Sarah Rothman – Maryland Grows

Leave (Some of) Your Leaves

October 7, 2025Maryland Grows1 Comment

Don’t feel guilty about being a little lazy this fall! Trying to remove every single leaf that lands in your yard is a lot of effort, without all of the reward.

Many municipalities will encourage leaf collection, sometimes distributing collection bags or vacuuming leaf piles from the street. The goal is to prevent leaves from making their way into our stormwater systems, where clogs could lead to flooding in the case of a large rain event. It is therefore important to follow local ordinances to relocate leaves in front and side yards that border a road. However, fallen leaves are valuable resources containing essential nutrients, and you may consider keeping some of this ruby and gold treasure for yourself!

Autumn leaves in reds, oranges, and yellows are beneficial as well as beautiful. Credit: Wikimedia Commons.

Here are some ideas for sustainably managing leaves in back yards and other areas where leaves are unlikely to be washed down a storm drain:

Move leaves onto garden and flower beds. You wouldn’t want to leave a thick mat of leaves on your lawn—it would kill the grass beneath; for that same reason, leaf cover is an excellent substitute anywhere you might put mulch. A layer of leaves (aim for at least two inches) will suppress weed growth in overwintering vegetable gardens, perennial flower beds, and around decorative shrubs. To stop the leaves from blowing away, wet them with a hose or watering can. These leaves will decompose slowly, releasing micronutrients back to the soil over time, retaining soil moisture, and helping your plants grow in the spring.

If you have more leaves than needed for your landscaping beds, simply mow over them where they fall. Mowing will break leaves down into smaller pieces, both preventing the thick cover that would smother your grass and speeding up the decomposition process. The additional organic matter will benefit your lawn.

Select a sheltered spot in your yard to leave fallen leaves intact. Insects need protection from the winter chill if they are to survive to the spring, and fallen leaves are excellent habitat! They insulate beneficial insects amidst the litter and in the soil below. Don’t mow the leaves in this sheltered spot, or you’ll shred the hibernating insects, too!

Ladybugs need warm, protected spots like leaf piles to survive the winter; as avid aphid-eaters, you’ll be glad to have them in your garden come summer. Credit: PickPik.

If you have a backyard composting system, fallen leaves are a great addition! The ideal compost mixture contains a combination of “greens” and “browns” (nitrogen-rich and carbon-rich materials, respectively). Dry leaves are considered “browns.” Crumbling them first will speed up the process.

Whatever options you choose, I highly recommend using rakes or at least electric mowers/leaf blowers over gasoline-powered yard tools, which are noisier and emit pollutants including carbon dioxide, methane, benzene, and other toxic and carcinogenic compounds that harm both human and environmental health. Now, I’ll ‘leaf’ you to it!

Manual yard tools like rakes are a more environmentally-friendly way to move leaves than gas-powered leaf blowers. Credit: Elenathewise, Getty Images.

By Sarah Rothman, Postdoctoral Fellow and Adjunct Faculty in the Department of Environmental Science and Technology at the University of Maryland. Read more by Sarah.

Mosquito Repellent Myths

September 4, 2025April 2, 2026Maryland Grows3 Comments

Post-Labor Day, you may be packing up your beach gear and saying an unofficial goodbye to summer, but at least one of the season’s staples is sure to stick around—mosquitoes. With the hottest days of the year behind us, and pleasant autumn temperatures ahead, you may find yourself spending even more time outdoors with mosquitoes than before.

Adult female mosquitoes can generally continue their activities—flying, feeding, laying eggs—in temperatures above 50˚F. The average September temperature here in Maryland over the past five years (2020–2024) was 70.4˚F. You could even continue to be bitten into October, as the recent five-year average temperature was 59.6˚F. It may not be until November, which had an average temperature of 49.1˚F over the past five years, that you will be rid of mosquitoes for the season.

There are many popular methods to repel mosquitoes, often marketed by stores or trending online, that claim to be supported by science, but that don’t really work. Many of these involve plants, and may be especially tempting for gardeners. Before you succumb to the hype, let’s take a look at some common mosquito repellent myths.

Aromatic plants

There are thousands of articles for gardeners that advise planting basil, beebalm, catnip, fennel, garlic, geraniums, lavender, lemon balm, lemongrass, marigolds, mint, rosemary, sage, and more to keep your yard mosquito-free. The idea is that, because mosquitoes use olfactory cues to find you, a heavily perfumed garden will mask your scent and prevent mosquitoes from detecting your presence. Yet, I have been bitten while watering my lemon balm, which is between my catnip and mint.

This myth persists based on assertions that these plants have been proven to repel mosquitoes—but there is a huge caveat. Scientific studies of the repellency of these plants test essential oils applied directly to the skin, not human proximity to a whole plant. Essential oils, the compounds that give the plant its scent, are extracted through pressing, boiling, distilling, filtering, and/or using solvents on the leaves, and are therefore more potent than the scent of a whole, untouched leaf on a live plant. I have searched and have yet to find a single scientific study that tests the efficacy of plants at repelling mosquitoes simply by growing nearby. Personal experience tells me the scent is not strong enough to mask my presence, especially since mosquitoes use additional cues like carbon dioxide exhalation, heat, and movement.

Essential oils

Many plant essential oils do demonstrate repellency when applied directly to human skin, but not as effectively as EPA-approved repellents containing DEET, picaridin, or oil of lemon eucalyptus (which is actually not an essential oil, but an extract that has been refined and enriched for its active ingredient). This is because mosquitoes have multiple chemical receptors that they use to find you, and essential oils block fewer of those receptors and for a shorter amount of time than DEET, picaridin, or oil of lemon eucalyptus. For example, one study found that participants using DEET did not get a single mosquito bite for six hours, while the complete protection time (no bites) of citronella oil was only 10.5 minutes and that of fennel oil was 8.4 minutes. Though the essential oils continued to show some repellency for a couple hours (fewer bites than using no repellent at all), you have to ask yourself if your goal is to be bitten less, or to not be bitten at all. Be warned that repellents containing only essential oils are not regulated the same way by the EPA, and manufacturers may tout the incomplete protection time on the bottles. One study that tested some commercially available repellents containing cedar, citronella, lemongrass, geranium, and peppermint essential oils found that the complete protection times were all below 20 minutes.

Citronella candles

Speaking of citronella, candles promising to repel mosquitoes rely on the same premise as above: the citronella smell will mask your own. This means the same limitations apply: not all of the mosquitoes’ chemical receptors will be blocked, and cues such as carbon dioxide, heat, and movement will still be available. Perhaps that is why one study found that citronella candles had no effect on mosquito attraction to human test subjects.

So, how to enjoy your time outdoors as the weather cools down? Scientifically-backed methods include wearing loose clothing and/or thicker fabrics that cover your arms, legs, and feet, so that mosquitoes can’t reach your skin; applying EPA-approved repellents; and sitting in the path of a fan, as mosquitoes are not very strong fliers and will be blown off-course by the moving air. The American Mosquito Control Association also recommends mosquito coils and Therma-cell devices that release a synthetic pyrethroid insecticide into the air, which can offer a radius of protection so long as it’s not windy; don’t use these in conjunction with a fan, or you will blow your protection away. Ideally, you will pair these repelling tactics with source reduction to prevent large populations of mosquitoes from sharing your outdoor space in the first place.

By Sarah Rothman, Postdoctoral Fellow and Adjunct Faculty in the Department of Environmental Science and Technology at the University of Maryland. Read more by Sarah.

Supporting Migratory Birds

June 5, 2025Maryland Grows2 Comments

With the return of warm weather, new plant growth, and emerging insects, you may have noticed the return of bird song, too. Migratory birds begin to return to their breeding grounds in May from locations as far south as Central and South America. Some species will stay in Maryland all summer to nest; others are passing through on their way to breeding grounds farther north. Maryland is under the Atlantic migration flyway, so we have quite a lot of migrating birds in our community, including orioles, warblers, swallows, and more!

Map of North America showing the four major bird migration flyways- the Atlantic Flyway, the Mississippi Flyway, The Central Flyway, and the Pacific Flyway. — Map by American Bird Conservancy

With bird diversity at its peak in our region due to the return of summer species and the temporary presence of northern species on their way home, it’s a great time to go birding! While the Maryland Ornithological Society’s annual May Count has already passed, you can check out their calendar for smaller birding events across the state, including meetings, tours, and educational opportunities.

You can also see which birds are coming and going through the BirdCast Migration Dashboard —enter your county to see how many birds flew over the previous night and cumulatively this season, as well as a list of species to expect this time of year. The Audubon Society hosts an interactive migration map where you can select a location and a bird species to watch where birds travel throughout the year, thanks to data collected from individually-tagged birds.

In addition to tracking their progress and contributing to community science efforts, there are steps you can take to help migratory birds on their journeys:

Keep cats indoors or supervise outdoor excursions, as free-roaming outdoor cats frequently prey on birds.

Reduce artificial light at night, as birds often fly at night to avoid predators, stay cool, and navigate by the stars and moon. Artificial light from the ground attracts and disorients migratory birds, disrupting their flight paths and increasing collisions with buildings. Minimize light pollution around your home by turning off outside lights, using red bulbs or red gel filters, and/or adding shades to direct light downward instead of up at the sky. You can also put path or porch lights on timers or install motion sensors, so lights are only on when you need them. Close your curtains or blinds at dusk to protect birds from indoor lights.

Lastly, an action where gardeners can excel: plant native vegetation to provide fruits and support insects for native birds with varied diets. For example, our Maryland State bird, the beloved Baltimore Oriole (Icterus galbula), which spends the winter in Central America, feeds mostly on insects like caterpillars, beetles, and grasshoppers, but also eats berries and nectar. A native plant like serviceberry (any species in the genus Amelanchier) would be an excellent choice; it can host 124 species of caterpillars and produces fruits that look similar to blueberries, another oriole favorite (any species in the genus Vaccinium). Native birches (like the sweet birch, Betula lenta), plums (like the American plum, Prunus americana), and oaks (like white oak, Quercus alba) also support higher caterpillar abundances, and red mulberry (Morus rubra) and native blackberry species (like the Allegheny blackberry, Rubus allegheniensis) grow attractive fruits.

With these actions, you can help protect our beautiful springtime singers!