A Guide to Hardening off Plants (and Why You Should)

Growing flowers, fruit or vegetables from seed is a rewarding process. While growing from seed can take a long time, during which you can become invested and attached to the delicate seedlings, it is a thoroughly rewarding process. The next step, transplanting outside, can sometimes see seedlings struggle, develop shock or even fail. Often this is because they haven’t been acclimatized properly. Hardening off plants properly helps to prevent this. While it may sound difficult, it really isn’t. This guide to hardening off plants will explain exactly what hardening off is and why you need to do it.

Take the time to acclimatize seedlings to their final growing position. 

What is Hardening Off?

Hardening off plants is basically the process of getting young seedlings, which have been started undercover, in a greenhouse or on a windowsill, acclimatized to outside conditions.

A gradual process, hardening off can take around two weeks depending on your weather conditions and the size of the seedlings. While this may seem a long time, it is well worth the effort. Suddenly transplanting delicate young seedlings into a colder, more exposed position can cause trauma, severely weakening or even killing specimens.

Acclimatizing seedlings helps to protect them from transplant shock. It also helps to create more resilient and productive specimens. 

You can either acclimatize your seedlings in an unheated greenhouse or cold frame or in a sheltered position in your garden. This guide will explain both methods.

When to Start Hardening Off your Plants

You can begin the process a few weeks before the last predicted frost date of the year. For most growers this will be in April or May but in the warmest USDA Zones it can be a few weeks earlier.

Flowering annuals such as fuchsias and pelargoniums can go out first. This is followed a few weeks later by beans, zucchini and tomatoes. Wait until late May, once all chance of frost has passed, before starting to acclimatize more tropical seedlings. Houseplants can also be moved outside during the summer months, when the air is at its warmest.

Hardening off Plants Undercover

An unheated greenhouse or a cold frame is a great way to acclimatize your seedlings because they have windows and doors. These can be opened gradually to slowly make the protected conditions more similar to the outside conditions.

Placing seedlings in a cold frame is a useful way to acclimatize delicate young specimens. 

On the first day place the seedlings in their unheated shelter for a few hours or, if they are already in the greenhouse, slightly open the window. After a few hours have passed, close the window or return the seedlings to their previous, sheltered position. Repeat this process the next day, exposing the seedlings to the outside conditions for a little longer.

Each day, gradually increase the amount of exposure that the specimens receive. As tempting as it may be, you should never increase the amount of time the seedlings spend outside by more than a couple of hours every day.

After a week you can leave the seedlings in their unheated, exposed position overnight, as long as there is no danger of frost.

Hardening Off in a Sheltered Spot

If you haven’t got a greenhouse or cold frame, you can also acclimatize your seedlings outside, in a sheltered position. Your chosen site should be protected from the wind. Wind speeds up evaporation which can cause seedlings and young plants to wilt incredibly quickly. To help prevent this, water the seedlings before you place them outside.

If the seedlings are in pots, place them close together in a bucket or crate. Clustering pots close together creates a windbreak around the foliage. It also prevents them from toppling over, damaging delicate seedlings.

Source: https://www.youtube.com/watch?v=yM0LgcsJQpM 

When selecting your position, avoid anywhere that enjoys lots of bright light. Sudden exposure to bright light can cause delicate foliage to burn or scald. A dappled shade position is ideal.

Start acclimatizing your seedlings about two weeks before you intend to transplant them into their final growing position. Ideally you should begin the process on a warm day. Temperatures should be above 45 ℉. The weather should be as still as possible and slightly cloudy.

On the first day, don’t put the seedlings out for more than two hours. As with seedlings you are hardening off undercover, gradually expose them to longer periods outside. You should also gradually increase the amount of direct light the seedlings are exposed to.

Gradually increase the amount of direct light that seedlings are exposed to.

Remember that shade and sun positions move throughout the day. If you aren’t able to watch the seedlings all day, use a shade cloth or row cover to protect them from potentially harmful direct light.

When night and day temperatures are consistently over 50 ℉, you can overnight the seedlings. A max-min thermometer is a useful way to monitor outside temperatures. The Brannan Digital Max-Min Greenhouse Thermometer, is easy to use and suitable for both garen and greenhouse use.

Acclimatizing Seedlings Overnight

While spring temperatures warm up gardeners in colder USDA Zones may continue to experience cool nights. If this is the case, you will also need to prepare your seedlings for exposure to early season cold nights. Tender seedlings, such as peppers and tomatoes are particularly sensitive and easily damaged by cold temperatures.

Towards the end of the acclimatization period, as you are preparing to leave seedlings out all night, cover them with a fleece or floating row cover. This protects the delicate foliage from chilly nights. Depending on how cold the nights are in your area you may need to continue covering the seedlings after transplanting.

Additional Tips

When hardening off plants outside, don’t place them directly on the ground where they can be attacked by slugs and snails or knocked over by pets and birds.

Always sow and grow more seedlings than you need, just in case. Should all the seeds germinate and develop into healthy specimens, you can give away or sell the excess seedlings.

Purchased plants may also need to be acclimatized. Particularly if they have previously been kept in sheltered conditions.

Don’t be in a hurry to finish hardening off your seedlings. Take the time to allow your plants to fully acclimatize before transplanting. This helps them to become stronger and more resilient. Correctly conditioned, mature  plants are also more productive.

Taking the time to acclimatize specimens helps to create resilient, productive plants.

Hardening off plants is a vital part of the growing process. Helping to create resilient, productive plants, hardening off, as you have seen, is nowhere near as complicated as it may seem.