What Does a Cool Spring Have to Do With Your Garden?

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We’re now a few weeks past the average last frost date in much of Wilkes County, but it does feel like spring is holding on a little longer with cooler than average temperatures on more days than not. This is making for pleasant spring gardening weather, reducing the risk of getting overheated while gardening, but it can have some implications in how things grow in the garden and what pests you may see.

According to weather data from 1956 to 2019 compiled through the North Carolina State Climate Office, the last average frost date in North Wilkesboro (elevation 1120 feet above sea level) is April 21, and first average frost in fall is October 19. This year, of course, we had freeze events on the nights of April 22 and 23, which damaged some of local the tree fruit and wine grape crop, and may have damaged cold-sensitive garden plants that were set out early. Those that remember last spring know that the later frosts are not to be ruled out! This year, many have reported damage to recently leafed-out trees, shrubs, and perennials. For the most part this damage was temporary and not significant, although by damaging flowers and flower buds, it could reduce some of the floral resource available to our local pollinators.

What about the ongoing cooler temperatures? The main things this cool weather may do is slow things down- both pests and our plants. Many spring gardeners grow cool-season plants such as lettuce, spinach, radishes, cabbage, and broccoli. These crops may be performing better than average this year, and harvests may be extended. Summer transplants like tomatoes and peppers are likely benefiting from cooler temperatures, which put less stress on the plants as they establish their root systems. However, some of the early fruit could be misshapen or show catfacing due to the cool temperatures.

Some of our crops, especially heat-loving plants like okra, squash, watermelon, and corn, may be slow to germinate and grow while it stays cool. Direct seeded crops can be slow to germinate and may germinate unevenly if planted when soil temperatures are less than optimal. In many cases, soil temperatures in the top couple inches of soil, where seeds are planted, are similar to our air temperatures. If our daytime high is 75 F and our night temperature drops to 50 F, then the average temperature for that day is somewhere around 60- 65 F. This is a common minimum temperature for germination for many heat-loving plants like corn, cucumber, and melons. However, these crops will all germinate much more quickly and evenly if the average daytime and soil temperatures are closer to their optimum germination temperature, which is, for many of them, 75-85 F. Darker soils will absorb heat and therefore may have more rapid germination.

Cooler temperatures mean that insect pests are slower to emerge and complete their life cycles. Plants are also less stressed, and therefore may be somewhat less attractive to insects. One pest that is favored by cool weather is slugs, which are a mollusk and not an insect. Slugs are likely to remain quite active as temperatures stay cool. Slugs will feed on foliage of greens such as kale and lettuce, and they will also eat seedlings of different plants, including beans. They leave a slimy residue when they feed. They are attracted to gardens with mulch or residue on the surface. To verify that you have slugs, place a cardboard square in your garden and look under the square in the early morning. Slugs will congregate there. To control them, many use a beer bait trap, but iron phosphate bait works well.

Diseases and blights are always of concern in local gardens, and similarly the cool temperatures may favor some diseases and reduce others. Common summer blights may not show up until later, which could help extend the harvest season. Others may be favored by cooler temperatures. Botrytis or gray mold is a disease that can affect flowers and new foliar growth, and it is more likely to infect during cool and cloudy weather. If you see a flower with gray mold growing, this is likely due to a botrytis infection. But, humidity plays a key role in disease development. Regular rain fall and higher relative humidity are necessary for most of our summer diseases to spread. Cool night temperatures throughout the season extend nighttime dews, which is another primary factor in disease development.

In summary, the cool season should be enjoyed by those who like to be outside while it’s not so hot! Those with spring gardens should enjoy a longer harvest window this year, with perhaps a little less pest management than normal. We should all be patient with our summer crops, perhaps even delaying planting of some. And, be sure to drink plenty of water, use sunscreen, and take breaks when working in the garden once temperatures do heat up.