

A: What you have here is not a milkweed. It is Tall White Aster, Symphyotrichum lanceolatum, and the good news is, it is actually what the Monarchs need more at certain times of the year than Asclepias. More on that in a minute, but first, a few notes and a caution about planting milkweeds.

As many people know, milkweeds are essential host plants for Monarch butterfly caterpillars. We commend people for adding milkweeds in their gardens to support butterfly conservation!

Twelve species of Asclepias are native to (all or part of) Maryland. There are other types of milkweeds you may find in garden centers or by way of purchased of seeds. One type you may see recommended for Monarchs is Tropical Milkweed, Asclepias curassavica. It is relatively easy to propagate and blooms late into the fall, so it is promoted as beneficial to migrating Monarchs.

However, we do not recommend planting A. curassavica, and neither does the Xerces Society. What the Monarchs need while they are here in Maryland is the milkweed that is native here. It provides what they need during the appropriate phase of their migration. A. curassavica is not native to most of North America.

The primary concern is not that a few butterflies will stay here too long in the fall because the A. curassavica stays green for so long, or that a few monarchs would get a parasite (OE disease) by aggregating around this particular species. The concern is that these are alien milkweeds, and no one has tested them for impact on our native milkweed populations.

If A. curassavica could hybridize with our natives, or if its seeds or plants carry a plant pathogen, there is a small but distinct possibility of irreversible harm to our native milkweed populations. If something bad happened to our native milkweeds, it could be devastating to all the Monarchs that migrate through the eastern United States. So we encourage people to hold off on the alien milkweeds. Asclepias tuberosa, A. syriaca, and A. incarnata are better choices.

Now, about that aster! Recent studies suggest that the cause of Monarch decline is not a lack of milkweeds at all, but a shortage of goldenrods and asters to provide nectar to fuel the adult butterflies’ return journey to Mexico (Agrawal, 2017). Tall White Aster can be a bit aggressive, but not awful. It spreads by short rhizomes. It is tolerant of compact soils and de-icing salts. If you would like to prevent these asters from reaching full height of about 6 feet, you can trim them back to about 3 feet in early July. Trimming some but not others prolongs the bloom season. And, it’s a monarch favorite!

By Sara Tangren, Ph. D., Sr. Agent Associate, Sustainable Horticulture and Native Plants, and Christa Carignan, Coordinator, University of Maryland Extension, Home & Garden Information Center

Monarch Butterfly Nectaring on Tall White Aster

References and Resources

Q&A: Is this giant hogweed or poison hemlock?

poison hemlock

Poison hemlock can be mistaken for giant hogweed

Q: I think I might have giant hogweed on my property, or maybe it is poison hemlock. How can I tell for sure?

A: Giant hogweed (Heracleum mantegazzianum) was found recently in Clarke County, Virginia, and it has raised awareness and concern about the plant – and rightfully so. The plant produces toxic sap that can cause very severe skin inflammation. We have received a lot of questions about it lately.

poison hemlock

Poison hemlock Photo: E. Nibali

What you have here is NOT giant hogweed. It is poison hemlock (Conium maculatum), which is much more common. The ferny foliage makes it possible to distinguish it from giant hogweed.

All parts of poison hemlock are toxic too. If you’re removing the plant from your landscape, treat it like you would poison ivy. Wear gloves and a long-sleeved shirt when you handle it. If you’re mowing the plants, wear a face mask as well.

With regard to concerns about giant hogweed, be aware that there are several other plants that look very similar to it. In addition to poison hemlock, there is common cow-parsnip, angelica, wild parsnip, wild chervil, Queen Anne’s lace, and golden Alexanders. Some of these plants also contain toxins, but none are as potent as giant hogweed.

In Maryland, invasive giant hogweed has been found in Garrett, Baltimore, and Harford Counties, where it has been controlled. It is possible to find giant hogweed elsewhere in the state, but the chances are much greater that you will encounter one of the look-alike plants.

If you think you see giant hogweed, avoid touching it. Take photos (particularly of the flowers and stems, if possible) and look at this identification guide online. If you are still unsure about the plant’s identity, send clear digital photos to our Ask an Expert service and we will help you.

Giant hogweed is a federally regulated noxious weed. Suspected sightings of it will be reported to the Maryland Department of Agriculture Plant Protection Division.

Additional Resources

By Christa K. Carignan, Coordinator, University of Maryland Extension Home & Garden Information Center

Have a plant or pest question? University of Maryland Extension’s experts have answers! Send your questions and photos to Ask an Expert.

Garden podcasts: best distraction from the heat

It’s hot out there! And sure, you don’t need me to tell you that, but that’s what gardeners (and humans) do: interact by complaining about the weather. If you were smart, you got as many of your gardening tasks as possible done by last month while temperatures were still bearable–though what with the cold rainy spring, this was a difficult year to get a jump on necessary jobs, and I can tell you I didn’t manage all of mine.

Even if you are fully and properly mulched and irrigated and fenced and planned down to the last seed and transplant, you still need to be outside doing some harvesting (and probably weeding and watering too). And let’s face it, it’s not pleasant. So perhaps, like me, you need a distraction from the sweat and mosquitoes? Let me suggest… podcasts! Read More

Monthly Tips for July

BegoniaOrnamental Plants

  • Chrysanthemums should be cut back halfway to encourage fall blooming. If not trimmed they will bloom later this month and not in the fall.
  • Pinch out the flower buds of asters, mums, goldenrod and other fall bloomers to keep plants bushy and prevent early flowering.
  • Although this is not the best time to divide and transplant perennials, it can be done if necessary. Divide and re-plant quickly in the early evening, keeping the root system moist at all times. Water the new divisions daily until they are established. (PDF HG 99 Dividing Herbaceous Perennials)


  • Crabgrass is growing very rapidly. While it is still young, it can be controlled with a post-emergent herbicide, or hand dug from the lawn.
  • Cicada killer wasps are becoming active. They are very large, 2 inches long, and resemble yellow jackets in coloration. Fortunately, these wasps are not aggressive and will not sting unless handled. No controls are recommended.
  • Typically, July is a very hot month and proper mowing of your lawn is critical to help it survive the summer. “Mow ‘em high and let ‘em lie” should be your slogan. Cut your cool-season turf (fescues and bluegrass) to a height of 3-4 inches and leave the clippings on the lawn where they will decompose naturally. Mow warm season grasses, like zoysia and Bermuda, to a height of 3 inches.

cucumber trellis raised bedVegetables

  • Cut back herbs through the summer to keep plants bushy and productive. Essential oils are most concentrated right before bloom. Now is a good time to propagate herbs by stem cuttings.
  • It’s time to begin thinking of fall vegetables. Seed for fall crops of broccoli, kale, turnip, and cauliflower should be sown in containers by the 3rd to 4th week in July. Late crops of squash, beans, and cucumbers can be direct sown through the end of July.



More tips from the Home & Garden Information Center

The Home & Garden Information Center’s horticulturists are available year-round to answer your plant and pest questions. In addition to gardening questions, we cover houseplants, indoor pests, and more. Send your questions and photos to Ask an Expert!

Beyond dead dirt: healthy soil is alive

This post is modified from an article originally published in The Delmarva Farmer (2/13/2018)

Most people would probably be surprised to know that bacterial cells outnumber human cells in our bodies by 10-to-1 and that just one teaspoon of healthy soil contains more than 1 billion bacteria and fungi (microbes for short). Yuck, right?  Well, not exactly.

SoilMicrobes have gotten a bad rap because the small fraction of bacteria and fungi that cause disease get all the attention.  In fact, most microbes are friendly, and neither humans nor plants can live without them.

Although the chemical and physical properties of soil have dominated discussion (and soil testing) in the past, the focus is now changing as soil is recognized as a living ecosystem.  With this change, it is becoming clear that sustained agricultural productivity requires farming practices that protect the soil and increase the diversity of life underground.  Home gardeners can also benefit from gardening strategies that protect and promote the living things in their garden soil.

Soil organisms are behind 90% of the services that healthy soil provides.  Microbes decompose organic matter and cycle those nutrients back to plants.  Healthy soil allows water to infiltrate after a rain while holding on to some of it for plants to use when it is hot and dry. Water that soaks into healthy soil is filtered, and soil microbes clean it further by degrading chemicals and toxins.  The microbes in healthy soil protect plants from disease and insects, provide plants with nitrogen, phosphorus, and water, and increase plant growth in a whole list of other ways.

Soil may seem like just “dirt” to the casual observer, but it is actually a highly structured environment built by the soil organisms themselves.  This habitat is composed of groups of water-stable aggregates, and these are what give healthy soil that good crumbly texture.  A soil aggregate is formed of tiny soil particles that are loosely stuck together by roots, fungal filaments, and sugary-gluey materials that leak from roots and soil fungi. Within a high-quality aggregate, there is plenty of space for the air, water and organic matter that plants and their microbes require.  In contrast, unhealthy soil is often blocky and compacted, low in organic matter and with little microbial life.

The sad truth is that most US agricultural soils aren’t very healthy. In the Midwest, tilling and leaving fields bare in the winter has allowed more than half of the region’s topsoil to be lost to wind and water erosion.  Tilling is very hard on soil.  It breaks up soil aggregates and exposes organic matter to air, allowing eager microbes to digest it, which releases carbon as CO2.   With routine tillage, life in agricultural soils has dwindled.  Now, much of the nation’s agricultural soil is indeed little more than dead dirt.


Here in Maryland, though, we can be really proud because our farmers are national leaders in soil health.  Over 70% of the acres planted to corn, soybeans, and grains are farmed using no-till practices.  This means that farmers skip the traditional spring and/or fall tilling.  Instead, they plant directly into the residue from a previous crop without disturbing the soil.  The crop residue protects the soil from erosion, and the lack of regular tilling allows soil organisms to go happily about their business within the soil aggregates that they built.

Maryland grain farmers also routinely plant cover crops, which provide a huge boost to soil health.  Winter cover crops protect the soil from erosion and soak up excess nutrients from the previous crop that would otherwise flow right to Chesapeake Bay.  But cover crops also feed the soil microbes, which depend on a constant supply of living roots.  Leaving ground bare over the winter essentially cuts off the food supply of the microbes that are so helpful to plants.  Other strategies like increasing the number of different crops in rotations or planting a mixture of cover crop species increase the diversity of soil organisms, further boosting soil health. When farmers “plant green” into living or recently terminated cover crops, the cover crop becomes mulch that fights weeds, holds water and adds additional organic material.

Cover crops - winter wheat and hairy vetch

Cover crops – winter wheat and hairy vetch

So how about your garden?  Have you been able to use no-till or cover crops?  If you’ve ever created new beds with the “lasagna” method instead of with your rototiller, then you’ve practiced no-till!  But what about your garden beds?  Do you rototill each spring?  Even turning over the soil can be somewhat damaging, though it is not as harsh as using a tiller and light digging can be useful to incorporate compost and manure into extremely poor soil.  However, tilling and digging are not necessary for the long term, and halting physical disturbance entirely can be a real boon to soil health.  Once you get beyond soil that has little organic matter or is very compacted, amendments placed on top filter down fairly readily.

No-till is relatively easy in corn and soybeans where weeds can be controlled by herbicides without damaging the crops.  But that doesn’t work in home gardens.

One promising strategy for no-till weed control is to use overwintering cover crops as mulch for spring crops.  Gardeners can easily dispatch the overwintering cover with a mulching lawnmower set to the lowest setting.  Then you can make a small hole for transplants or a slit for seeds and just plant into the undisturbed soil.  Adding more mulch around the transplants or seed row will help prevent regrowth of the cover crop and prevent germination of weed seeds.  Mulched leaves are great for this, so is straw.

“Living mulch” is another no-till weed-control strategy.   Here’s how it works.  First plant a cover crop like red clover early enough in the fall to get established.  Then mow it in spring to reduce competition with your crop but not low enough to kill it.  After mowing, plant your seeds or (even better) transplants immediately without tilling as described above.  The difference between this and pure no-till is that the cover crop will start growing again around your vegetables.  A legume like red clover will not only crowd out weeds, it will fix nitrogen and reduce your fertilizer needs.  Win-win!


No-till and cover crops work because they prevent disturbance of the all-important soil organisms and provide them with food during the winter while leaving their habitat intact.  Adoption of these strategies is just beginning in the gardening world.  However, I suspect we will be hearing a lot more about them as people become more aware of the value of nurturing soil organisms.

Why not try planting a cover crop in your empty beds this fall?  Then in December when you are warm in front of your fire, you can think of your army of soil microbes happily carrying on in your winter garden amid a tangle of living cover crop roots.

Sara Via, Professor & Climate Extension Specialist, UMD College Park

What Can I Do About All These Weeds?

Ground Ivy

Ground ivy or creeping Charlie (Glechoma hederacea). Photo by Betty Marose

The famous quotation about the certainties of life which we all know includes death and taxes should also mention weeds! They are sprouting up all over. Even the most meticulously tended landscapes are not immune.

Where to Begin?

The first step is identification. You need to know your opponent. Control is more attainable if you know whether it is a grassy or broadleaf weed. Is it an annual, perennial or biennial? When does it germinate? Fall, spring, or summer?

Begin by browsing through the Home & Garden Information Center weed galleries where you will find photos, lifecycle information, and control suggestions for a large collection of weeds commonly found in Maryland. If you are still not sure, send a clear digital photo of the entire weed(s), preferably with flowers or seed heads, through Ask An Expert and we will get back to you.

Pick Your Battles

Clover and dandelions are common lawn weeds but they are not amongst the worst offenders. They provide pollen to pollinators throughout the growing season, particularly in urban areas and early in the season when blooms are scarce.

But, of course, fight back against invasive plants. Invasive species damage the natural environment by displacing more desirable plants that have evolved to provide food and shelter for insects, birds, and wildlife. Some invasives (English ivy, Wisteria vines) can kill mature trees. Other invasives such as Japanese barberry have been shown to support higher populations of ticks that can carry human pathogens.


Crabgrass (Digitaria sanguinalis). Photo by Betty Marose

Prevention Strategies

Lawns – A healthy, dense turf is your best defense against weed infestations. Something as simple as mowing fescue lawns to a height of 3-3 ½ inches will help prevent crabgrass from moving in.

Gardens – Mulch or plant groundcovers to cover bare soil and when digging or pulling weeds to minimize soil disturbance. When weed seeds are brought to the soil surface they will germinate.


Chickweed (Stellaria media)

Vegetable gardens – Spread an organic mulch (2 to 4 inches of grass clippings, finished compost, or newspaper covered with straw or shredded leaves) to keep weeds at bay.  Or lay a synthetic mulch (black plastic or landscape fabric), cultivate, or use a weed trimmer to keep weeds cut back on a regular basis. Looking to the fall, plant a cover crop to keep winter weeds down and improve the soil.


Pigweed (Amaranthus retroflexus). Photo by Betty Marose

The Last Line of Defense

Herbicides are one tool for weed management that should be used as a last resort and not the first go-to solution. Check to make sure the weed you are controlling is on the herbicide product label and read and follow the directions.

Additional Information

By Debra Ricigliano, Lead Horticulturist, Home & Garden Information Center

Have a plant or pest question? University of Maryland Extension’s experts have answers! Send your questions and photos to Ask an Expert.

2018 Home & Garden Information Center Survey – Your Help is Needed!

surveyThe University of Maryland Extension Home & Garden Information Center (HGIC) is conducting a brief survey of its Ask a Gardening Expert service. This service is conducted by Maryland Certified Professional Horticulturists, employed by HGIC, who answer home gardening, lawn care, and pest management questions that are submitted through the HGIC website.

I invite you to participate in this 5-10 minute survey.

Your participation will help us better understand your use of our service and how you prefer to receive information. Results of the survey may help us improve our service for future users. The survey will close on July 31, 2018.

Thank you for your time and valuable input.

Christa K. Carignan, Coordinator, Digital Horticulture Education, Home & Garden Information Center