Timing Spring Vegetable Planting in a Changing Climate

Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu

Success with spring planted vegetables depends on getting seeds or transplants into the field at the right time. Timing plantings correctly is getting trickier since the average growing season length for Delaware has increased by 27 days since 1895, with most of the increase occurring since 1990. On average, the last spring frost is occurring 19 days earlier and the first fall frost is occurring 8 days later. However, the spring weather conditions in any given year are unpredictable which can make the season stressful or downright discouraging. Plantings can be damaged by cold weather, or on the other end of the spectrum, unseasonable hot weather can stress cool season crops. Here are some tips to increase your chance for success with spring planted vegetables in a changing climate.

Know Likely Frost/Freeze Free Dates for Your Site
Frost can begin to form when air temperatures drop close to 32 °F. A freeze occurs when temperatures reach 32 °F or lower and a hard freeze occurs with sustained temperatures of 28 °F or lower. Some crops will tolerate a frost or freeze, some will tolerate a hard freeze and some will not tolerate either. The likelihood of frost or freeze after a certain date can be estimated based on past years’ temperatures. The table below shows the probability of a freeze/frost or a hard freeze at 4 locations in Delaware based on recorded temperatures for the 30 years from 1991 to 2020.

Freeze and Hard Freeze Probabilities for Four Locations in Delaware for 1991-2020.

You should also consider conditions that are specific to your location. Urban sites retain more heat creating a microclimate, termed a “heat island”, with a longer frost-free period. Areas near large bodies of water or with topography that allows cold air to settle into lower elevation areas will be protected from freezes when temperatures near 32 °F. Conversely, low areas where cold air collects or flat areas where cold air cannot drain will be more at risk of late spring freezes. Knowing how your site deviates from the average for your area will help you decide if you can plant cold sensitive crops earlier in the season or if you should be more cautious. Even though the average growing season length has increased in Delaware the degree of increase varies by location in the state.

Use Historically Recommended Planting Dates as a Guide
Recommended planting dates for spring vegetables are available for commercial growers in the Mid-Atlantic Commercial Vegetable Production Recommendations. These planting dates are based not only on a crop’s tolerance of cold weather but also on their time to maturity and desired harvest window. For example, Brussels sprouts are cold tolerant, but they are transplanted in June for fall harvest because they require a long growing season. To adapt to warmer spring conditions and a longer growing season, choose dates earlier in the planting window for cool season crops, especially those that are prone to quality problems when exposed to heat (i.e. lettuce, cauliflower, broccoli). In many years, warm season crops can be planted early in their recommended window to achieve earlier harvest.

Variety Selection
For cool season crops that might be impacted by unseasonably hot conditions in April, May and June, choose varieties with heat tolerance. Heat tolerant lettuce varieties will be less prone to bolting and turning bitter tasting. Heat tolerant broccoli and cauliflower are less prone to quality reducing physiological disorders. Another strategy to avoid damage from heat stress is to plant several varieties with different days to maturity. Planting several varieties makes it less likely that all the plants will be at the heat susceptible growth state during a period of hot weather. A similar approach is to make sequential plantings of the same variety during the entire planting window.

Use Season Extension Aids for Very Early Plantings
Many growers use added inputs to modify the environment for seedings and increase the chance of success with early season plantings. This includes plastic mulch, floating row covers, low tunnels, and rye windbreaks. See the past WCU article by Gordon Johnson “Transplanting Warm Season Crops in April” for a detailed discussion of these practices.

Check the Forecast Before Planting
Before deciding on an early planting date, check the weather forecast to determine if predicted conditions will support plant growth. This is especially important if you are planting cold sensitive crops early. If you are in a freeze prone site, note any predicted clear nights which could result in frost even if temperatures are not forecast to drop to 32 °F. Try to plant at a time when temperatures are expected to be warm for a few days afterward. When direct seeding, warm temperatures promote faster germination resulting in less exposure to insects and diseases that impact seeds. Transplanting at the beginning of a warm period allows plants to quickly root into the soil, improving their stress tolerance. Also avoid transplanting right before forecasted strong winds which can quickly dry out transplants that have not yet rooted into the soil.

Acclimate Transplants
Make sure that transplants have the best chance of survival by acclimating them to higher light and lower humidity conditions before planting. Transplants should be moved from the greenhouse to an outdoor location in partial sun and protected from excessive wind. Maintain transplants in these conditions for a week before planting to the field. Move or cover transplants if very cold or windy conditions are expected.

Be Ready to Frost Protect
Even though some recent spring seasons have been warm we have still had years with very late frosts (such as in 2020) so be prepared to protect sensitive plants if freezing conditions are predicted. Some crops like potatoes and sweet corn will regrow if the plants are still small when frost damaged. Other crops like tomatoes, peppers and melons, will not regrow if plants are frozen. Row covers can be used to protect frost sensitive vegetables that have already been planted in the field. Covers should be applied in late afternoon before air temperatures fall to retain heat accumulated in the soil.