How to Harden Off Seedlings Without Losing Any Plants

How to Harden Off Seedlings

Harden off seedlings over 7 to 10 days. Set them outside in shade, out of the wind, for 2 to 3 hours on the first day. Add a few hours and a little more sun each day until they are spending 10 to 12 hours outside, then leave them out day and night for a couple of days before planting. Keep tender seedlings indoors on windy days and whenever the temperature is below 45°F. That sequence comes from Nebraska Extension and University of Illinois Extension, and the other extension services cited here describe the same pattern.

Seedlings started indoors have never met real weather. Indoors they grow in still air with water always on hand. Nebraska Extension names the four things an unprepared transplant meets outdoors: cold temperatures, wind, lower amounts of water and strong sunlight. Hardening off introduces them in small doses so the plant can adjust before it has to live in the bed full time. Our companion guide to hardening off seedlings without shock covers what changes inside the plant and what happens when hardening goes too far.

When to Start

Count back from your planting date. University of Maryland Extension advises beginning 1 to 2 weeks before setting plants out in the garden, and both University of Illinois Extension and Penn State Extension say about 2 weeks. The process itself takes 7 to 10 days in Nebraska’s and Iowa State’s guidance and 10 to 14 days in Penn State’s, so starting 2 weeks ahead leaves room for a few days of bad weather.

Pick a mild, calm stretch to begin. For tomatoes and other frost-tender crops, the planting date depends on soil and night temperatures, and our guide to when to plant tomatoes outside covers how to find it.

Change the watering at the same time. Reduce how often you water to slow growth, but do not let the plants wilt. Maryland, Penn State, Illinois and Nebraska all give that instruction in nearly the same words. Penn State’s reason is that frequent watering produces lush, weak growth, which is the opposite of what a transplant needs.

A 10-Day Hardening Off Schedule

The extension sources describe phases and hours without assigning them to numbered days. The plan below arranges those phases across 10 days, the upper end of the 7 to 10 day range.

1. Days 1 and 2. Put the trays outside in a shaded place, sheltered from strong wind, for 2 to 3 hours. Illinois suggests the warmer part of the day, typically between noon and 5 PM. Bring them back in afterward.
2. Days 3 to 5. Each day, lengthen the time outside by a few hours and move the trays where they get more sun. Introduce direct sun gradually.
3. Days 6 to 8. The seedlings should now be outside for 10 to 12 hours a day, in the light they will get in the garden. Hold at this stage for several days and bring them in at night.
4. Days 9 and 10. Leave them outside for the full 24 hours for a couple of days, provided the nights are warm enough for the crop.

After that, plant. For a 7 day version, the lower end of the range, drop one shaded day and one day from each of the two middle phases, and keep the two final nights out.

Watch the plants more closely than you did indoors. Iowa State University Extension recommends checking them at least once or twice a day and watering thoroughly when the soil surface is dry to the touch. Small pots dry quickly in wind and sun, and a tray that was fine at breakfast can be wilting by mid-afternoon.

Temperature Limits for Tender and Hardy Seedlings

Three numbers settle most decisions about whether the trays go out and whether they stay out.

• 45°F. Do not put tender seedlings outdoors when the temperature is below this, or on windy days. Maryland and Penn State both use this figure.
• 50°F. Seedlings may be left outside overnight if the temperature will not fall below this, according to University of Maryland Extension.
• 60°F. Warm-season plants prefer nights at least this warm, according to Nebraska Extension.

Crops differ in how much cold they accept. Nebraska Extension notes that hardy cool-season plants can take temperatures in the 40s, so they can stay out on nights that would set back a warm-season crop. Warm-season plants are the ones to hold to the 50°F and 60°F marks, and Nebraska adds that they cannot stand below-freezing temperatures even after hardening off.

When to Pause and Bring Them In

The schedule bends to the weather. Iowa State’s rule is to move the plants indoors if strong winds, a severe storm, or an overnight frost threatens them. A missed day costs nothing. Resume where you left off, or repeat the last stage if the break lasted more than a couple of days.

Wilting is the other signal. Because watering is reduced during this period, a plant can tip from slightly dry to stressed on a bright, breezy afternoon. Water it, move it back to shade, and give it an easier day before pushing on. On any day below 45°F, tender seedlings stay inside regardless of where they are in the plan.

Using a Cold Frame

A cold frame takes much of the carrying out of the job. University of Maryland Extension calls it an excellent environment for hardening off transplants. The seedlings sit outdoors in real light, the frame blocks the wind, and the lid can be opened a little more each day to do the work of moving trays in and out.

The same temperature limits apply inside the frame as outside it. Check a thermometer in the frame on the first cold night instead of assuming, and close the lid when the forecast drops toward 45°F for tender plants. A lightweight row cover over hoops gives a similar buffer in the bed after planting, and our guide to row covers compares the fabric grades.

Transplanting Day

Choose the gentlest conditions available for the move into the ground. Penn State Extension says cloudy conditions are best, and that late afternoon or early evening is also acceptable because temperatures are cooler. Iowa State gives the same advice: set plants in the garden in the evening or on a cloudy day if possible.

Water the seedlings within an hour before transplanting. Penn State notes that this keeps the root balls moist and intact, which makes them easier to slide out of the cells without tearing roots. Plant, water them in, and keep an eye on them for the first few days as you did during the 7 to 10 days of hardening.