The Spring Warm-Up Begins

David Graper
Horticulture Specialist & Director of McCrory Gardens

Above: Prostrate knotweed is one of the earliest weeds to germinate.

Do you like to watch the weather on TV? I think that most gardeners try to watch the weather whenever they can. Of course now we can get up-to-the-minute updates on our smart phones but I still want to catch the weather forecast every day if I can. The weather and especially the temperature are very influential in what happens in the garden and we have little control over it.This week we are finally seeing some nice warm, almost early summer-like temperatures. It is amazing to watch the transformations that occur with the warmer weather. The coats disappear to be replaced by t-shirts and shorts. What looked like barren soil for the last few months suddenly shows signs of life emerging from below.Temperature is what drives that transformation.

Air temperatures and soil temperatures are linked but as you would expect, soils warm up much more slowly than the air and show little variation over time while air temperatures can easily vary by 30 degrees or more in a single day. There are many factors that influence how quickly soils warm in the spring. Dry, sandier, more exposed soil will warm more quickly than wet, clayey and shaded soils. Mulches and turf grass shade the soil and keep it cooler longer into the spring.

This can be beneficial in keeping some plants dormant a little later into the spring to avoid spring freeze damage to tender shoots. If you look around, you will probably see where tulips have been planted up close to a home, particularly on the south side of the home, the tulips are growing quite rapidly. While other sites, that are more shaded or mulched will see later tulip emergence and growth.

Soil temperature can play a more important role in the early spring growth of many different kinds of plants than air temperature. This is particularly true for seed germination of vegetables as well as weeds.Radish seed can germinate in soil as cold as 40° while watermelon seeds need soil temperatures close to 75°. Prostrate knotweed can germinate at a range of soil temperatures ranging from 35° all the way up to 85°. Crabgrass, a common lawn weed, will start germinating when soil temperatures reach about 62° in the top 1-2” of the soil, which usually coincides with the time when lilacs begin blooming.

Vegetable seedlings and transplants also need certain temperatures to germinate as well as grow and produce fruit later in the season. Spinach, peas, radishes, cauliflower and head lettuce can be grown with minimum soil temperatures of 40 to 45° but have an optimum soil temperature of 60 to 65°. But they also will not tolerate high air temperatures that can occur in mid-summer so they need to be planted early in the spring or in some cases, planted again in late summer for a fall crop. Broccoli, onions, cabbage, chard, leaf lettuce, potatoes, carrots, turnips, beets and kohlrabi are in the next group that can be planted and will tolerate warm summer temperatures. Warm season vegetables like beans, sweet corn, squash and pumpkins need at least 50° soil temperatures while cucumbers and muskmelons need at least 60° and tomatoes, eggplant, pepper and watermelons need 65-75° soil temperatures.

Source : SDSU