Like most people, you, too, probably get anxious to plant your seedlings into the ground as soon as they sprout. However, if it’s lush growth you’re after, you should give your seedlings the time to acclimatize (harden off) before you kick them out of the comfy nursery conditions.
In this blog post, we’ll show you how to harden off seedlings so they can become stronger and more resistant to unfavorable weather conditions, pests, and diseases.
So, read on to learn about the “when,” “how,” and “how long” of this delicate process.
What Does “Hardening Off Seedlings” Mean?
Hardening off is the process of gradually acclimating seedlings to outdoor conditions, which makes them stronger and prevents them from suffering a shock when they’re finally transplanted outside.
This step is especially important if you’re trying to grow herbs, flowers, and vegetables.
Whether you’ve germinated the seedlings yourself or you’ve purchased them from a local nursery, hardening them off should be a vital part of your planting schedule, as it’s crucial to increasing your seedlings’ chances of survival.
What Happens If You Don‘t Harden Off Seedlings?
Skipping the “getting used to” phase can prove fatal for your seedlings. The indoor conditions where they’ve been germinated differ greatly from the rough outdoors, and the drastic change can cause your young plants to wilt and die.
Intensive sunlight, sudden drops in temperature, persistent rain, strong winds, pests, and diseases can all damage your fragile seedlings to the point of no return. Even in the best-case scenario, your seedlings might be faced with stumped growth for weeks.
So, make sure to dedicate at least a week’s time to slowly introduce seedlings into your garden to prevent the transplant shock.
When to Harden Off Seedlings?
Most seedlings are ready to go through the process of hardening off two weeks after germination. Since the growing season for most plants begins in spring, hardening off should start after the last frost date.
A good rule of thumb is to wait until your seedlings are at least a few inches tall and have sprouted a pair of true leaves.
How to Harden Off Seedlings Before Planting Them?
There are many proven ways to harden off your fragile seedlings:
Method 1: Use a Cold Frame
Pro gardeners who don’t like to move pots in and out of their basements/garages/homes during the acclimatization process typically use the cold frame method to harden off seedlings.
A cold frame is simply a bottomless box (usually a wooden one) with a clear lid on top. The lid that can be opened and closed as you wish allows sun and air to reach and nourish your seedlings but also protects them from harsh weather conditions.
You can use this gardening aid for hardening off plants and accustoming seedlings to specific light intensity, humidity levels, temperature fluctuations, wind, etc.
Here’s how to use it:
- Place the seedlings in the cold frame.
- Open the cold frame lid a certain amount and keep it open for a while. How long you’ll keep the lid open and how wide the opening will be will depend mostly on the weather conditions and the type of plants you’re hardening off. It’s best to start with a smaller opening for a smaller amount of time.
- Little by little, increase the amount of time your seedlings spend with an open lid.
Tip — If you can’t be around to keep opening and closing the lid throughout the day, start by opening the lid just a crack when you first start hardening off seedlings and then slowly increase the opening on each subsequent day.
Don’t forget to close the lid in the evening when the temperatures drop!
Method 2: Gradual Exposure
You can also harden off seedlings quickly by gradually exposing them to the elements by taking them outside for a certain amount of time:
- Choose a warm day to bring your seedlings outside for the first time. The temperature should be above 45 °F, and the location should be shielded from wind, sun, pets, and pests.
- Exposure to outdoor conditions should last about an hour on the first day. Extend the time spent outdoors by one hour every day.
- After the seedlings have spent the allotted period of time outdoors, bring them inside and put them in a well-aerated, warm place.
- Pay attention to the light intensity outside. Start your seedlings with the morning sun, and gradually work toward exposing them to the midday sun. It’ll probably take the seedlings two to three days to get ready to come out of the shade.
- Progressively adapt the seedlings to weather conditions like cloudy weather and wind.
- Keep tender seedlings indoors on particularly windy and cold days (when temperatures drop below 45 °F).
Method 3: Hardening Off Seedlings in a Greenhouse
If you have a greenhouse, you can use it to your advantage.
By keeping your seedlings in the greenhouse, you can control the amount of sunlight and wind they’re exposed to. This will help them acclimate better and avoid transplant shock when you move them outdoors.
Place the seedlings in the greenhouse every day, extending the amount of time they spend in there every day. Start with just a few hours of exposure, and increase the amount of their stay by two hours every day until the plants are ready to be transplanted.
How Long Will It Take to Harden Off Seedlings?
It typically takes two or three weeks to harden off seedlings.
The factors that influence the length of the hardening-off process the most are the temperature and the type of plant.
For example, if the initial growth of the seedling occurred in particularly warm temperatures, the hardening-off process might last longer than usual. Furthermore, hardy plants acclimate to outdoor conditions way faster than tender ones.
When to Put Seedlings Outside?
Two to three weeks after you’ve started the hardening-off process, you can pack your seedlings’ bags and transplant them into their new home. Choose a day with mild weather (a slightly cloudy day will also work) to move your plants outside.
Tip — Use your better judgment and what you know about the hardiness of your plant to decide when it’s the right time to move the hardened-off seedlings outdoors.
How to Harden Off Seedlings Quickly — Tips and Tricks
Here are some useful tips that’ll help make the hardening-off process faster and more successful:
- Check the weather forecast before you start. Choose a week with mild weather changes.
- Be patient and introduce changes (e.g., exposure to sunlight, wind, etc.) very gradually. Don’t rush your plants, and let them take their time to acclimate to the outdoor environment at their own pace.
- Put seedlings in places inaccessible to snails, animals, and insects that can damage young plants.
- Keep watering the seedlings as usual throughout the process. Before they become established, seedlings are vulnerable to any extreme changes, including changes in moisture.
- Create a false breeze with the help of a fan to acclimate your seedlings to the wind. Just make sure the airflow is indirect to avoid drying your seedlings off.
Transplanting Seedlings Into the Garden — Checklist
After hardening your plants, make sure you’re well prepared for planting them outdoors:
- Choose a cloudy day to transplant your seedlings (late afternoon sun will do, too).
- Prepare your the planting area for the seedlings’ arrival. Get rid of any invasive weeds and enrich the soil with compost or fertilizer.
- Water the seedlings profusely one day before moving them (and do so right after transplanting them into the garden soil).
- Choose the right neighbors for your seedlings. Even if you harden off vegetable seedlings right, they may not do so well next to incompatible plants. In the world of plants, not everyone can be everyone’s neighbor, so choose companion plants wisely.
- Keep plants sheltered for two or three days until they’re well-established in the garden. Don’t expose them to direct sunlight, wind, low temperatures, and other harsh weather conditions straight away.
- Mulch around the seedlings to keep precious moisture inside the soil.
Conclusion
No matter which method you choose, it’s important to remember to harden off your seedlings gradually. Start with exposing your seedlings to the outside world for short periods of time and gradually increase the exposure as your plants become acclimated to the outdoor conditions.
By following these tips, you can ensure that your seedlings are strong and healthy when it’s time to plant them in their forever home outside.
FAQ
How big do seedlings need to be before hardening off?
There’s no one-size-fits-all answer to this question. The general rule of thumb is to wait until your seedlings are at least a couple of inches tall and have developed a pair of leaves.
Once they’re big enough, start the acclimation (hardening off) process. It’ll take at least one week (ideally two or three) before it’s safe to transplant the seedlings into your garden or larger containers.
When can I leave seedlings out overnight?
To err on the side of caution, leave your seedlings out overnight after several days of hardening off, only when the nighttime temperatures are above 50 °F.
How long does it take to harden off seedlings?
Again, there’s no definitive answer to this question. While hardy plants may take only a week to harden off, more tender ones may take two or three weeks.
The length of the process will depend on a number of factors, including:
- The type of plant you’re growing
- The climate you live in, etc.
For more relevant details and information on how to harden off seedlings properly and how long to wait until they’re ready to be moved into your garden, check out the sections above.
