By Pat Hitchcock, U.C. Master Gardener of Napa County
When should home gardeners start planting their spring or summer vegetable patches? A recent warm spell with near record-breaking daytime temperatures might have inspired you to consider getting some seeds and plants into the ground.
Your dreams of fresh produce might have been encouraged if you saw all the vegetable seedlings at local nurseries. However, nature is probably not finished with winter yet. Many of those nursery seedlings will not thrive if we have chilly days and nights ahead.
A look at the data can be enlightening. Here are the average monthly high and low temperatures for Napa: February, 62°F and 42°F; March, 67°F and 44°F degrees; and April, 71°F and 45°. But these temperatures are averages, which means that many days have lower lows or higher highs. The record shows that our area has a 90 percent chance of experiencing frost in January, dropping to a 10 percent chance in April. So there is still some chance of frost in the next eight weeks.
What does this mean for vegetables? Annual vegetables belong to several different plant families, but they may be loosely divided into two kinds: cool season and warm season. Cool-season vegetables grow best and produce the highest quality crop when average temperatures are 55°F to 75°F, and they usually tolerate slight frost when mature. In contrast, warm-season vegetables require long, hot days and warm soil to mature. They grow best when average temperatures are 65°F to 95°F, and they are damaged by frost.
March and April are much better for cool-season vegetables than for warm ones. Not only are the temperatures better suited to cool-season produce like broccoli, but they are anathema to warm-season vegetables. If we get frost, these tender plants would need to be protected from exposure that would otherwise kill them. The soil right now is pretty cold as well, and the heat-loving vegetables would not grow much until both soil and air temperatures warm up.
Fortunately, there are a lot of cool-season vegetables and many of them mature in 45 to 70 days. The list includes radishes, beets, carrots, lettuce, kale, spinach, Asian greens, broccoli and cauliflower. So you can plant these cool-season vegetables now and be finished with harvesting in May, in time to plan warm-season veggies. If you have limited garden space, this is one way to maximize production.
In Napa Valley, May is the first month when the average night temperatures hit 50°F, while daytime highs are 76°F on average. Over the summer and into early fall, daytime temperatures climb to the 80s and low 90s and nighttime averages stay in the 50s. These temperatures provide a four- to five- month growing season for the vegetables that need warm soil and warm air to thrive.
The long growing season is helpful because many warm-season plants need a lot of time to mature. In many cases, the part we eat is a fruit, botanically speaking. Melons, tomatoes, winter squashes and pumpkins are all fruits that we eat when ripe. Sweet corn, green beans and summer squash are immature fruits.
May is also the month when soils have warmed enough to plant warm-season produce. Although nurseries sell seedlings of many vegetables for you to transplant, several warm-season crops do better when started from seed planted directly in the ground. These include beans, squashes, cucumbers, melons and pumpkins. The minimum soil temperature needed for these seeds to sprout is 60°F. If you plant the seeds in cold soil, they will not sprout and may rot. In warm soil, the seed will come up readily, rapidly establish a good root system and grow well.
Is it time to plant vegetables? Yes, as long as you choose cool-season plants. Hold off on summer's heat-loving crops until the weather is warmer.
Workshops: U. C. Master Gardeners of Napa County will lead a workshop on “Growing Spring and Summer Vegetables” on Saturday, March 12, from 9:30 a.m. to 11:30 a.m., at the University of California Cooperative Extension office, 1710 Soscol Avenue, Napa. Learn the requirements for success with summer vegetables from seed to starts. Topics include soil types and temperature; when to plant seedlings; how to water, fertilize, harvest and manage pests and diseases. On-line registration (credit card only); Mail-in/Walk-in registration (cash or check only).
The workshop will be repeated on Sunday, March 13, from 2 p.m. to 4 p.m., at the Yountville Community Center, 6516 Washington Street, Yountville. Register for the Yountville workshop at the Parks and Recreation Department at 707-944-8712 or on its web site.
Compost Workshops: on March 5 starting at 9 am and again at 11 am and Worm Composting on March 19 at 9 am. Preregistration is required. Find more information and registration links at our website http://ucanr.edu/ucmgnapa
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. U. C. Master Gardeners of Napa County ( http://ucanr.edu/ucmgnapa/) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.