Gardening in Maryland: zones, frost dates and when to plant

From the western counties to the Lower Eastern Shore

Maryland’s growing conditions change from west to east. University of Maryland Extension (UME) counts about 155 frost-free days in the far western counties and about 230 on the Lower Eastern Shore, and says the season is getting longer overall as the climate changes.

UME’s Vegetable Planting Calendar for Central Maryland is the standard reference, and its “When to Plant Vegetables” guide explains how to adjust it:

• Southern Maryland, the Eastern Shore and the Baltimore area: plant a week or two earlier than the calendar.
• Western and Northern Maryland: plant a week or two later.

Begin with the frost dates for your city above. UME also points gardeners to Purdue’s Freeze Date Tool for county-level frost probabilities.

Reading the UME calendar

The calendar marks each crop with a planting method. Direct seeding in the garden is the default wherever a crop does well that way. Crops that need a long time to mature compared with their preferred season are started indoors and transplanted, and UME notes that raising seedlings indoors requires a grow light. If you buy transplants instead, use the transplant dates. Some crops are marked as able to go in as seedlings for an earlier harvest.

The crop list goes beyond the usual tomatoes and beans. Alongside them are callaloo, jute leaves, waterleaf, hibiscus, bitter melon, ground cherries and tomatillos.

UME divides vegetables into two groups:

• Cool-season crops such as lettuce, spinach and broccoli. Plant them in spring for a spring and early-summer harvest, then plant again in midsummer and early fall for a fall crop.
• Warm-season crops such as tomatoes, peppers and beans. Plant them in late spring and summer. Tender crops such as basil, tomatoes, cucumbers and peppers are damaged or killed by freezing temperatures, so pick the last of them before the first significant frost.

Soil temperature is the best guide for direct seeding. Beans, cucumbers, melons and similar crops need soil of at least 60°F to germinate. Cool, wet soil slows germination and leaves seeds more likely to rot. UME suggests a soil thermometer, an online soil temperature map or the Maryland Mesonet for checking local conditions.

Midsummer heat and fruit set

UME’s page on poor blossom and fruit set lists the causes: long stretches of dry or wet soil, very high or very low temperatures, too much shade, too much nitrogen and hot, dry winds. Picking crops irregularly also tells plants to slow flower and fruit production. The temperature limits UME gives:

• Pepper flowers and small fruit drop when nights fall below 60°F or stay above 75°F, or when days pass 95°F.
• Bean plants stop flowering, or their flowers die, above 85°F.
• Eggplant sets fruit poorly until nights stay above 55°F.

UME’s remedies are adequate sun, water and nutrients plus shelter from wind. Wetting the foliage during hot, dry spells eases heat stress, and placing tomatoes and peppers where they get late-afternoon shade can raise yields over the season.

Clay, compaction and poor drainage

UME’s guide to common soil problems names heavy clay, compaction and poor drainage as a frequent combination. Soil with more than about one-third clay can be sticky when wet and brick-hard when dry. Clay compacts easily, and compacted soil leads to weak roots, root and crown rots and higher acidity. UME advises against working or walking on wet soil and suggests raised beds to lift plants above slow-draining ground. It also notes that gypsum adds calcium and sulfur but will not loosen Maryland’s clay. On new home sites, topsoil has often been stripped or mixed with fill during construction. Topsoil sales are not regulated in Maryland, so ask a supplier for soil test results before you buy in bulk.

Soil testing without a university lab

The University of Maryland no longer runs a soil testing lab. Its Home and Garden Information Center (HGIC) keeps a list of recommended labs instead. UME recommends testing vegetable gardens every three years and says fall is a good time to sample, because amendments then have time to work before spring. Every vegetable garden, wherever it is, should also be tested for lead. Results give baseline pH, nutrient levels and organic matter, plus recommendations for fertilizer and pH adjustment. For help reading the report, send it with your questions to UME’s Ask Extension service. Use the same lab each time so results compare cleanly.