Spring is fleeting and summer is long. To mark the advent of spring, follow this climate-conscious resource and tip sheet informed by some of the mid-Atlantic’s best gardeners and educators who are out to help us turn upheaval into opportunity.
What climate zones mean
The USDA Plant Hardiness Zone Map classifies cold hardiness of regions in 10-degree increments according to running averages of minimum recorded temperatures. Baltimore has various classifications including 7a to the north, 7b to the south and a warmer 8a in downtown. In a perfect world, this map would be updated more frequently and its zones would be more exact, but they are still extremely useful to gardeners.
For example, if you read that the Norway Spruce is classed as suitable from zones 2 to 7, the 7 meaning it is on the bubble of its cold hardiness range, then resist planting it. Look around town. Baltimore is now studded with big, dying, heat-stressed former “living Christmas trees.”
For information about how cold hardiness zones tie into the greater picture of climate change, Maryland State Climatologist Alfredo Ruiz-Barradas points to NOAA’s climate summary for Maryland and the District of Columbia. This observes a 2.5 degree increase in temperature since the beginning of the 20th century and projects further increases.
Other tangible and predicted future impacts of what meteorologists are calling “the weirdification of weather” include stormier storms, hotter hot spells, warmer nights, more rain in winter and spring, and sea level rise accompanied by salinity problems in coastal soils.
For a small state, Maryland has a lot of geological regions. Understanding which is where can help you avoid planting mistakes. Baltimore straddles the fall line (picture the path of I-95) between the Atlantic Coastal Plain and the inland Piedmont. Most of the city and county is located in the upland Piedmont, and southernmost Baltimore City and Anne Arundel County in the Coastal Plain. Frederick County, by contrast, is in the transition between the Piedmont and Blue Ridge provinces. Swamp-loving plants, for example, such as pin oaks, are not good choices for most upland Piedmont locations.
“Compost improves any soil,” says Barbara W. Ellis, an Eastern Shore gardener and author of Chesapeake Gardening and Landscaping: The Essential Green Guide. Adding organic matter mimics natural soil replenishment, a process that aids water retention in the sandy coastal plain and assists drainage in whatever native soils are left by developers in the Piedmont’s clay-rock uplands. Across a home garden, school or any cultivated landscape, soil building can be done in a variety of ways, from saving autumn leaves to visiting local municipal facilities for compost and chips.
Baltimore City’s Camp Small is a great source for wood chips, mulch and custom woodwork. Hollins Organic, whose website has a handy mulch calculator, delivers compost and mulch. The University of Maryland’s Extension program urges all urban gardeners who plan to create vegetable gardens on impacted ground to have their soil tested, not least for lead and other contaminants.
Top dressing soil with grass clippings, leaves and wood chips is key to insulating soil against heat and cold and holding moisture, but beware of just dumping chips. Because early clear outs and mulching can smush ground dwelling native bees and wipe out chrysalises before butterflies have taken wing, the Xerces Society recommends waiting until after tax season to clear out last year’s summer wildflowers. Once it is time to start forking a (maximum 3″ deep) protective blanket over exposed soil, Ellis cautions against piling mulch up against the trunks of plants, particularly against trees and shrubs. It causes rot. She’s also not a fan of shredded bark. “It makes a solid mat, like a roof over the soil.”
Natives versus exotic plants
The benefits of native plants (in the right places) over exotics are as obvious as they are profound. Maryland’s State Climatologist notes, “native plants require fewer resources than foreign ones.” And they are better adapted, says the City of Alexandria’s Natural Resource Specialist Rod Simmons.
“Most of our native flora — and especially that of the Coastal Plain and Piedmont — have adapted over millennia to long periods of catastrophic drought and heat, as well as wet periods and extremes of cold (for the region), and so are much better suited for planting during climate change than non-native/exotic plants,” he said.
When to plant
The National Gardening Association has a frost calculator about when it’s safe to plant the seeds and seedlings of tender annuals. But, when it comes to planting timelines, what’s right for salads and tomatoes can be perilously short, or even downright wrong for shrubs and trees, which take longer to become established.
“People buy trees and shrubs in spring because they buy them when they’re in bloom,” says Ginny Rosenkranz, a University of Maryland Master Gardener Program coordinator on the Coastal Plain. It’s likely that garden center stock will have been manipulated into early bloom, which will give gardeners a little bit more time to get them in the ground before the heat sets in. “In that sense, get them when you see them,” she said.
But for those who miss the tiny spring window for planting trees and shrubs, fall is the better bet. “The advantage in the fall is you get more moisture in the soil to support root establishment,” says Christa Carignan, digital horticulture education coordinator for Maryland’s extension program. “In the spring it gets so hot quickly around here, if you’re putting in a brand new plant and it’s already getting really high heat exposure, it’s really fighting to get established.”
Carignan also stresses that gardening in the face of climate change isn’t just about planting, but protecting what’s already in the ground, particularly the long-term investment that is a mature shade tree. In a dry summer, this may mean slow watering and resisting crowd-planting around its root zones.
The University of Maryland Extension’s Home and Garden Information Center is an invaluable resource for gardeners.
- Interested in going grass-free? Here’s how to tackle reducing your lawn.
- Want to create a townhouse garden? They teach small space design.
- Looking for a resilient hedge? They have a guide all about privacy screens.
- Want to irrigate with rain instead of municipal supplies? See them about a rain garden.
- Want to do your part to reduce polluted runoff into the Bay? Check out their Bay-Wise Program.
- Want to take classes? Enroll in the Master Gardener program.
- Adkins Arboretum, 12610 Eveland Rd., Ridgely, 21660, (410) 634-2847
- Cylburn Arboretum, 4915 Greenspring Ave., Baltimore, 21209, (410) 367-2217
- Irvine Nature Center, 11201 Garrison Forest Rd., Owings Mills, 21117, (443) 738-9200
- Longwood Gardens, 1001 Longwood Rd., Kennett Square, Pennsylvania 19348, (610) 388-1000
- US National Arboretum, 3501 New York Ave., NE, Washington, D.C. 20002, (202) 245-4523
- Chesapeake Gardening and Landscaping: The Essential Green Guide by Barbara W. Ellis
- Common Invasive Plants, Maryland Department of Natural Resources
- Conservation Landscaping, University of Maryland
- The Green Book for the Buffer: An Illustrated Guidebook for Planting at the Shoreline, Maryland Department of Natural Resources
- An Illustrated History of America’s Plant Hardiness Zones by Tom Packer
- Missouri Botanical Garden Plant Finder
- Native Plants for Wildlife Habitat and Conservation Landscaping, Chesapeake Bay Watershed, US Fish and Wildlife Service
- Recommended Native Plants for Maryland, University of Maryland Extension
- Trees for American Gardens by Donald Wyman
- USDA Plant Hardiness Zone Map
- City of Alexandria Flora & Natural Communities, edited by Rod Simmons, City of Alexandria
- American Horticultural Society
- Garden Club of America
- Horticultural Society of Maryland
- Maryland Native Plant Society
Emily Green is a freelance writer based in Baltimore. Her writing on urban gardening may be found at www.chanceofrain.com.