Open Grow Guide

Hardening off seedlings

By Open Grow Guide Updated

Seedlings raised indoors need time to adjust before they move into the garden. Acclimating plants to the garden environment prevents damage from sudden changes in light intensity and temperature. Plants that are not adequately hardened are less likely to survive transplanting. Day and night temperature changes, more air movement and wind, and full light stimulate plants to build carbohydrate and nutrient reserves and strong cell walls. This guide walks through the steps, from the first fan to the first afternoon outdoors to planting day. The timelines in the facts range from 5 days to three weeks, so each range is given as published.

When: Spring (Mar–May)

When to do it

Plan to start at least two weeks before the day you transplant into the garden. Some growers put plants outside for 5 to 7 days prior to planting. Another schedule increases outdoor exposure over 7 to 10 days, a few hours a day. A longer routine runs for two to three weeks. Conditioning can begin once seedlings are around 3 in (7.6 cm) tall. A fan can start when plants have two true leaves, and it continues until transplanting. Do not put tender seedlings outdoors on windy days or when temperatures are below 45°F (7°C). By the end of two weeks, unless freezing temperatures are forecast, seedlings can stay outside in a sunny spot until transplanting. Melon seedlings should go into the garden only once soil temperatures reach at least 65°F (18°C).

What you need

A small fan is the first tool, placed near seedlings to produce a gentle sway. A fan causes higher transpiration rates that dry seedlings out, so moisture management is important. For the first outings, choose a shaded spot protected from wind. Bright, indirect sunlight under a tree or on a covered porch also suits the early days. At night, a sheltered porch, shed, or garage area can hold the plants, away from gas and engine exhaust. A coldframe can serve as a temporary mini-greenhouse, with a lid you vent and adjust. A lath house or shade cloth lets you shade transplants for a few days before planting. Have a starter fertilizer solution ready, about one-half cup per plant at planting time. Water is the last item, as plants should be watered well before transplanting.

Step by step

  1. Step 1

    Start hardening off seedlings at least two weeks before transplanting

    Two weeks is the minimum time you need before moving them outdoors for good.

  2. Step 2

    Begin conditioning seedlings once they reach about three inches tall

    This height marks the point when they are ready to start growing outdoors.

  3. Step 3

    Set a fan near the seedlings once they grow two true leaves

    Aim it to create a gentle sway, and keep it running until you transplant them.

  4. Step 4

    Watch soil moisture closely while the fan runs

    A fan raises the seedlings' transpiration rate, which dries them out faster than usual.

  5. Step 5

    Place the seedlings outdoors during the day to begin outdoor exposure

    Bring them back inside before the sun goes down.

  6. Step 6

    Start with just a few hours outdoors in the shade

    Choose the warm part of the afternoon, and keep the spot sheltered from wind.

  7. Step 7

    Skip outdoor time on windy days

    Also keep seedlings inside whenever the temperature drops below 45°F (7°C).

  8. Step 8

    Bring the plants back inside before nighttime temperatures begin to fall

    Move them in while it is still mild outside, not after dark.

  9. Step 9

    Place plants on a sheltered porch, shed, or in the garage at night instead

    Keep them away from gas appliances and engine exhaust in that space.

  10. Step 10

    Increase outdoor time by a little each day

    Add a little more direct sunshine each day than the day before.

  11. Step 11

    Move plants into sunlight for longer periods each day over 10 to 14 days

    This gradual schedule steadily raises their sun exposure day by day.

  12. Step 12

    Increase direct sun exposure gradually rather than all at once

    A gradual increase avoids injuring the plants' leaves.

  13. Step 13

    Continue the outdoor conditioning routine for two to three weeks total

    This full window is what the hardening-off period covers.

  14. Step 14

    Try a shorter schedule: set plants outside for 5 to 7 days before planting

    Some growers use this schedule instead of the longer one.

  15. Step 15

    Increase outdoor exposure over 7 to 10 days as another option

    Add just a few hours of outdoor time each day during that stretch.

  16. Step 16

    Move the transplants outside for about a week

    Check on them daily during that week.

  17. Step 17

    Start hardening off indoor-grown plants a week before transplanting

    Increase the hours they spend outdoors each day, and water them less often.

  18. Step 18

    Place the plants in bright, indirect sunlight at first

    A spot under a tree or on a covered porch works well.

  19. Step 19

    Reduce how often you water to prevent lush, weak growth

    Even so, do not let the plants wilt.

  20. Step 20

    Keep watering reduced, since this also gives some hardening effect

    Still, do not let the plants wilt excessively.

  21. Step 21

    Avoid hardening off transplants by cutting back on fertilizer

    This often stunts plants and leaves them struggling to establish in the field.

  22. Step 22

    Lower humidity gradually by increasing the time between mistings

    Raise airflow around the plants before you raise the light intensity.

  23. Step 23

    Decrease watering frequency gradually as the next step

    Follow that with a gradual increase in light intensity.

  24. Step 24

    Check that hardening is complete before you transplant

    Plants that are not adequately hardened are less likely to survive the move.

  25. Step 25

    Leave seedlings outside in a sunny spot by the end of two weeks

    Bring them in only if freezing temperatures are forecast.

  26. Step 26

    Use a coldframe as a temporary mini-greenhouse for hardening off

    Vent the lid a bit farther each day to get the plants used to wind and cold.

  27. Step 27

    Adjust the coldframe lid as needed on cold nights

    This protects the plants from freezing temperatures.

  28. Step 28

    Check the stems before transplanting

    Transplants are ready when their stems feel firm but are not yet woody.

  29. Step 29

    Watch for woody stems as a warning sign

    Woody stems show growth has slowed, and these plants may struggle to resume active growth.

  30. Step 30

    Plant transplants before they flower

    Planting before flowering supports rapid growth and establishment.

  31. Step 31

    Shade transplants for a few days before you move them to the garden

    Use a lath house or shade cloth to provide that shade.

  32. Step 32

    Decrease watering slightly in the days before transplanting

    Stop short of letting the plants wilt.

  33. Step 33

    Water the plants thoroughly before transplanting

    Do this right before you move them to the garden.

  34. Step 34

    Transplant in late afternoon, or choose a cool, cloudy, calm day

    Either timing is the preferred choice for moving transplants outdoors.

  35. Step 35

    Firm the soil around the roots after you set each transplant

    Water with a starter fertilizer solution, about one-half cup of solution per plant.

  36. Step 36

    Expect some wilting when hardened-off plants first meet full sun after transplanting

    They generally recover within a day or so.

  37. Step 37

    Harden off melon seedlings before planting them in the garden

    Transplant them only once soil temperatures reach at least 65°F.

  38. Step 38

    Acclimate plants fully to the garden environment

    This prevents damage from sudden changes in light intensity and temperature.

  39. Step 39

    Expose seedlings to day-and-night temperature swings, more air movement and wind, and full light

    This stimulates them to build carbohydrate and nutrient reserves and strong cell walls.

Mistakes to avoid

Skipping the routine is the biggest risk, since plants that are not adequately hardened are less likely to survive transplanting. Putting tender seedlings out on a windy day, or when temperatures are below 45°F (7°C), is another error. Sunlight needs a slow build, because exposing plants to direct sun too fast can injure them. Cutting fertilizer to harden plants is also a mistake, since this often results in stunted plants that do not establish well in the field. Cutting back water works only in moderation, because plants should not be allowed to wilt excessively. Waiting too long matters as well, as woody stems are a sign that growth has slowed and the plants may have difficulty resuming active growth. Firm but not woody stems are the sign of readiness. Transplants should also be planted before flowering. A fan dries seedlings, so forgetting to manage moisture is a risk. Some wilting after planting is expected, as hardened-off plants generally recover within a day or so. Adjust the coldframe lid as needed to protect plants from freezing temperatures.

How this page was made

  • Written with AI assistance from the sources listed on this page. AI is never the source of a fact.

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