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

Delaware’s growing regions

Delaware has the lowest average elevation of any state. NOAA’s state climate summary classifies all of it as coastal zone, and the Atlantic Ocean and Delaware Bay keep temperature extremes in check. Average annual temperature runs about 5°F warmer along the southern coast than in the north. There are two main gardening regions:

• The Northern Piedmont. UD Extension notes that some Delaware soils formed from metamorphic rock under Appalachian oak forest. The north of the state is also its coolest part.
• The Middle Atlantic Coastal Plain. Other soils formed from loose sediments under oak, hickory and pine forest. Average temperatures are highest along the coast in the south.

The jet stream often runs near the state, especially in winter and spring, and the storms that follow it bring frequent rain and changeable temperatures. Strong winter nor’easters are common. NOAA also notes that annual rainfall swings widely from one year to the next.

What limits Delaware gardens

Soil pH that drifts. UD Extension says Delaware’s native soils tend to be slightly to moderately acidic, and they become more acidic as a landscape ages. Its soil lab reports a median pH of 6.1 for home lawn and garden samples. There are exceptions. New homes can have alkaline spots where concrete, stucco and other lime-rich building materials ended up in the soil. Freshly cleared wooded lots and exposed subsoil can be quite acidic. Bagged mixes used to fill raised beds are often very acidic too, frequently below pH 5.5.

Drainage and site. With frequent rain in the cool months, drainage matters. UD’s planning fact sheet tells gardeners who have a choice of site to pick the best well-drained soil available, away from trees and buildings for maximum sun.

Summer workload. UD Extension notes that a garden is easy to keep up in the first weeks of spring and that the real work of weeding, thinning and replanting begins once the weather turns hot. UD’s advice is to garden a small plot well rather than a large one poorly.

What UD Extension recommends

University of Delaware Cooperative Extension’s fact sheet Planning a Vegetable Garden is built around a planting and harvest calendar (HG-34.1). It also includes a Suggested Garden Plan and Planting Dates for a Home Vegetable Garden prepared by the New Castle County Master Gardeners. The core advice:

• Plan on paper first. Draw the plot to scale. Put perennials such as asparagus, rhubarb and strawberries along one side, and keep tall corn where it won’t shade beets and carrots.
• Stretch the season. Use succession plantings of the same crop, plant quick crops between slow ones, and follow early crops with late ones. UD says a well-planned garden can keep producing for seven or eight months.
• Plan a fall garden. The suggested plan follows spring crops with midsummer plantings of broccoli, cabbage, cauliflower, kale and Brussels sprouts for fall harvest, plus late lettuce, spinach, radishes and beans. Spinach sown in fall can overwinter for an early spring picking.
• Wait on tender crops. For tomatoes, peppers, eggplant and melons, the Master Gardeners’ rule of thumb is to plant about two weeks after the date when the chance of another frost drops to 10 percent. They define frost as 35°F or lower.
• Rotate and choose resistance. Don’t plant related vegetables in the same spot in back-to-back years, and pick disease-resistant varieties.

The Master Gardeners also note that published last-frost dates for Delaware disagree, by up to a couple of weeks depending on the station and reference used. Use the frost dates for your city above as your reference.

Soil testing and liming

UD’s Soil Testing Program recommends testing vegetable gardens every 1 to 2 years. Its routine home garden test reports pH, lime requirement, organic matter and nutrients, and includes a screen for lead. Kits are available at all Delaware Extension offices, and buying the bag covers the routine analysis.

If the report calls for lime, mix it into the top 6 to 8 inches several months before planting. If the recommended rate is more than 100 pounds per 1,000 square feet, split it into smaller applications a few months apart. UD warns that permanently lowering the pH of truly alkaline soil is close to impossible, so in those spots choose plants that tolerate higher pH.