Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
As we move from September into October, frost becomes a factor in harvest and recovery of vegetables. Later in the fall, freezes can become a concern. In the past, the first frost on inland sites generally occurred by the third week in October in the middle of Delmarva. However, this has changed. From 2019-2021 the first temperature below freezing in the Laurel, DE area occurred on November 12, 18 and 14 respectively. Prior to that the frost dates were Oct 22 in 2018 (29.5), Nov 10 in 2017 (27.3), Oct 26 in 2016 (31.3), Oct 18 in 2015 (31.6), Oct 20 in 2014 (31.9), Oct 24 (29.6) in 2013, and Oct 13 (28.9°F) in 2012. The first hard freeze (below 28°F) in the Laurel area has remained in Mid-November occurring on Nov 12, Nov 18, Nov 14, Nov 12, Nov 10, Nov 12, Nov 15, and Nov 8 from 2021 to 2014 respectively. Coastal areas will see a delay in frost. For example, Kitts Hummock, near the Delaware Bay, had first frosts on Nov 14, Nov 18, Nov 2, Nov 10, Nov 10, and Nov 12 over the last 6 years. Are frost free Octobers now the normal in Delaware? Data would say yes.
Light to moderate frosts will not affect cool season vegetables such as cole crops, lettuce, and spinach. Some cool season crops, such as beets, Brussels sprouts, broccoli, kale, and collards will handle freezing conditions. In contrast, cauliflower, once frozen, can 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”.
Some beets can withstand temperatures as low as 12°F
Peppers will tolerate frosts in the fall, tomatoes will not
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 in warm season crops. For example, harvest all tomatoes (ripe, breakers, and mature greens) prior to a frost.
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 on spring frost protection).
Brussels sprouts are frost and freeze tolerant to 20°F.
Lima bean harvest is minimally affected after a light frost. However, after a freeze, lima beans must be harvested within 48 hours.