hardening transplants

Hardening Transplants

Guest Article By Cheryl Moore-Gough:

Springtime weather is gorgeous! The sun is shining, there’s little wind, and you’re in the mood to plant something! So, you run to the local greenhouse and purchase a flat of transplants, run home, and tuck them into your garden. Smart, yes? NO.

Taking a plant out of a luxurious growing situation and putting it directly into a harsher one can be hugely detrimental to its health. Such is the case if you purchase a plant from a greenhouse and plant it directly to the garden, without giving it the chance to acclimate, by taking a little extra care before setting it into its permanent home.

The pros call this acclimation process “hardening” and it’s important you follow these simple instructions to give your new plants the optimal chance of surviving our oftentimes harsh spring conditions.

Hardening Transplants

  • Select healthy transplants carefully with a maturity well-within your area’s length of growing season.
  • Take your transplants home and slowly introduce them to your outdoor conditions. Remember that a plant in a greenhouse has been experiencing optimal conditions – perfect light, humidity, lack of wind speed, and temperatures. Outdoor conditions have excessive, and oftentimes harsh sunlight, dry air conditions, high winds, and less-than-optimal temperatures at night.
  • When you get home, place your new plants in a protected spot with dappled sunlight. Be sure they are watered well. If your new plants show any signs of “flagging” (drooping) bring them indoors to a more protected spot until they recover. Then try them outside again.
  • Gradually increase the exposure to sunlight and time outdoors, at least over several days and preferably for a week or so.
  • Plant your transplants on the afternoon of a cloudy, still day. Consider protecting your plants from cold nighttime temperatures with cloches, HotKaps, or Wall O’ Waters. If you use transparent cloches, be sure to remove them during the day so the temperatures don’t get too hot, or are vented to let the heat out! HotKaps should have a vent torn in the top for the same reason. The water inside the channels of the Wall O’ Waters insulate from high heat during the day, and release that heat during cold nights, thereby moderating our fluctuating temperatures.

A Note About Installing Wall O’ Water:

The easiest way to place and fill a Wall O’ Water is to use a 5 gallon bucket. Plant your transplant, then place the bucket upside down over the plant, making note of the location of the bucket’s bale. Fill each channel of the Wall O’ Water, starting by filling opposite channels, until they are all filled within 3-4 inches of the top. Take care of your back while doing this! Take breaks and rest if you’re feeling a catch in your get-along!

When all the channels are full, reach into the Wall O’ Water, grab the bucket where it sits on the ground (find the bale and hold onto it as well) and pull the bucket out. The top of the Wall O’ Water will rest against itself, forming something of a teepee. This is what will protect your plant! When you water, be sure to open this top and apply water to the plant inside.


Other ways to moderate cold nighttime temperatures include:

  • Covering your crop with a floating rowcover, also effective in deterring flea beetles and other early season flying pests.
  • Some people use Christmas lights under the rowcovers for additional heat if it’s expected to be really cold.
  • Placing gallon milk containers, filled with water amongst your transplants during the day will also absorb heat and release it during the night.
  • Planting transplants inside old tires is another idea. These tires can also absorb the sun’s heat and release it at night, but can become mosquito hatcheries if water gets trapped inside.

Cheryl Moore-Gough M.S. Plant Sciences retired in 2008 as the Montana State Extension Horticulturist, after leading the Montana Master Gardener program and writing numerous Extension yard and garden publications. She is now an Adjunct Professor at Montana State University with the Department of Plant Sciences and Plant Pathology where she teaches vegetable production and plant biology, and was Rocky Mountain Gardening (formerly Zone 4) magazine’s technical editor for horticulture. She is an accomplished author of seven books and her articles have been published in many acclaimed horticultural magazines. Producer and host of the Northern News Network’s daily Northern Gardening Tips radio program which is in its 21st year, Cheryl speaks throughout the western United States on a wide variety of yard and garden topics, including seed saving, vegetable production, and many more seasonally appropriate topics.