Get Stronger Plants: Hardening Off Seedlings.

In zone 3, our growing seasons are incredibly short compared to the rest of the world, meaning we have to start most of our seedlings inside so they are able to grow enough that we can harvest them before the fall frost. The challenge with this is that the seedlings get used to a temperature-controlled, humidity-controlled, perfect environment. The first time they experience the wind and the sun’s harsh rays can be a huge shock for them if they aren’t ‘warmed up’ to the outside world first. Skipping the hardening off stage of gardening can often end in disaster (and tears).

What is hardening off?

Hardening off is a phrase used to describe the slow transition of converting your indoor plants to outdoor plants. This phase of gardening helps plants adjust to changing temperatures throughout the day, the strength of the sun’s rays, and other challenges such as insects and wind.

Depending on what type of seedling you have or what kind of spring you’re having, hardening off should take approximately one to two weeks. I’ll admit, I am usually a ‘tough love’ approach gardener that fully embraces the survival-of-the-fittest mentality, however, I have found that hardening off my plants is truly worth my time and effort.

You can harden off your garden plants with or without a greenhouse. Either or is completely acceptable, but if you are opting to forgo the greenhouse, you may need to pay a little closer attention to the weather.

After your plants are successfully hardened off, they are ready to be planted permanently in the garden. This does not mean that they will survive a frost! A late frost can still wipe out your efforts at this point so you’ll have to keep a close eye on your weather app. Any nights that show temperatures low enough to create a frost, you will need to protect your plant babies with a drop cloth held up with some stakes.

Schedule for hardening off without a greenhouse:

Day 1: 4 hours in the shade outdoors, then bring plants back inside.

Day 2: 1 hour in the sun, 3 hours in the shade, then bring plants back inside.

Day 3: 2 hours in the sun, 4 hours in the shade, then bring plants back inside.

Day 4: 4 hours in the sun, 5 hours in the shade, then bring plants back inside.

Day 5: 6 hours in the sun, 6 hours in the shade, then bring plants back inside.

Day 6: 8 hours in the sun, remainder in shade. Bring plants inside overnight.

Day 7: All day in the sun, bringing plants back inside overnight.

Schedule for hardening off with a greenhouse:

How you harden off your plants using a greenhouse will be dependent on what kind of greenhouse you’re using. A portable, glass, or plastic greenhouse all trap heat and let in sunlight differently.

If you’re using a small, plastic, portable greenhouse, this is an example schedule:

Day 1-3: 4 hours direct sun through plastic. Remainder of the day in the shade. Bring plants inside at night.

Day 4-5: 6 hours direct sunlight, remainder of day in shade. Cover the greenhouse with a blanket overnight and leave it outside.

Day 6 – 7: 8 hours direct sunlight through plastic, remainder of day in shade. Leave the greenhouse uncovered overnight, unless temperatures are freezing.

If you’re using a glass greenhouse, you will still have to bring your plants inside since the glass does not filter the sun’s rays enough to prevent burning. You’ll follow the same hours in sunlight as above, but bring the plants inside out of the sun afterwards rather than relocating the greenhouse. When you get to your final days of hardening off, you’ll leave the plants outside full time, with the greenhouse door open during the day and closed at night.

Contrary to a glass greenhouse, a plastic greenhouse is much better at protecting your plants from becoming sunburned. You can more your plants directly to your greenhouse from indoors, leaving them there full time. To complete the hardening off period, you’ll need to leave the greenhouse door open during the day and close it at night.

Tips:

• Know your last frost date. You can find these approximate dates in the Farmer’s Almanac for your area. Since you typically don’t want to plant out until the last frost has passed, you can time your hardening-off process to align with when you want to plant.

• Find a spot in your yard where you have access to both sun and shade so that you don’t need to carry your plants too far and risk breaking them.

• Wind is good for seedlings, it helps strengthen the stems. BUT, harsh winds can be damaging so you may have to brainstorm some wind protection if your yard doesn’t naturally have some.

• I can hear some of you reading the hardening off schedules and saying, “I have a full-time job, how am I supposed to move my plants around so much?” I hear you. My recommendation is to start the hardening off process on days when you know you’ll be at home to move them back inside (say a weekend). After they’ve spent two days getting experienced to the outside world, you are likely safe to leave them outside for a full day in the shade. Alternatively, I would recommend investing in a small, cheap, portable greenhouse that you can put your plants in without fear of sudden torrential downpours, wind gusts, or birds eating your seedlings while you’re away.

Hardening off your seedlings is a little extra work, but I promise that in the end it is worth it! With the extra few steps, you’ll have stronger and healthier plants that stand up to Mother Nature’s tests much better than they would have otherwise.