You can harden off leggy seedlings with some extra care and a few changes to the normal process. Leggy plants need more wind shelter and support stakes to get through without snapping. The good news is that even stretched seedlings can become strong transplants with the right approach.
I ended up with leggy tomato seedlings my third year gardening when my grow lights sat too far above the plants. They stretched tall and thin reaching for the light while their stems stayed weak and floppy. I thought they were goners but decided to try hardening them anyway to see what would happen to them.
In my experience, those leggy seedlings made it through hardening and into the garden just fine. The key was keeping them in calm sheltered spots throughout the whole process. I used bamboo stakes to prop up the tallest ones so they would not flop over and snap during outdoor sessions.
Leggy seedling hardening takes more patience because your plants start out at a big disadvantage from the start. Their thin stems snap in even light wind that normal seedlings would handle fine without any help. They dry out faster because the tall thin shape loses water quick. You need to baby them more and accept the process will take longer.
You can help strengthen leggy stems before hardening starts by brushing them with your hand each day indoors. This triggers the same thickening response that wind causes in healthy plants outside. Spend two to three minutes brushing your seedlings morning and evening for a week before outdoor time starts.
Keep your stretched seedlings outdoor transition focused on calm sheltered spots the whole time. The north side of your house or a covered porch works well for leggy plants from start to finish. Skip the windier spots you might use for normal seedlings since your weak stems cannot take the stress.
Some leggy crops can be buried deeper when you transplant them which helps fix the problem for good. Tomatoes grow new roots all along buried stems which turns that extra length into extra root system for you. Bury your leggy tomato seedlings up to their first true leaves and they will come out stronger in the end.
Pinching back the top growth on leggy plants can help them grow bushier instead of taller over time. Snip off the top inch or two once your seedling has several sets of true leaves. The plant will branch out from lower on the stem and grow more compact going forward into your season.
Read the full article: The Complete Guide to Hardening Off Seedlings
