Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
As we move into October, frost becomes a factor in harvest and recovery of vegetables. Later in the fall, freezes can become a concern. The first frost on inland sites generally occurs by the third week in October in the middle of Delmarva. However, this can vary quite a bit. For example, the first temperature below freezing in the Laurel DE area occurred on Oct13 (28.9°F) in 2012, Oct 30 (30.7°F) in 2011, Nov 1 (31.6°F) in 2010, Oct 20 (31.5°F) in 2009 and Oct 20 (29.8 F°) in 2008. The first hard freeze (below 28°F) in the Laurel area occurred on 11/5, 10/31, 11/14, 11/6, and 10/31 from 2012 to 2008 respectively. Coastal areas will see a delay in frost. For example, Kitts Hummock, near the Delaware Bay, had first frosts on 11/6, 10/31, 11/2, 11/6, and 10/31 over the last 5 years.
Light to moderate frosts will not affect cool season vegetables such as cole crops, lettuce, and spinach. Some cool season crops, such as broccoli, kale, and collards will handle freezing conditions. In contrast, cauliflower, once frozen, will deteriorate quickly. Warm season vegetables vary considerably in their ability to tolerate a light frost. For example, pepper is more cold tolerant in the fall than tomato which is severely damaged by frost. Pumpkins and winter squash will have leaf and vine kill with light frost but fruits will remain marketable. Heavier frosts and freezes will damage the fruit. Sweet potatoes must be dug quickly after a frost kills vines and will suffer root damage if soil temperature drops below 40°F. We often have significant acreage of beans still out in the fall. Snap beans and lima beans will have leaf damage but still can be harvested with a light frost. It is when temperatures drop below 28°F and pods freeze that harvest recovery is affected. When lima beans are frosted, you may have several weeks to get into the field and harvest. However, if there is pod freezing, the harvest window drops to a few days, depending on the day temperatures, before seeds start to “sour”.
For unprotected frost sensitive vegetables, it is important to follow weather forecasts closely for risk of frost or freeze. Clear sky conditions after a cold front moves through will be the highest risk for frost or freeze. When risk is high, growers should harvest all marketable produce ahead of the frost or freeze with crops like tomato (ripe, breakers, and mature greens).
Floating row covers offer the best protection of sensitive vegetables against frost and freeze injury, depending on the thickness of the row cover, expect 2-6°F degrees of protection. Moist soil also can store some heat, lessening frost, and sprinklers can be used for fall frost protection (see past articles of spring frost protection).