The soil temperature in my garlic patch registered 38 degrees last weekend when I pulled the winter’s mulch off the bed. While the pale garlic tops looked pretty pathetic, the temperature told me that it will soon be time to plant the cool-season vegetable crops.
Vegetables are broken into two general categories: cool and warm season. The warm-season vegetables include tomatoes, peppers, eggplant, all squash, beans and cucumbers. All these are generally planted near the end of May when the danger of frost has passed and soil temperatures have warmed. Cool-season vegetables thrive in the cooler early spring weather and include broccoli, cabbage, cauliflower, peas, onions, spinach, lettuce and other leafy greens, radishes, beets, potatoes and carrots.
Language on a seed packet like “plant as soon as the ground can be worked” or “plant several weeks before the last frost date” are indicative that the seed is a cool-season crop.
In general, the ground can be worked when a handful of soil is squeezed into a ball. If that ball easily falls apart when it is poked, the ground is dry enough to begin planting. Clayey soils will take longer to dry than loamy or sandy soils.
Spokane’s traditional last frost date is May 15. This is a generalized average though and it can vary a lot across the region. Hangman Valley, Deer Park, and Cheney can be as much as two weeks later because of microclimates and altitude than the Spokane Valley or the South Hill. Even then some cool-season vegetables are more frost resistant than others.
Cole crops like cabbage, broccoli, and cauliflower need the cooler weather to grow rapidly and stay sweet. They can survive a low 20s frost with ease. It may be better to plant transplants as soon as the ground can be worked to get a jump on the season. A fall crop can be planted by seed in late July for a late September harvest.
Potatoes need 50-degree soil to get a good start. This is slightly warmer than other cool-season crops. By folk tradition they were always planted on Good Friday. That works well here if Easter is late. However when Easter is in early April, it can take them a long time to emerge. If there is a hard freeze, the emerging leaves can be damaged but do recover.
Radish, spinach, some lettuces and peas need to be planted early because they are day length sensitive. That means they sense the lengthening light as we move to the summer solstice and do their growing. Peas planted earlier than later in the spring will bear more heavily. In the case of radishes, some lettuces and spinach once the day length begins to shorten after the solstice, chemicals in the plants signal the plant to set seed. The plants then bolt and send up a flower stalk and become nearly inedible. This is also why spinach and radishes don’t do well when planted in the fall.