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This one mistake when hardening off seedlings could cost you the health of your plants. Whether you're growing vegetables in pots or planning to transplant delicate flower seedlings into your garden beds, it's important to do the hardening off like you mean it.
What some gardeners don't know is that the purpose of hardening off your plants is not just to protect them from frosts in areas that are prone to them. Hardening off, it turns out, is as much about protecting your plants from the devastating effects of sunburn.
Luke Marion, gardening expert and founder of MIgardener (opens in new tab), explains the process behind hardening off seedlings in a recent YouTube video, stressing the importance of protecting young seedlings from sun scald and sun burn – both detrimental to your plants' chances of reaching maturity and producing flowers or fruit.
Although the two look slightly different, the root cause is the same – overexposure to strong sun when the plants aren't acclimatized to it. 'The leaves fry just like your skin fries on a really sunny day,' Luke explains. If you're learning how to grow flowers from seeds, it's very important to prevent leaf burn from happening, as there's not much you can do to reverse the damage once it's been done.
Fascinatingly, the process of hardening off – only taking out your seedlings outside for short periods of time to begin with – allows plants to build up resilience to the sun, almost like a 'natural sunscreen', as Luke describes it.
The traditional method of hardening off plants involves leaving them outside in two-hour increments: 'Bring them out for two hours, bring them in for the rest of the day; bring them out for four hours the next day' and so on, until the plants can be left outside all day. 'But,' says Luke, 'what if I told you there's a way to do it that's a lot less hassle?'
If you have a lot of plants, bringing them all outside, then bringing them all back in by lunchtime to avoid scorching them, is a lot to do on a daily basis for well over a week. And as for taking the plants outside during the peak UV hours ('roughly between 1pm and 4pm') – that's leaf burn almost guaranteed.
Instead, says Luke, choose a week of warm, overcast weather, and harden off your plants outside the whole time, without bringing them back in. 'The sun is still stronger than any commercial grow light available, but it's not strong enough to cause leaf burn.' Luke cautions that you will need at least three overcast days – 'two is pushing it', as your plants may not have enough time to acclimatize to strong UV radiation.
Some plants will need longer than others. If you're not sure, 'look for those sturdy, green leaves', says Luke – if you don't have that, your plant needs longer to harden off, so don't leave it out if the sun does come out until it's developed stronger leaves.
Need to know how to transplant seedlings too? You'll find all the advice you need in our guide.
Anna is a keen urban gardener, with David Austin roses and Japanese acers among her favourite plants. She moved into the world of interiors from academic research in the field of literature and urban space a couple of years ago. She's always been interested in how people make houses into homes, and how our concepts of what's stylish change over time.
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