Hardening Off Cannabis Seedlings: A Step-By-Step Schedule
Quick answer: Hardening off is the process of exposing seedlings gradually to outdoor conditions before they’re planted outside. Start 1 to 2 weeks ahead in a shaded, sheltered spot, add an hour or two of sun each day, water a little less without letting plants wilt, and bring them in on cold or windy days.
What Is Hardening Off?
University of Maryland Extension defines hardening off as exposing seedlings gradually to outdoor conditions. The process encourages a change from soft, succulent growth to a firmer, harder growth, so young plants can handle the conditions they’ll meet once they’re planted outside.
Cannabis seedlings started indoors are used to ideal growing conditions. Outside, South Dakota State University Extension notes, plants have no protection against the wind, possible subfreezing temperatures, direct sun and fluctuating moisture. Exposing them slowly is what lets them thrive there. If you’re starting from scratch, how to germinate cannabis seeds covers the first step.
What Changes Inside the Seedling
Hardening slows the growth rate, mostly through less frequent watering, cooler air or both. SDSU Extension describes three changes that follow:
- The cuticle thickens. This protective leaf surface helps prevent water loss on hot, windy and sunny days.
- Carbohydrates build up. They become reserves for new root growth after planting.
- The tissues hold less freezable water, which gives properly hardened early-season crops greater cold tolerance.
What Happens If You Skip It
University of Vermont Extension warns that without hardening off, tender young plants would likely become scorched, shocked, and bowed to the ground by elements they never met indoors. Seedlings moved straight into the garden have a good chance of not surviving the transition.
When to Start Hardening Off Cannabis Seedlings
Begin 1 to 2 weeks before planting outside, says University of Maryland Extension. Other extension guides land in the same range: 5 to 14 days depending on the crop (University of Maryland), 7 to 14 days (South Dakota State University), and at least one week even when you need to speed things up (University of Vermont).
Work back from your planting day. For hemp grown for essential oils, Penn State Extension advises planting after risk of frost, and Oregon State University notes that seeding too early can expose seedlings to frost. Season-by-season sun patterns are covered in how much sunlight outdoor cannabis plants need.
A Day-by-Day Hardening Off Schedule
- Pick a shaded, wind-protected spot, ideally dappled shade that’s also sheltered from a downpour.
- Keep the first visit short. University of Vermont Extension suggests an hour or two in the early morning or late evening, while South Dakota State University suggests three or four hours when temperatures are warmest. Bring them back indoors each evening.
- Increase exposure to sunlight by an hour or two each day, and add a few more hours outside each time.
- For the last one to two days, leave the seedlings outdoors day and night if nighttime temperatures look favorable.
| Stage | Where the seedlings sit | Time outside |
|---|---|---|
| First day | A shaded spot out of the wind | An hour or two, or three to four hours at the warmest time of day |
| Middle days | Gradually more direct sun | An hour or two more sunlight each day, and a few more hours outside each time |
| Last one to two days | Outdoors day and night | All day and overnight, if nighttime temperatures look favorable |
Sources for the table: University of Vermont Extension, South Dakota State University Extension and University of Maryland Extension. A cart or wagon makes the daily trips easier when you have a lot of seedlings to move.
Temperature, Wind and Rain Limits
Don’t put tender seedlings outdoors on windy days or when temperatures are below 45°F, University of Maryland Extension advises. South Dakota State University also says to keep plants inside if daytime temperatures are projected to be below 45°F, or if there’s a threat of strong winds or rains.
Cold matters most before the process is done. Maryland’s guide notes that even cold-hardy plants will be hurt if exposed to freezing temperatures before they’re hardened. Leaves that take a beating from wind are covered in cannabis wind burn.
Watering and Feeding While Hardening Off
Reduce the frequency of watering to slow plant growth, but don’t allow plants to wilt. Less water is part of how hardening works, since it slows growth while the tissues toughen. For the basics, see watering cannabis plants.
Hold off on feeding, too. South Dakota State University Extension advises that if plants appear healthy, you should avoid fertilizing during hardening, and that a plant short of a nutrient should get an appropriate dose. For help reading leaf color, see 8 common cannabis deficiencies by leaf symptoms.
Signs a Seedling Needs to Slow Down
Go slowly in the beginning. If the plants protest by wilting or flopping over, take them back inside until the following day, then carry on with the schedule.
Don’t park seedlings somewhere dark between sessions. Transplants should not be stored in unlit garages for prolonged periods, SDSU Extension warns, as they may start to stretch or yellow from lack of sunlight.
Keep the goal in mind. Hardening is meant to slow growth, and Maryland’s guide cautions that carrying it to the extreme of actually stopping plant growth can do significant damage to some crops.
Using a Cold Frame
A cold frame provides an excellent environment for hardening off, according to University of Maryland Extension.
Planting Out After Hardening Off
Once seedlings are sufficiently hardened after one to two weeks, they’re ready for planting. Choose a cloudy day, and plant early in the morning or evening rather than in the hottest part of the day. Make sure plants are watered thoroughly after planting.
If there’s a frost advisory in the forecast, it’s better to wait a few more days to plant. You might still need to protect plants from strong winds and extreme cold even after they’re hardened, and water them each morning until the roots are well established in the garden soil.
For digging, root-ball handling and spacing, follow the steps in transplanting marijuana plants.
Frequently Asked Questions
How Long Does It Take to Harden Off Cannabis Seedlings?
Plan on 1 to 2 weeks. University of Maryland Extension gives a range of 5 to 14 days depending on the crop, South Dakota State University suggests 7 to 14 days, and University of Vermont Extension says to spend at least one week even if you need to speed things up.
Can Seedlings Stay Outside Overnight While Hardening Off?
Not at first. Bring them back indoors each evening during the early days. South Dakota State University Extension says that by the last one to two days of the process, plants should stay outdoors overnight if nighttime temperatures look favorable. Keep them in whenever daytime temperatures are projected to be below 45°F.
Should You Water Less During Hardening Off?
Yes, a little. University of Maryland Extension advises reducing the frequency of watering to slow plant growth, but not letting plants wilt. Slower growth is part of how seedlings toughen. If a seedling wilts or flops over, take it back inside until the following day, as University of Vermont Extension suggests.
Should You Feed Seedlings While Hardening Off?
Usually not. South Dakota State University Extension advises against fertilizing during hardening if plants appear healthy. If a seedling looks short of a nutrient, give it an appropriate dose. After planting, the same guide notes you may also use a dilute starter fertilizer solution when watering them in.
Do Greenhouse Seedlings Need Hardening Off?
Yes. University of Maryland Extension describes hardening off as gradually acclimating an indoor- or greenhouse-grown plant to the harsher spring conditions in the garden. The same gradual schedule applies: a shaded, wind-protected spot first, then an hour or two more sun each day.
Is It Okay to Keep Seedlings in a Garage at Night?
Not for long. South Dakota State University Extension warns that transplants should not be stored in unlit garages for prolonged periods, as they may start to stretch or yellow from lack of sunlight. The same guide’s routine is to bring plants indoors each evening, then back out to a protected spot the next day.
Sources
- University of Maryland Extension, Hardening Off Vegetable Seedlings for the Home Garden
- University of Maryland Extension, Planting Vegetable Transplants
- South Dakota State University Extension, Harden Your Transplants Prior to Planting Your Garden
- University of Vermont Extension, Hardening Off for Healthier Vegetable Transplants
- Penn State Extension, Mitigating Production Risks in Hemp for Fiber, Grain, and Essential Oils
- Oregon State University Extension, Soil, Seedbed Preparation and Seeding for Hemp in Oregon (EM 9239)
