What Does It Mean to Harden Off Plants? The What, Why, When, and How!
Every year when spring comes around, I am waiting in anticipation to start my garden and plant out all the seedlings I have so fondly attended to.
Now, before you get too overly excited and rush to get your seedlings into the ground, those babies need to be hardened off first or they will get the shock of their lives!
And this is where patience and commitment will become your new best friend. If you stick with those two, your seedlings will forever be thankful!
What Does it Mean to Harden Off Plants?
Hardening off or acclimating is vital for gradually adjusting your seedlings to the outdoors when you are ready to transplant them. By hardening off, you slowly expose your vegetables, fruits, and flowers to the outside elements to ensure their survival.
Seedlings started indoors, whether in a greenhouse, on your windowsill, or under a grow light, are grown in the ideal conditions where you control the temperatures, the light, and moisture levels. They are also not much affected by wind, rain, and extreme temperature changes such as frost.
When the plants are used to these pleasant and cozy conditions, moving them outside without any warning or weaning can cause transplant shock. Causing distress to the plants or even killing them.
It's like having to go out during a cold winter's evening when you've been snuggling up under your favorite blanket in front of the fireplace. Nobody likes to do that!
Why You Should Harden Off Seedlings
Plants have a protective waxy coat on their leaves that helps to repel water, reduces the rate of dehydration, and protects them against harmful ultraviolet light.
But when seedlings have been grown indoors, they haven't fully developed this waxy coat. By gradually exposing them to the outside elements, they build up their defenses and have time to adapt to the new environment.
Hardening off will prevent shock that could set back or halt the plant's growth, prevent sunburn (scalding), and help your seedlings or young plants grow into healthy, vigorous adult plants.
Hardening off your plants may seem like a tedious and time-consuming process, but it is actually easy and worth the effort.
Your plants will thank you, you'll be happier with the results, and you wouldn't waste money by losing expensive plants.
Trust me, I have learned this lesson the hard way!
When to Start Hardening Off Plants
It is best to start hardening off about one to two weeks before the final frost date for your area and when the weather is more favorable.
Determine when you need to transplant your plants in the garden by referring to the seed package instructions on planting times and outdoor conditions. It also helps to know when your last frost date is expected. Back up two weeks from the planned planting date — that's when you'll need to start hardening off.
This Spring Planting Guide provides guidance for when some popular crops can be planted outdoors for zone 7 (where we live). It is also a good guideline for zones 6 and 8.
How to Harden Off Seedlings
Hardening off should take between a week and sometimes up to two weeks, depending on the weather and plant variety.
An unheated greenhouse or cold frame is great for hardening off as these have windows or doors that can be gradually opened with time and closed at night or during cold spells.
If you have no intermediate home available, move your plants between a shady, sheltered spot outside in the garden and your garage or home.
Hardening Off Steps
The main aim of hardening off is to gradually increase the time outside and light intensity each day to make the plants used to outdoor conditions.
Remember to monitor the weather forecast, especially initially, and keep plants indoors if strong storms or low temperatures are expected.
Hardening off plants outside
If you don't use a greenhouse or cold frame to harden off your plants but rather outside, you can follow this suggested schedule:
Day 1: 4 hours in the shade outdoors when the temperature is 40 degrees or above in an area that is shady and sheltered from the wind, and then bring plants back inside.
Day 2-3: 1- 2 hours in the sun and 3-4 hours in the shade outdoors, and then bring plants back inside for the night.
Day 4-5: 3 hours in the sun and 4-5 hours in the shade outdoors, and then bring plants back inside for the night.
Day 6-7: 4 hours in the sun and 5-6 hours in the shade outdoors, and then bring plants back inside for the night.
Day 8-9: 5 - 6 hours in the sun and 6-7 hours in the shade, and then bring plants back inside for the night.
Day 10: 6 - 7 hours in the sun and the remainder of the day in the shade. Bring plants back inside for the night.
Day 11-12: All day in the sun outdoors and bring in overnight.
Onwards: Let your plants stay in full sun and outside overnight as long as night temperatures are above 40 degrees. If it goes below that or cold snaps are expected, rather be safe than sorry and move the plants indoors.
Hardening off plants in a cold frame
Using a cold frame can help you easily harden off seedlings without the effort and hassle of needing to move your seedlings in and outdoors.
Cold frames protect your plants against low temperatures early in the growing season, strong winds, and harsh sunlight.
Start by placing your seedlings into a cold frame outside in the shade with the lid slightly open for a few hours. Then, close the frame at night and cover it with a blanket for added heat.
As with the steps above, increase the number of sunlight hours and gradually raise the lid within the next few days. After about 10-12 days, you should leave the frame open in mostly full sun for most of the day.
You may still need to close the cold frames at night but without a blanket. And when night temperatures start to rise, and all risk of frost has passed, can you leave the frame open at night.
Hardening Off Tips
Avoid Bright Sunshine
Seedlings or young plants exposed to sudden bright sunshine will cause the leaves to scald. That's why it is best to start hardening off on a cloudy day with fair temperatures in a shady spot. And from here on, you will increase their sun exposure every day.
Keep in mind that the shade spots will move over the course of the day, so pay attention to where you place them, especially if you will be out for the day.
Choose a Sheltered Spot
When your seedlings are standing out in the open, the wind will speed up evaporation and cause plants to dry and wilt quickly. Choose a sheltered spot such as a corner or solid crates to protect them from wind and drying out.
Water the seedlings before placing them outside, so there's less risk of drying out.
Crop Variety and Nights
If you live in a region with cold winters, you need to be extra vigilant for cooler nights early in the growing season.
Warm-season crops such as peppers, tomatoes, cucumber, and melons are easily damaged by cold temperatures. They prefer night temps at 60 F degrees or higher.
You'll need to harden these seedlings off for longer and bring them inside at night for two to three weeks instead. At the end of the hardening off period, when you will leave them outside at night, use a fleece or row covers to protect the plants at night.
Cool-season crops such as cabbage, broccoli, Brussel Sprouts, lettuce, spinach, peas, and onions will tolerate cooler night temperatures around 45 F degrees better, and light frost won't damage them easily.
Nursery Greenhouse Plants
Seedlings or young plants bought at your local nursery are usually larger and stronger than those grown at home. Some of them have also already started to harden off.
But in some cases, they may have spent all their time indoors, and you need to harden them off for a few days. Check with the nursery to make sure.
Planting Day
Finally, after two weeks or so, your seedlings are ready to be transplanted! I advise planting your seedlings out in the morning on a cloudy or moderate temperate day.
Also, check the weather for the next few days to ensure no heavy rain, cold snaps, or frost is forecasted.
If the weather looks unpredictable, instead, hold off and plant at a later stage. You can just continue with hardening them off outdoors.
You may be tempted to skip hardening off your seedlings as it seems like too much effort, but I would strongly advise against it.
Whether you live in a hot climate, or a cooler climate, extreme heat or cold can significantly damage the seedlings you’ve worked so hard on growing.
Hardening off is definitely worth the extra effort and will ensure bountiful harvests or blooms later on!
For even more successful gardening and harvest, the quality of your soil is just as crucial. As well as using an organic fertilizer.
These blog posts will guide you in optimizing the growing conditions:
Organic soil recipe for increased germination and plant growth
Love, Annette xx