Once you decide what to plant, the next issues to consider are whether you will be growing your own transplants, which are plants you raise indoors – whether inside your house or in a greenhouse – or will you directly seed into your garden?
All plants have minimum as well as optimum soil temperature requirements for seed germination. They also have specific requirements for best growth. If you plant seeds in soils that are too cool it can cause, at best, slow germination and at worst can lead to a seed that decays or is destroyed before starting to grow.
The first crops most people will consider planting are cool-season crops, which grow best when both soil and air temperatures are cooler such as in late winter to late spring. The minimum soil temperature for good seed germination for crops such as cabbage, cauliflower or broccoli, is 40 degrees Farenheit, while the optimum is 80 to 85 degrees. Again, I am talking here about soil temperatures and not air temperatures.
By developing your own transplants you get an early start on growth, which will potentially give you more success later, allowing these plants to form larger heads before the summer heat arrives. These vegetables require between five to seven weeks to reach a good transplant size, which means having four to six mature leaves and a good root system. As such, there is no better time than right now to start growing them since they would reach this size about the right time to plant – which is in mid-March.
Other cool season crops such as lettuce or spinach require at least 32 degree soil temperatures. As the soil warms more, so does the success with starting seeds planted directly into the ground outside. That doesn’t mean you can’t also grow transplants of those vegetables, but it could be less of a concern, especially if you are using row covers to help the soil warm faster.
So what about warm season crops such as peppers, tomatoes, eggplant, and onions? For most of these plants it rarely pays to directly seed them into the outside soil, unless you are growing in a greenhouse. That’s because while the minimum soil temperature for good germination is 50 to 60 degrees, the plants are really only going to grow well once soil temperatures reach 80 degrees. By that time it will be late spring, so it’s best to have plants to put in the ground rather than trying to seed directly.
We do see negative effects with early plantings of warm-season crops since outside air temperatures don’t typically translate to warm soil temperatures. That means that even if we don’t get a late cold snap that kills those early plantings, the cooler soil temperatures can really slow and even stop growth. So just because nurseries are selling warm-season plants does not in any way mean it’s time to actually plant them outdoors.
Tomato transplants can reach the size to plant outside in four to six weeks, so determine when you expect to have the plant in the ground. If, for example, you plan to plant by the first of May then you would want to plant seeds indoors at least somewhere around the last week in March to the first of April.
That being said, most people don’t have the best facilities to grow plants indoors and yet they often want to grow bigger plants to put outside. If that’s the case, you likely would want to start planting indoor seeds earlier. Both eggplant and pepper require a little more time, from six to eight weeks, to reach a good size to plant outside. Onion seeds, which are different than sets, require up to eight to 10 weeks. For those who want to store onions, remember that an onion grown from seed will store much better than an onion grown from a set.
Thought for the day: “Life expectancy would grow by leaps and bounds if green vegetables smelled as good as bacon.” —Doug Larson
Have an idea you’d like Mike to consider writing about? Want more information about these topics? Call the Utah State University Extension Grand County office at 435-259-7558 or email Mike Johnson at firstname.lastname@example.org.