Seedlings can be planted now, but its suggested to install a hoop system or use a movable cold frame to protect the plants from frost. I use a cold frame in my garden, as heavy spring snows in Colorado can crush young plants.
Below are a list of annuals and vegetables that tolerate colder temperatures. Note that when planted early plant should also be protected from cold, drying winds. Winds can cool off soil temperatures and stunt plant growth. Some use hoops with heavy clear plastic to protect from winds. Another option is to plant early plants in containers that can be moved into a garage.
Light Frost (L): 28 – 32 F Heavy Frost (H): < 28 F
- Flowers: primroses (L), sweet allysum (L), ranunculus (L), pansy (L), dianthus (L), sweet peas (L)
- Leaf Vegetables: spinach (L/H), lettuce / arugula (L), kales (H), chard (H)
- Cruciferous Vegetables: cauliflour (L), cabbage (H), Brussel sprouts (H)
- Root Vegetables: radish (H), beets (L), Onions (H)
- Herbs: parsley (H)
- Legumes: peas (L/H)
I placed my seeds based on guidelines for the square foot garden. Within 1 sq foot , I created 4 rows with 4 seedlings per row for radishes and beets. Lettuce got a 3 x 3 spacing. For easy spacing, I created templates from cardboard. Holes are made at the right width apart.
Tropicals come later
I will keep my tropicals (tomatoes, peppers) inside until I can reliably sustain 65- 75 temps in my mini-greenhouse. At temps below 65 you may see tomato leaves turn purple from stress as you see in my indoor seedlings sitting next to the leaky aluminum framed windows. Want more information on planting tomatoes in Colorado? See this guide from CSU Extension and the Colorado Master Gardener Program.
Winter squash will also need to stay in a warm environment until soil temperatures reach 65 degrees. These plants here already have buds! Better get to pinching as we have a few weeks to go.
Tomato Temperature gauge
From: Gardenweb Forum
- 92F = This is the temp at which pollen starts clumping and blossoms begin to drop.
- 70F to 92F = This is the goldilocks zone. Tomatoes grow prolifically, flowers set readily, plants need maximum fertility in the soil. The high end of this range is optimum for spread of several foliage diseases.
- 65F to 72F = the best temperature to grow seedlings.
- 50F to 65F = this is the beginning of cold stress. Tomato plants in this range grow slowly, often produce anthcyanins (turn purple), and become pale green from loss of chlorophyll function.
- 32F to 50F = This is the range where normal tomato plants show severe cold stress. Leaves shrivel, turn yellow, wilt, stems lose turgor, roots stop absorbing water. If your plants get this cold overnight, you can reverse the effects by raising the temperature above 90 degrees the next day for the same length of time they were too cold!
- … don’t even think about it with North American Cultivars..
- 0F to 15F = A few Russian cultivars are able to handle temps this low for brief periods of time. This is the low end of the range that wild tomato species S. Habrochaites, S. Chilense, and S. Lycopersicoides can withstand. <– might need to find me some of these!