Especially in the spring, it is important to realize when you can sow seeds directly in the ground.  While I used to wing it, this year I bought the 7 inch soil thermometer from Botanical Interests. Do a search and find one that you like online.
While the snow is on the way out (I hope!), I wanted to see what the temperature was in my raised beds.  The beds without any fabric covers over it had a temperature of 40 degrees Fahrenheit.  The beds that I had two covers over pvc hoops had a temperature of 50 degrees Fahrenheit. The two covers that I use are All Purpose Garden Fabric which I purchase from Gardener’s Supply Company and 6 mil plastic sheeting on top.
So why is the temperature important??
Some vegetables can germinate with a soil temperature of at least 40 degrees Fahrenheit which includes arugula, lettuce, mustard, and spinach.
At 45 degrees Fahrenheit, you can sow bok choy, carrots, Chinese cabbage, fava beans, peas, radishes, snow peas and sugar snaps.
Other vegetables can germinate at 50 degrees Fahrenheit such as arugula, beets, leeks, mache, onions, shallots, spinach and swiss chard.
At 55 degrees Fahrenheit, you can sow broccoli, brussels sprouts, cabbage, cauliflower, celery, celeriac, kale, kohlrabi, parsnips, romanesco, rutabagas and turnips.
I got most of the above temperatures from the Territorial Seed catalog. I did notice some different temperatures when I checked out the High Mowing Seed catalog. I think the above will help get you started.
Depending on where you live, you may want to start sowing broccoli, brussels sprouts, cauliflower, romanesco, celery and celeriac indoors. When I have room, I will start my greens, onions, shallots and kohlrabi indoors too. This year, I am going to direct sow some of these crops outside under double cover and see how they do.
Since we might still get more snow, I will leave the covers on. If it gets real sunny or hot, I will vent the plastic on the ends.
While we are talking about the minimum temperature for germinating, we will talk about the maximum temperature next time so you can plan your succession crops.
