How to Harden off Seedlings – The Lazy Way

After weeks, even months, of nurturing and caring for seedlings (either indoors or in a greenhouse), there’s nothing more satisfying than seeing them finally planted in the garden. But we can’t just move our baby plants out in the elements and call it a day – they need to go through a process called “hardening off” first.

So what is hardening off? Put simply, your young plants need to develop a “thicker skin” so they can withstand radiation from the sun, stronger winds, and varying temperatures. Seedlings need to be gradually accustomed to the sun so that they don’t suffer from sunburn or transplant shock.

Traditionally, hardening off seedlings involves moving them outside, then back inside for 7 to 14 days, for incrementally longer periods. You can find complex schedules to teach you how to do this – but let’s face it – moving dozens of trays and pots twice a day can get really tedious.

Fortunately, there’s an easier approach that many gardeners use, including me: the “lazy” hardening off method, and this article will teach you how to “cheat” and get your plants in the ground in half the time and effort.

Do you really need to harden off seedlings?

Before we even get started, you’re probably wondering – do you really need to go through all this hassle?

The simple answer is: while some of your seedlings will do just fine when you move them from indoors straight into the ground (particularly early spring crops), their growth will become seriously stunted from the shock, and some of them might even die.

No matter how healthy and vibrant your seedlings look indoors or in the greenhouse, they still haven’t experienced the full strength of the sun and need to develop a protective cuticle to survive it. New growth on hardened off plants looks quite different, too – leaves take on a leathery, waxy texture and are much darker in color.

Hardening off your young plants is the only way you can protect them from sunburn (leaves become pale and light yellow), or transplant shock (seedlings look droopy and wilted for a few days).

When to start hardening off seedlings

So how big should seedlings get before you decide to harden them off? While you can move your seedlings in the ground at any stage, most seedlings have an ideal transplanting window you should be aware of:

• 4-6 weeks from sowing for cool-season crops
• 8-12 weeks from sowing for warm-season crops

Knowing this, it’s best to start hardening off cool-season crops at the 3-week mark, and warm-season crops anywhere after 7-8 weeks, depending, of course, on the weather.

Check the temperature before you move your seedlings outside for prolonged periods of time, especially overnight. While cool-season crops can handle frosty nights, you don’t want them to experience hard frosts at this stage. It’s best to get early spring crops outside when the temperature is consistently above 40°F (5°C) in the daytime, and ideally above freezing at night.

The same goes for warm-season crops – tomatoes, peppers, eggplants, etc – check the weather and temperature and make sure you’re past the last frost date. Temperatures above 55°F (13°C) are best for hardening off these plants – in fact, the warmer, the better. These weather conditions usually coincide with mid-May for many temperate gardening zones, but be sure to check your weather forecast.

Hardening off seedlings the lazy way

Lazy isn’t always bad – I like to see it as more efficient. If it involves less running around and faster results, why not give it a try? Let’s dive into the details:

Lazy hardening off – METHOD 1

1. *Essential step*: Check the weather for three consecutive days of overcast skies. The temperature can be slightly cool, but it’s usually not an issue as long as it’s above 40°F (5°C) for cool-season crops and above 55°F (13°C) for warm-season crops.
2. Choose a sheltered location to protect your seedlings from strong gusts of wind. Place your seedlings against a fence or wall, ideally in a corner.
3. Elevate your seedlings from the ground – place them on a table or on shelves. You’d be surprised how quickly slugs find their way to your plants if you leave them on the ground.
4. Leave the seedlings out for the night. It’s okay, they’ll survive – but if you’re nervous about this, you can bring them inside for the first night.
5. After 3-4 days, your seedlings are ready to go in the ground.

This “overcast skies” method works great because the sun still sheds enough light to stimulate the plants (think of how you can get a tan on a cloudy day), but it’s not strong enough to damage their leaves.

Lazy hardening off – METHOD 2

I use a slightly different approach because let’s face it, we can’t always get 3 overcast days in a row AND the ideal temperature. Also, I don’t always keep an eye on the weather – something I should work on, for sure.

1. Find a spot in your yard that gets morning and evening sun, but is shaded the rest of the day. This could be under a large tree, on a porch, etc.
   *Tip: If you have a garden parasol, use it to create shade whenever you want and control sun exposure.
2. Choose a sheltered location to protect your seedlings from strong gusts of wind. Place your seedlings by a fence or wall, ideally in a corner.
3. Elevate your seedlings from the ground – place them on a table or on shelves. You’d be surprised how quickly slugs find their way to your plants if you leave them on the ground.
4. Only expose seedlings to direct sunlight for a couple of hours in the mornings and evenings for the first 3-4 days. Leave them out in the sun for a few full days after that. This may involve some moving, and it’s more convenient if you have a parasol that you can open and close.
5. Leave the seedlings out for the night. It’s okay, they’ll survive – but if you’re nervous about this, you can bring them inside for the first night.
6. After 7 days, your seedlings are ready to go in the ground.

For both methods, it’s best to avoid windy days and rainy days. Move your seedlings inside in case of unexpected frosts or storms.

Hardening off cool-weather seedlings

I used to try to get my cool-weather seedlings in the ground as early in spring as possible. To achieve that, I would use a cold frame to transition my seedlings from indoors to the garden. While I think it’s a great tool to protect your seedlings from frosty nights, cold frames only work if you keep an eye on them.

Never leave your cool-season seedlings in a closed cold frame during the day as it can get too hot in there, and you’ll find many of the more fragile seedlings (like lettuce) dried up or drooping.

The same principles apply to hardening off cool-season crops – get them out during a longer stretch of overcast days and keep them away from slugs. If the temperatures go well below freezing, either take them back indoors, move them in a cold frame at night or throw a couple of layers of horticultural fleece (row covers) to keep them warm.

I’ve tried all these methods, and while the cold interferes with how fast things grow (a.k.a not growing much at all), sunburn and transplant shock are a result of insufficient sun exposure, even in apparently hardy cool-season seedlings.

In the end, I’ve decided to simply wait and sow my early crops a little later, but it’s a matter of trial and error that will probably take decades to perfect.

FAQs

Can you harden off plants in a greenhouse?

Greenhouses are a great way to start your seeds and grow your seedlings. They get cold during the night and warm during the day, so your plants are used to extreme temperature variations. Ventilation and opening doors and vents can ensure some amount of wind and air circulation, so your seedlings become more sturdy.
However, there’s a big difference between a greenhouse’s sheltered environment and the outdoors. While your plants might need less time to get adjusted compared to indoor-grown seedlings, you still need to expose them to the sun and wind for at least 3 to 4 days.

Can you harden off plants in the rain?

A light drizzle is fine (less watering to do), but if you notice heavy rain, it’s time to move your seedlings under a roof. Rain can fill up your seedling trays with water and cause root rot, not to mention you then have the extra task of emptying out all the trays. Don’t harden off your seedlings if you’re not home to watch them for a few days.

Do you need to harden off transplants from a nursery?

Normally, sellers should provide you with strong healthy seedlings that have already been hardened off. Before buying, check with your nursery if it’s okay to plant your seedlings in the garden straight away. Also, check the top leaves – new growth should appear dark green and leathery – a sign that the plant has been exposed to the sun.
If you’re not sure, you can always follow the lazy hardening off methods for a few days.

What happens if you don’t harden off your seedlings?

Delicate seedlings that are still young and have thin leaves will most likely dry up and die if you transplant them in the garden without hardening them off. The older a plant is, the more likely it is to survive, but it will go through transplant shock and slow down its growth to focus on healing and expanding its roots. Be sure to check our articles on transplant shock and sunburn to learn more.

Conclusion

Hope you’ve enjoyed this article and got some peace of mind knowing that you can harden off your seedlings with a little less work. Seedlings are like babies – they need constant attention and care, but the good news is that we’re in control of their well-being.