Make your plants (almost) Invincible: The fine art of “Hardening Off”

What exactly is “Hardening Off”? I thought plants were supposed to be green and juicy? But when a green and juicy (and also very wimpy) plant meets frigid March winds, bad things happen.

Ironically, most plants, even tropical plants, can withstand much colder temperatures than most people assume. The trick is all about how the plant experiences the cold. The term “Hardening Off” was coined by old gardening curmudgeons to explain the process of preparing a tender, wimpy plant for the cold hard realities of outdoor life. This article will explain in detail how to make your plants cold-tolerant. Here’s the real dirt on the subject:

  1. Never expose your plant to a temperature drop of more than 10 degrees F in one day. In other words, if your plant has been inside your house at 65 degrees for the last two weeks, then the coldest temperature your plant can handle is 55 degrees. Allow your plant to adjust to this new temperature for 2 days, then drop the temperature again. Repeat the process until your plant is used to the temperatures of the current outdoors.
  2. The wind is more damaging than the cold. Most plants grown inside houses or greenhouses have weak stems because they do not experience wind inside. Planting these tender plants outside on windy days, even when it is warm, often results in damage. To eliminate this risk, put your plants outside on warm, sunny days (only during the day). Bring the plants inside at night. The first day, the plants should be protected from almost all wind. The next day, allow the plants to experience gentle wind. Each day, expose the plants to increasing amounts of wind.
  3. Covering your plants with plastic gives 5 degrees of cold protection. Covering also give protection from the wind. Covering does not work miracles and you should still follow the first two points above.
  4. Know your plant. Some plants are more sensitive to cold than others. All plants have absolute limits: Potato plants will die at temperatures below 25, no matter how much you do to protect them or strengthen them. In general, perennials (live many years) and cold-weather annuals are able to tolerate more cold abuse than other plants. Good examples of cold-loving plants would be: Pansies, spinach, cabbage, kale, and snapdragons. Tropical plants and summer-blooming flowers are generally more sensitive and will need more delicate care.
  5. If there is a frost warning for your area, move your plants inside for the night or cover them. Frost occurs on clear (no clouds) nights when the temperature drops below 32 degrees. The cold causes the insides of the plant tissues to freeze. When water freezes, it expands. This expansion literally rips the plants apart on a microscopic level, which effectively kills them. Cover your plants with heavy fabric (blankets, sheets, plastic) if you have already planted them.
  6. Frost in spring is more damaging than frost in fall. Here in Virginia, we sometimes get blessed with a warm fall… And often the first frost is very light. Simply cover your plants with a sheet or plastic, and see which ones survive. You may be surprised to find that your tomatoes are still alive into November! Fall weather is a perfect example of the “Hardening Off” process: Warm days, gentle breezes, and night temperatures that are slowly dropping day by day.

Happy Gardening! If you have any questions, stop by our greenhouse to see us, or email us at contact@windmillheights.com

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