How to Harden Off Your Seedlings – Green Planet Naturals

How to Harden Off Your Seedlings

Are you excited to start gardening again? We know that we are! The days are growing longer and warmer. The birds are chirping. Most of the snow has melted, the ground is muddy and – most important of all – the indoor seedlings are (hopefully!) coming along nicely.

As you probably know, gardeners typically harden off seedlings in April or May. This step prepares the plants for life outside by helping them adjust to outdoor weather. After all, there’s a big difference between the gentle spray from a watering can and the pounding of a hard rain. Delicate seedlings can easily burn in bright sunlight or suffer the effects of strong winds and chilly evenings. Such drastic changes can shock plants, often killing them. That's why it's important to always harden off seedlings, even if it's tempting to transplant them immediately.

It takes dedication to properly harden off seedlings, but the concept is simple. All you need to do is spend a couple of weeks setting your seedling trays outside for small periods of time, gradually increasing the number of hours each day. Here are some basic guidelines to follow if you are new to gardening or need a refresher.


It is ideal to begin the hardening off process at least one week prior to the predicted last frost date in your area. If you haven’t checked it yet, the Farmer’s Almanac website has this handy information chart detailing frost dates across the US. If your city’s last frost date has already passed, never fear! Your seedlings won't be getting the earliest start possible, but it's not late to harden them off and plant them anyway.


On the first day, put the seedlings outside for one hour in a partially sunny area that is protected from strong winds. Bring them inside afterwards. The next day, leave them out for two hours. The day after that, leave them for two or three hours. Continue to increase the time by one or two hours every day for one week. As time goes on, you can expose the seedlings to more sunlight and wind.   

At the end of the first week, leave the seedlings outside for a full day and bring them inside at night. Continue this routine for another five or six days. Now they are almost ready. Leave them outside for one full day and one full night. The next evening, you can plant them.

Not all gardeners follow the same routine. Some prefer to harden off their plants more slowly or quickly than others. It partly depends on the weather in your area and what you find works best for you. However, the general guidelines remain the same: start with small increments of time and gradually increase them each day.


Your seedlings should be sufficiently acclimated by now. According to reader tips from the Birds and Blooms website, they should also have at least four leaves and be firmly rooted in their starter containers or soil blocks, if you happened to use our soil block kit. If they only have two leaves or seem poorly rooted, you should delay the transplant date. Continue to keep them outside in a tray until they become sturdy enough to plant.

If possible, plant the seedlings on a cloudy day. Otherwise, plant them late-afternoon and provide them with shade for the first couple of days. Doing so will make for a smoother transition period.

Growing your own plants from seeds is a rewarding experience. Doing so means taking a few extra steps in the spring, but hardening off seedlings is fairly simple to do and well worth the effort.


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