Whether you’re starting your own plants from seed or buying seedlings or transplants from a local greenhouse, you need to prepare your little plants for their new home in your garden. This preparation process is called “hardening off”. In this post we’ll talk about how to harden off plants including some different ways to do it.
Hardening off seedlings is vital to your tender plants’ health. While growing plants indoors from seeds is a great way to get your garden started early and save money, it also teaches your plants that life is easy.
Your small plants have been haven’t been exposed to the wild and changing elements outside, so you need to get them ready for the change. However, transplanting plants has to be approached carefully so that they can live through the stress and thrive.
Why You Need to Harden Off Plants
Your seedlings have had it easy for their entire lives.
They were planted in the perfect environment, soil and environmental temperatures were monitored, and optimal soil moisture was achieved. The precise amount of light was given to them.
Compared to what awaits outdoors, your seedlings have been living at an all-inclusive resort. However, when they have to live out in the elements, things aren’t so gentle.
Hardening off plants helps prepare them for what’s to come. If this step isn’t taken, your tender seedlings won’t be able to handle the harsh sun, break from the winds and rain, and be unable to stand up to the other rigors of nature.
What Hardening Does
Hardening helps toughen up your seedlings gradually before transplanting. It takes a little time, but it’s worth it.
- slows down the growth rate of the plant to prevent it from being spindly
- thickens plants’ cuticle and waxy layers so they lose less water when exposed to the sun
- stimulates root development
- increases the amount of carbohydrates in the plants’ tissues for food reserves
- reduces the amount of freeze-prone water in the plants’ cells
- causes the plants to develop more lignin in the cell walls to strengthen them
If you don’t harden off your plants, seedlings can succumb to transplant shock. Even if they survive, their growth will generally be stunted and they won’t produce as abundant of a harvest.
How to Harden Off Plants
Note that the following tips are mostly for the typical “move plants in and out” hardening off method. You can also set up a cold frame and then you can do all of this without bringing your plants in and out–and just cover the cold frame as needed to shield the plants from the elements.
Different plant species required different hardening periods, but in general, these time frames should work well for most young plants.
1. Check Plants’ Frost Dates
Some plants—broccoli, brussels sprouts, and cabbage, for example—may be OK to harden off and plant while there’s still a risk of frost. Other plants, like tomatoes, typically can’t handle those cold temperatures. Here are lists showing the frost tolerance of most vegetables.
2. Stop Indoor Watering and Fertilizing
About a week before your plan to take your seedlings outside, suspend any supplemental watering or plant food. This is the first step to toughening them up for the great outdoors.
3. Add Some Wind
Before taking your seedlings outside, consider turning a fan on low (not high!) nearby to simulate wind. You don’t need to have it on all the time–just at various times during the day or night is fine.
You can even start by just waving your hands near the seedlings to get the used to a gentle breeze.
4. Choose Your First Hardening Off Location
A shady spot next to your home, a table under a tree, under a table on your deck, or even inside a cold frame can all work well. You can even put the plants in a deep bin to shelter them from the wind.
A garage or shed is another great option to consider.
5. Place Seedlings Outside for an Hour
In the mid to late afternoon, move your seedlings to your hardening off spot for just an hour or two. Afternoon sun is weaker than morning sun, so it’s better to start with that.
Make sure that the plants sheltered from too much wind as well as from pests and animals that might eat them.
6. Move Plants Back Inside
After an hour or two, bring your seedlings back inside. If they’re in a cold frame, close and secure any openings.
7. Lengthen Daytime Hardening Off Time
Each day, gradually leave your seedlings outside for an hour more than the previous day and also gradually set them in the direct run for more and more time. You should also move them to a less sheltered location—either further away from the house, out from under the, or outside of the cold frame, depending on where you have them.
8. Extend Nighttime Hardening Off Time
Once your seedlings have been outside in the daytime hours for a few days, it’s time to get them used to outdoor nightlife. Try to leave them outside for gradually longer periods of time but make sure to really keep an eye on the temperatures to make sure that they aren’t going to dip too low.
9. Protect Seedlings As Needed
If the temperatures dip too much while you’re hardening your plants off, you might need to bring them inside, close your cold frame or cover the plants with row covers. You can also use covers if there’s a hard rain. You can even make a makeshift cover if need be.
10. Address Wilting as Needed
If wilting occurs during the hardening off period, you can sprinkle the plants with a little water to revive them. A water sprayer like this one works great and is one of my favorite gardening tools.
Once your plants are handling a full day of sun and the elements without getting stressed and the temperatures are not dropping too low at night, your plants are ready for transplanting.
Although you’ve toughened up your plants by hardening them off, you still need to be gentle when transplanting to give them the best start possible.
When to Transplant
The best time to transplant your seedlings is on a cloudy and warm day, ideally one that isn’t too windy. Your plants will already go through some shock just by moving from their original container to the ground, so try not to stress them with too much wind or sun. However, just do that best that you can with this.
How to Transplant
At this point, transplant your plants according the specific directions for each plant. For example, setting out squash in threes and fours on a hill or 3 feet apart in rows or planting your tomatoes so that about three quarters of the plant is in the ground and 2-4 foot apart depending on the variety.
Dig a hole that’s a little bigger than the root ball for your plant, fill the hole to the plant’s root line and then pat the dirt a bit with your hand.
After hardening off plants and transplanting them, remember to water them deeply. That extra push of water will help them avoid transplant shock and set them up to be as healthy as possible as they take their final steps from indoor to outdoor living.
In the first few days after planting in the garden, check on your plants regularly. A little wilting in the first day is normal, but plants should quickly perk up. And while some may adjust just fine and not need any special care, others may need supplemental watering. Also monitor temperatures carefully; unexpected temperature dips are rare but do happen, and you may need to cover some plants to protect them.
I hope that these tips help you to harden off your plants successfully and that they help you have a great harvest!