Hardening off tender transplants to plant in the garden. | The Old Farmer's Almanac

Time to Get Tough: Hardening Off Plants

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Robin Sweetser
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We sell hundreds of vegetable starts each spring and always remind our customers to be sure to harden off the plants before planting them in the garden. Surprisingly, many people have no idea what hardening off means.

Tough Love

One of the most stressful transitions for a tender transplant is to go from life in a cozy greenhouse, where it has been sheltered from the elements, to life in the great outdoors. Our sales greenhouse is unheated so the plants have been subjected to some cold night temperatures but they have not been exposed to direct sun, pounding rain, or gusty winds. If the plants have not had time to gradually acclimate, they can suffer and even die so don’t be in a rush to get them planted. Instead give them a few hours every day in a protected spot out of direct sun and bring them in at night. Each day expose them to a little more sunlight and wind until they are able to be outside all day. Pay close attention to their need for water since young plants in small pots dry out fast. If you aren’t home to tend your plants the easiest way to harden them off is to put them in a cold frame that you can open during the day and close at night

or place them under a protective layer of reemay— the spun-bonded polypropylene sheeting used for floating row covers. This material is lightweight and lets sunlight, water, and air in. It captures some warmth, raising the temperature a few degrees above the outside air. You can make a tunnel by bending lengths of plastic pipe or heavy wire into hoops and pushing the ends into the ground and covering them with reemay.

Think of it as a halfway house for plants. After 5-7 days of this treatment they should be able to survive a move to the real world.

Don’t Kill ‘Em With Kindness

One lady complained that her tomato plants all died after she planted them. She proudly explained that she dug holes, filled them with manure, and planted the tomatoes in the enriched holes. The manure turned out to be the culprit. She had gotten some fresh chicken manure from her neighbor. Chicken manure is extremely high in nitrogen and heats right up. Those poor plants had their roots fried! Fresh manure of any kind is a big no-no in the garden. National organic standards state that we should not plant in any area that has had fresh manure applied on it for 90 days for crops whose edible parts don’t touch the soil such as tomatoes or peppers and 120 days for foods that grow in the soil such as beets, carrots, etc. Fresh manure can harbor parasites and dangerous pathogens such as E. coli so to be on the safe side, manure should be aged for at least one year before it is used in the garden.

Less Means More

Some folks think that if a little fertilizer is good then a lot of fertilizer is even better but this is not the case. If peppers are given too much high nitrogen fertilizer the plants will be lush and beautiful but will have few if any fruits. Hot peppers develop more heat if they are grown under adverse conditions—poor dry soil, heat and unrelenting sunshine will produce much hotter peppers than cool rainy weather and rich soil.

Rather than relying on chemical fertilizers it is better to feed the soil so that it can feed your plants because soil life supports plant life.

Balancing Act

One rule of thumb is to feed your soil as much organic matter as you remove in harvested crops each season.  Instead of dumping on lots of expensive amendments you might not even need, let your soil tests be your guide. Get your soil tested every 3-5 years and follow the recommendations. The single most important thing you can do to make soil nutrients available to your plants is to adjust your pH level, a measure of soil acidity with 7 being neutral. Higher numbers are more alkaline and lower are more acidic. Plants can’t absorb nutrients if the pH isn’t correct. Most plants like slightly acid soil between 6 to 6.5. Potatoes love more acid conditions between 5-5.2.

Toughen Up Those Tender Transplants

 Think of your plant as if they were your children. Practice a little tough love now and they will be ready to face the outside world soon.

See our video on how to harden off plants.

About The Author

Robin Sweetser

Robin has been a contributor to The Old Farmer’s Almanac and the All-Seasons Garden Guide for many years. Read More from Robin Sweetser

2023 Gardening Club