Post by Johno
Every plant needs a suitable climate in which to grow. Season extension means providing that climate before or after it occurs naturally. This can mean heating or cooling. For any specific crop, you want to obtain optimal conditions as fast as possible, and keep them for as long as possible.
Row covers under snow at 5 degrees below zero Fahrenheit.
The English used heated glass structures to house orange trees and other exotics through the winter. The French used cloches, small bell-shaped glass domes, to keep vegetables warm until the weather was ideal. These days we have other materials that are much lighter and less likely to shatter from impact, though glass is still a viable option.
A wonderful material available today is called the floating row cover. It comes in different weights for varying degrees of frost protection, the heavier weights offering the most protection at the cost of light penetration. Water and air pass freely through this material, so you don’t have to worry about removing them on warm days or do much watering unless there hasn’t been enough rain. They also protect from drying winds. The first time I used this material I was amazed at the rate of growth of the salad greens underneath. But don’t think you can start hot weather crops weeks early with this material. It offers a few degrees of protection, so it’s mostly for close calls and cold hardy crops.
For serious protection and very early starts, a hot house is ideal. But short of that expense, hoop houses or low tunnels covered with clear plastic are great for season extension. Hoop houses are tall enough to walk in, and low tunnels are just tall enough to provide a tropical mini-climate for the plants starting underneath. The first time I used a low tunnel to get an early start on corn, the seed germinated in three days, while it was still quite chilly outdoors. The temperature was a balmy 90 degrees Fahrenheit. But the drawback is that temperatures can really soar on unseasonably warm days, so you need to monitor the weather daily and open the ends to draw some of the heat out on occasion – more and more so until it’s time to remove them. You can get ventilated plastic specifically designed for this purpose, too. I also use low tunnels to protect tender perennials and to grow cool-hardy crops throughout the winter.
Another method commonly used for an early start is to cover prepared beds with black plastic and plant directly through small holes in it. It heats the soil up to a more suitable temperature quickly. This can be less of a good method in the south, where the summers are so hot it can work too well. But even in this situation it works for crops that will be finished before triple digit heat arrives.
Heat can be as much of a problem for cool-loving crops as cold is to heat-loving crops, turning them bitter of preventing germination. Shade cloth or lathing can help cool the soil.