Three important factors should be considered before planting seeds in early spring:
Amount of daylight hours-In the winter the sun is weaker in the northern hemisphere and we have less daylight hours. If you plant seeds too early either outside or in a greenhouse, the seedlings will be spindling when they germinate. Once we have 10 hours of daylight (we currently have over 10 hrs), we can start planting our cold hardy seeds. So in Santa Fe, we now have enough daylight hours. But wait, there are several more factors we need to consider before we plant seeds.
Soil temperature-If you are thinking of planting OUTSIDE, forget about it, your soil is probably frozen so of course you can’t plant anything! Even if it’s not frozen, it’s probably still too cold to plant outside. However it will warm up soon. How can you tell what temperature your soil is? You’ll need a soil thermometer. I prefer a compost thermometer that is about 18″ long so you can check both the soil and a compost pile. A soil thermometer is invaluable, as different veggies like to germinate at different soil temperatures. Insert it about the depth of the root zone of the plants, about 4”-6″ in the soil to see how warm it is. Notice the chart above gives an optimum range for each veggie. If you have a cold frame, hoophouse or greenhouse your soil is probably much warmer already. So are you ready to plant? Not quite. There is one more factor to consider.
Air Temperature-The air temperature is also important and is the main thing people think of in considering when to plant seeds. It’s too cold at night to plant most veggie seeds outside or even in a greenhouse without extra protection BUT there are some wintergreens that are very cold hardy, some even hardy below 32°F at night. Even in an unheated cold frame or greenhouse, the temperature dips below freezing at nights so if you have a one, I suggest you put some row cover (winter weight-.9-1.0 mm.) over your beds. If you don’t have a greenhouse and will be planting outside in early spring, definitely put row cover over it at night but don’t forget to check your soil temperatures too.
I’ve compiled a list of these very cold hardy crops that can be started in a greenhouse now if the soil temperature and daylight hours are good. Many of these cold hardy crops can be planted outside as soon as the soil warms ups in March. For the list go to my blog at: http://giantveggiegardener.com.
Thank you for this great post! I am very interested in the soil temperature for vegetable seeds chart- this is exactly the information I need for a school project! I was wondering if you might be able to tell me if there was a publication you got this table from?