Creating Warmer Micro-Climates

Harvesting winter carrots that were protected by a low hoop house

A four-season harvest requires maximizing warmth. These are some techniques I’ve picked up to add or preserve heat at plant level.

 

  • The key concept is that it is a four season harvest, not four season growing. Winter crops are planted from mid- to late-summer and complete a significant portion of their growth before deep winter sets in. Once winter has really arrived, crop growth is greatly reduced; however, as long as a plant doesn’t die it can still be harvested as fresh, nutritious food.
  • Because Central and Southern France are on the same latitude as New England, New England gets as much solar radiation as southern Europe. (The Gulf Stream produces warmer winters in Europe than the Northeastern U.S. receives.) Adopting techniques used in Europe to maximize solar gain in New England results in enough solar gain to keep winter-hardy crops going all winter.
  • Winter wind is a great enemy of plants; the bitter blowing air dries out the plants and kills them. There are a variety of ways to protect plants from the wind: cold frames, plastic covering tunnels, solid or semi-permeable wind breaks are prominent methods used in Europe – some for generations – and increasingly in the northeastern U.S.
  • A wind break protecting south-facing vegetable plots creates a micro-climate where temperatures are 5 to 7 degrees F warmer than outside of the protected area.
  • East German Poster Promoting Windbreaks, 1952

    A wind break provides between 2 and 7 degrees increased warmth; the amount of gain gradually diminishes further from the break, as far as 8 times the height of the wind break.

  • Short windbreaks can be placed along the north side of garden plots to block wind, reflect solar radiation, and increase the warmth of the soil on the south side of the mini-wind break, making warm-weather crops viable earlier in the spring season.
  • Canting garden plots 5 – 7 degrees toward the south will increase winter solar gain up to 20%.
  • Dark soil captures more solar radiation than light soil. Soil can be darkened by good cultivation practices – that is, by mixing in good amounts of fully matured compost. Soot can also be spread on the garden to increase winter gain with no harm to the soil.
  • A low plastic blanket, just 6 or 12 inches above the plants, helps trap the warmth stored in the earth during the summer, effectively creating a warmer micro-climate at plant level. Combining a blanket or cold frame with an overarching wind-stopping plastic tunnel or cold greenhouse can increase the temperature at plant level as much as 40 degrees. In trials in Maine, Eliot Coleman measured air temperature at plant level when the outside temperature was -25 degrees F; it was 15 degrees F next to the plants.
  • In Europe, plastic tubes of water are placed along the ground between rows of winter plants. Those tubes collect and store solar radiation during the days, and release it in the form of added heat at night.
  • A simple pup tent of plastic can be used to create a micro greenhouse over newly transplanted early season crops. Simply stretch a wire between posts and drape plastic over it; anchor the side of the plastic to the ground by covering them with earth, or holding them down with sandbags. Pinch the ends closed with clothes pins.