Crop Protection

In California, we are blessed with a climate that allows for nearly year round gardening. In most regions of the United States, however, you will need to moderate the winter climate to enjoy fresh harvests throughout the winter months. There are a variety of plants that are hardy enough to survive most any winter if properly protected.

Hardy plants don't mind the cold, but they are affected by the alternating freeze / thaw, wet / dry, and gale / calm conditions. For this reason, you will not need to provide heat (for hardy crops) in your protective structures. In fact, too much heat can be detrimental. What you will need to provide is protection from sudden weather changes. The aim of crop protection in the winter is to lessen climatic changes and consequently lessen crop stress. The first step to lessening crop stress is to cover the plants. Clever gardeners have come up with a variety of techniques to cover their plants and extend their growing season.

The Cold Frame

In the world of season extension, the cold frame is king. This low-tech tool is the simplest and most effective way to moderate the garden climate. Basically, it is a box with sides and a glass top (light), but no bottom, which sits on the soil. It can be made to any dimensions desired by the gardener. Cold frame technology has been around since before glass. Early forms of the cold frame used sheets of mica instead of glass. This simple tool was the foundation for the early development of intensive commercial horticulture.

A single layer of glass creates a microclimate in which the nighttime temperatures can be from 10 to 20 degrees warmer than the outside air. The daytime temperatures will generally be 10 to 15 degrees warmer. On sunny days the temperature may need to be regulated by venting. Plants just soaked by a winter rainstorm before freezing will be more stressed than plants that are dry. The glass roof of the cold frame protects plants from excess moisture. Plants are affected by the wind-chill factor. The stress of winter wind alone can mean the difference between life and death for hardy vegetables. Even minimal wind protection can make a significant difference in minimizing plant stress. The cold frame protects plants from the drying and chilling effects of winter wind.

Managing the Cold Frame

The two most important factors in managing a cold frame are temperature control and watering.

Temperature Control: This involves venting excess heat out the top (light). The light can be opened by hand or with an automatic mechanical arm. How much or how long you vent the top depends on the temperature inside the frame. Prop the sides of the lights rather than the fronts and angle them away from the prevailing winds. Keep track of the temperature using a thermometer. Place the thermometer in a small, white, slatted box placed on the soil in the center of the frame. In the spring, aim for about 70 degree daytime temperatures and in the fall try to maintain 60 – 65 degrees. It is better for temperatures to be cooler rather than warmer in the frame. In the winter, your crops are basically done growing. For this reason, you don't want to create warm conditions. Frame temperatures in the 30's and 40's are fine.

Watering the Cold Frame: The key to successful watering is to do very little. Transpiration rates in the winter are low, so waterings should be infrequent. During the dead of winter, when conditions are cold and rainy, you probably won't need to water at all. Attempt to time your watering in the morning so leaves can dry before nightfall.

Starting Seeds in the Frame: Even in warmer climates, such as California, a cold frame is an excellent tool for starting seeds in early spring. Here's how to do it:

1. In the cold frame, spread 2 – 3 inches of potting soil in the area you wish to use. Treat the area as you would a flat and evenly sow your seeds.
2. When the seedlings have developed one set of true leaves, transplant them to another section of the cold frame, which also has a 2 – 3 inch layer of potting mix over the soil.
3. When the outside temperatures are warm enough and the seedlings are established, harden off the plants either by venting more or completely removing the light. Plants started in cold frames require less hardening off than plants started in artificially heated environments.
4. Transplant seedlings into the garden.

Cloches, Tunnels, and Row Covers

Simple covers for the soil can extend the growing period at both ends of the growing season. They will warm the soil before sowing and help it from becoming too wet to sow. They shelter fragile young plants in cold weather, reducing plant stress and allowing for easier establishment. The fabrics available as row covers allow enough light and water through for normal growth. Another benefit of row covers is that they are very effective as physical barriers against insect pests.

Covered Tunnels: Simple tunnels can be made using wire, plastic, or PVC hoops and a length of row cover (usually sold under the product names Reemay, Tufbell, or Agribon). Place the hoops over a garden row or bed and cover with the fabric. The sides and ends of the fabric can be secured with garden staples or by mounding up soil down the length of the row.

Floating Rowcovers: Perhaps the simplest form of crop protection is a floating row which is nothing more than a length of row cover fabric laid over a crop and secured around the edges with garden staples or soil.

Cloches: An overturned plastic bottle with the top cut off can protect small plants in the early stages of growth. This easy to find resource is can be found in a variety of sizes. Cloches will need to be vented on sunny days.

Greenhouses

A well-managed greenhouse is the ultimate in plant protection. Unheated structures extend the growing period by up to two months. With protection and good light, seeds can be sown even earlier. If fitted with a heating system, even tender plants may be grown year round.

Elliot Coleman has had tremendous success extending the season in his garden in Maine using a moveable greenhouse of his own design. Basically, it is a simple system of hoops covered with plastic that attach to fixed side rails in such a way that it allows the whole structure to be slid or moved down the length of a garden space. This type of "low tech" technology has been used in Europe for over a century. The moveable house does not harbor the pest and disease problems associated with the artificial conditions of a normal greenhouse. Moving greenhouses allow for a more diversified rotation of crops. When uncovered, the soil benefits from periodic exposure to the revitalizing effects of sun, rain, and winter temperatures.

Cold frames under these simple structures add additional protection and allow for increased plant options and length of growing season.

Rain Management

Areas of the garden can be protected from rain with tarps or other plastic sheets. Lay the tarp over the area to be protected and secure the sides with garden staples or soil. The water shed by the tarp will need to be diverted out of the garden in a way that causes the least amount of erosion. Generally this method is only practical in small areas due to the amount of water potentially diverted by the tarp. It does allow for the protected soil to be worked earlier than unprotected soil in the spring.