I have included both minimum and optimum soil temperature for germination of seeds. The minimum is the lowest temperature at which the seed will sprout but it often takes a very long time, sometimes as long as a month for some seeds. The optimum temperature is the temperature at which the seed sprouts the fastest…often taking just 2-7 days depending on the seed. The importance of soil temperature is often the most over looked aspect of gardening and the most common reason why seeds just will not grow.
Soil temperature is more important than air temperature when planting seeds or seedlings. In the Spring, you can have a warm spell of temperatures in the 70’s while the soil temperature is still in the 40’s. Every vegetable has a preferred soil temperature for seeds or transplants. A soil thermometer is essential for determining the proper planting time. Planting too early, before the soil has had time to warm up, can lead to seed rot, slowed germination, poor growth and disease.
For example, cucumber seeds usually take less than a week to germinate in a soil of 70 degrees F. They could take two weeks at 60 degrees F. Peas will germinate in temperatures as low as 35 degrees F but it will take at least 3-4 weeks, but you can soak the seeds in water for 24 hours indoors at a temperature around 80-85 degrees F to speed the germination process and then plant the soaked seeds outdoors…even if the soil is at only 40 degrees F the soaked pea seeds will more than likely be up in 10-14 days. Tomato transplants need a soil temperature above 60 degrees F for growth. Setting pepper plants out before the soil temperature is 70 degrees F could stunt their growth for the entire growing season.
Correct temperature readings need to be observed on three consecutive mornings. Readings should be taken at a depth of 1 – 2 inches for seeds and 4 – 6 inches for transplants.
Crops that will germinate in the coolest soils (40 degrees) include arugula, fava beans, kale, lettuce, pac choi, parsnips, peas, radicchio, radish and spinach seed. As of right now, I have already planted all of this in my garden 😀
With a soil temperature above 50 degrees, Chinese cabbage, leeks, onions, Swiss chard, and turnips can be planted.
When the soil warms to 60 degrees, warm season and many cool season vegetables can be sown, including beans, beets, broccoli, Brussels sprouts, cabbage, carrots and cauliflower. But be forewarned – beans will not tolerate any frost and may have to be planted again if the temperature goes below freezing.
Wait until the soil warms to above 70 degrees to plant warm season vegetables including tomatoes, eggplants, peppers, cucumbers, squash, corn and melons. Tomatoes and peppers are slow-growing and often take at least 6 weeks to grow to the stage where you can plant them out in the garden. At the other end of the spectrum squash, cucumbers and corn grow quickly and are easier to start outdoors from seed .
Fertilizing can be anything from a side dressing of compost or partially composted, mostly dry chicken or goat hay up to watering with a homemade compost tea or fish emulsion…it really depends on the feeding demands of the plant and the quality of the soil where I have placed it.
I live in hardiness zone 6A and we always get plenty of rain so, as long as the plants aren’t spaced too close together, I don’t usually have to worry about constant watering except for the plants that really love their water…like cucumbers or melons. For some plants it is best to not water near the time of harvest, for example, potatoes will not be as tasty or store as well when watered near the time of harvest.
As for crop tenderness, generally those plants labeled as “hardy” can survive a frost or two and even a freeze….those labeled as “very tender” get chills if you so much as whisper the word frost. “Semi-hardy” can survive a light frost or two but don’t leave em’ out all winter and “tender” means that they almost always die with the first hard frost of the year.