Extension Educator, Horticulture
Mother Nature bats last. And sometimes she carries a particularly big bat. This week's weather was a surprise. Once March 1 arrived snow shovels and winter coats were history or at least buried and long forgotten. I even had calls from people who had already planted half their vegetable garden. I wasn't convinced winter was over. I had seen the report that in other El Niño years we received more snow in March than in February. Snow is not necessarily a bad thing, at least for plants. If particularly cold temperatures are predicted (20's and below), then snow is the best mulch we could have to protect the emerging plants. Just in case you are as confused as the robins, here is a run down of timings for spring activities.
The first vegetables that can be planted are the very hardy vegetables. These plants withstand freezing temperatures and hard frosts without injury. They can be planted March 25-April 10. These include: asparagus crowns, cabbage seed, collard, broccoli transplants, kohlrabi, leek seed, leaf lettuce, mustard greens, onion seed and sets, peas, Irish potatoes, radish, rhubarb plants, spinach and turnips. Spinach and lettuce seeds may even be planted in the fall or on late snows. Their seeds can germinate in cool soils of 45 degrees F.
I like taking advantage of these early planting dates. Most of these crops develop higher quality when grown during cool weather and for many they get their growth and produce a crop before all the insects come out to devour them.
In contrast we should wait until mid-May for warm-loving vegetables such as tomatoes, squash, peppers, sweet potatoes, watermelon, okra, lima beans, eggplants, and cucumbers. These plants don't grow well and their seeds often won't germinate well during cool weather. Their seeds germinate best at warm temperatures of 70 degrees F and above. Some annual flowers will withstand light frosts. These include annual phlox, blanket flower, cornflower, cosmos, lobelia, globe amaranth, marigold, morning glory, pansy, petunia, pinks, strawflower, sweet alyssum and sweet pea. I recommend planting these after mid-April. Annual flowers that can be injured by light frosts include ageratum, cockscomb, coleus, flowering tobacco, geranium, impatiens, moss rose, nasturtium, periwinkle, scarlet sage, verbena, wax begonia and zinnia. Planting date for these is mid-May.
However, there are ways to stretch the season by providing frost protection. Now before dreams of orange trees dance in your head realize frost protection will help only a few degrees. One fairly new innovation in early season vegetable production is the floating row cover. Row covers are a type of fabric made of white ultraviolet-light-stable spun bonded polypropylene plastic. They are very lightweight and allow air, light and water penetration. In areas without much wind the cover can be placed directly over the crop and allowed to float over it as it grows. In particularly windy areas a hoop support may be necessary. The protection provided may amount to a 4 degree F difference in temperature on frosty nights.
During daylight hours additional heat is trapped and stored in the soil which can increase both top and root growth of plants. This increased growth contributes to earlier harvest and higher early yields. Covers can also protect against airborne insects. However covers will have to be removed on insect pollinated plants such as squash. Covers can be reused and can stretch the season in the fall also. Covers can allow for planting a few weeks earlier than usual.
When used along with black plastic mulch which can warm the soil temperatures 5 to 10 degrees, the row covers can dramatically increase the soil temperatures and therefore crop earliness of warm loving plants.