This week, I'd like to spotlight tips for two specific vegetables that you may be starting now--tomatoes, and the cabbage/broccoli family. (AKA cole crops--as in coleslaw.) These are probably the most popular vegetables for starting indoors for transplant later.
That word "transplant" is one of their key similarities. Both tomato and cabbage/broccoli seedlings have many small, flexible, fibrous roots. Their root systems increase by making more and more small roots, which form a mat around the plant, rather than the existing roots getting larger and deeper. What's more, new roots can sprout from any stem that is touching the ground, not just from the original root. This is a key characteristic for plants that cope well with disturbance. In nature, disturbance might come from erosion, flooding, landslides, or animal activity.
In the garden, a prime disturbance is transplanting. Tomatoes and cabbages often show increased growth when transplanted, because their roots are stimulated to new growth by disturbance. Setting the seedlings a bit deeper at transplant gives the roots more soil to colonize, and takes advantage of their ability to make roots from the stem (adventitious roots.) Other plants with different root types resent transplanting. I have written about the various root types HERE
Even if temperatures are warm enough to start seeds of these vegetables outside, you save space, water, and nutrients by starting them in containers (pots, flats, module trays, etc) or in a nursery bed. It also gives you more time to prepare their final destination in the garden.
As I write, snow is falling and my road is closed. That brings to mind another factor in seedling success--heat.
The plants themselves differ widely in hardiness--a tomato plant can't take any frost at all. The seeds need soil to be at least 60 degrees for sprouting, and sprout fastest at 85 degrees. Given that moist soil will be about 10 degrees cooler than the air, heat mats do improve germination, even in the house. Their relative, peppers, need even more heat and will be stunted for months if they experience too much cold as seedlings.
Kale and cabbage are mainstays of the winter garden, famous for frost-hardiness. However, if you've ever let one go to seed, you might have noticed that after overwintering, they bloom in spring and the seed matures in midsummer. Their seeds fall onto warm soil. Biennial kale and cabbage need a minimum of 55 degrees for sprouting, and sprout fastest at around 85 degrees--just like tomatoes.
Broccoli is the same species as kale, but bred into an annual form that sprouts and blooms all in one season. It will germinate in cooler soil--around 45 degrees--than it's biennial relatives.
Many annuals from Asian and Mediterranean regions are better adapted to cold soil, because in their homelands, they needed to grow and finish before the soil dried up in summer. Peas, lettuce, Asian greens, favas, turnips, endive, and cima di rapa sprout best and are tastiest in cool weather. Started in modules or flats, you can be eating them in just over a month in spring weather, and even faster if you start some in a tub in a window indoors, where you can clip young leaves as they form.