Question: I want to plant my vegetables early this year so my tomatoes have more time to ripen. How soon can I plant tomatoes outside?
Answer: Tomatoes and other warm weather vegetables and flowers can all be damaged by frost. Our normal last frost date in eastern Idaho varies from May 15 to early June, depending upon elevation. Late frosts can occur even after these dates.
Tomatoes and other tender vegetables can be planted before these dates if protected from frost. Wall-O-Water and similar water jacket plant protectors can protect from 10 degrees of frost or more. Best protection is when the tops are covered temporarily on cold nights.
Row covers may only give 3 or 4 degrees of protection unless temporarily covered by insulating blankets. My favorite row cover is the floating row cover that does not need wire support. Floating row covers also contain tiny holes so water flows through.
Both Wall-O-Water and row covers heat the air around plants so that they grow faster. They both need to be opened or removed when temperature rises to the high 70s.
Another good way to get a head start is to buy and transplant small plants into larger pots and bring them inside at night. By alternating cool bright outside daytime conditions and warm inside night time conditions plants develop strong, compact growth. It takes about two to three weeks to grow a 4 inch plant into a one gallon size.
I start my tomatoes from seed and use this alternating warm night/cool day regime to develop strong, sturdy plants.
Another way to speed growth of tomatoes and other warm weather vegetables like peppers, squash and melons is to cover the soil with red plastic mulch. Red plastic mulch can speed maturity by up to two weeks.
SOIL TEMPERATURE IS IMPORTANT
One of the main reasons why warm weather vegetables grow slowly when planted early is cool soil temperature. Soil temperature is also important for seed germination. Warm weather vegetables like green beans and sweet corn are normally planted from seed directly in the ground. Corn will germinate at a temperature of about 50 to 55 degrees. The soil usually reaches that temperature by early May. Green beans require a soil temperature of about 60 degrees. I usually wait until mid-May to plant beans.
COOL WEATHER VEGETABLES
Virtually all the root, leaf and flower bud vegetables are frost tolerant and do well when planted early. Their seeds will germinate at 50 degrees or lower. April is a good time to plant seeds of pea, carrot, beet, turnip, radish, onion, lettuce and spinach. Started plants of cabbage, broccoli, cauliflower, kale, and pak choi can also be planted now.