How to Harden Off your Plants in a Week

You have so carefully planted and cared for your seedlings indoors and now you want to prepare them for surviving the transition to outdoors. How do we go about moving them from inside, into our garden without them dying? They need to be carefully hardened off (climatized) to sun, wind and cool temperatures. Skipping this step all too often ends in disaster.

Hardening off plants for early spring planting on my back deck.

What Does Hardening Off Plants Mean?

The phrase "Hardening Off Plants" means slowly transitioning plants to adjust to the cooler temperatures during the night and the strength of the sun and the wind so that they can be moved from in a house or in a greenhouse to outdoors without shocking the plants.

The hardening off process should take approximately a week. To move plants directly from your house into your garden would likely give your plants a shock that will either kill them or stunt them and cause the plants to stop growing for a few weeks, thus losing all that hard work of getting ahead by starting seedlings early in your house.

Following are methods for hardening off your plants with no greenhouse as well as hardening off plants using different types of greenhouses. Take care not to break your plants when moving them in and out of the house during the hardening off phase. If you notice the leaves begin to get white or yellow spots on them, take a break from the direct sunlight and back up two days in your hardening off schedule. If at anytime the temperature is expected to fall below freezing or if high winds develop, bring plants indoors.

After completing the week of transition your plants are ready to be permanently planted in your garden. This does not mean they will survive a frost. If there is a late forecasted frost, cover your plants with a blanket, towel or plastic for the duration of the freezing temperatures. These are guidelines and may be adjusted as needed. (Find tips for covering your plants here.)

How to Harden off Plants Without a Greenhouse

If you are not using a greenhouse to harden off your plants, follow this schedule.

Day 1 - 4 hours in the shade outdoors and then bring plants back inside.

Day 2 - 1 hour in the sun and 3 hours in the shade outdoors and then bring plants back inside.

Day 3 - 2 hours in the sun and 4 hours in the shade outdoors and then bring plants back inside.

Day 4 - 4 hours in the sun and 5 hours in the shade outdoors and then bring plants back inside.

Day 5 - 6 hours in the sun and 6 hours in the shade outdoors and then bring plants back inside.

Day 6 - 8 hours in the sun and remainder in shade. Bring plants in overnight.

Day 7 - All day in the sun outdoors and bring in overnight.

How to Harden off in a Greenhouse

For each of these 4 methods, leave your greenhouse open during the day (as long as the temperature is above freezing) and closed at night.

Hardening off in a Small, Portable Plastic Greenhouse

Keep the front of the greenhouse open during the day and closed at night.

Day 1 to 3 - Four hours of direct sunlight through the plastic and the remainder of the day in indirect sunlight. Bring plants in at night.

Day 4 and 5 - Six hours of direct sunlight through the plastic and the remainder of the day in indirect sunlight. Cover greenhouse with a blanket and leave plants out at night.

Day 6 and 7 - Eight hours of direct sunlight through the plastic and the remain of the day in indirect sunlight. Leave greenhouse uncovered at night unless freezing temperatures are expected.

Hardening off in a Greenhouse with a Plastic Roof and Glass Sides

Using a greenhouse to harden off your plants makes the hardening off process significantly easier. The greenhouse filters out the strength of the sun's rays and keeps the plants warmer at night. If you have a poly carbonate or plastic roof to your greenhouse, you can move your plants directly from your house into your greenhouse without bringing them in at night.

Day 1 to 3 - Four hours of direct sunlight through a glass window and the remainder of the day indirect sunlight.

Day 4 and 5 - Six hours of direct sunlight through a window and the remainder of the day indirect sunlight.

Day 6 and 7 - Eight hours of direct sunlight through a window and the remain of the day indirect sunlight.

Hardening off Plants in an all Glass Greenhouse

A clear glass roof allows stronger sunlight through than the plastic roof ones. If you do not have a shady spot in your greenhouse for your young plants follow this schedule.

Day 1 to 3 - In the greenhouse for 4 hours direct sunlight through the glass window then bring inside.

Day 4 and 5 - In the greenhouse for 6 hours in the direct sunlight through the glass window then bring inside.

Day 6 and 7 - In the greenhouse for 24 hours with it open during the day and closed at night.

Hardening off Plants in a Fully Poly Carbonate or a Permanent Large Plastic Greenhouse

The plastic will filter the rays of the sun and protect your plants from becoming sunburned. You can move your plants from indoors directly to the greenhouse without having to bring them back and forth at all. Make sure you leave your greenhouse open during the day and closed at night for a week to allow the plants to adjust to the outdoor temperatures.

Remember, once your plants are planted outdoors, keep an eye on the night time low temperatures. If you have cold sensitive plants cover them when a frost is expected.

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