This week we received two questions about the same problem: spindly or stunted seedlings under grow lights. One gardenerd said her seedlings were still leggy, even though she situated the grow light 2-3″ above the leaves. The other gardenerd said her seedlings were just not growing. Period.
Let’s address the first issue: spindly seedlings.
Timing–or rather–time, is everything when it comes to grow lights. Because grow lights don’t replicate the sun’s rays completely, it’s important to leave grow lights on long enough to do the sun’s job in an artificial environment. That means 12-16 hours. Usually the latter.
We use a vacation lamp timer to turn our lights on at 6 a.m. and off at 10 p.m. to prevent legginess. So it doesn’t matter if we’re home or not.
Darkness is also Important. Don’t leave the lights on 24 hours. Plants need darkness to restore, just like we do. If you leave the lights on all the time, it messes with plants’ circadian rhythm and can stunt growth as well.
Location is Everything – grow lights should sit no more than 3″ above the leaves of seedlings. Anything further away will cause legginess as the plants reach for the light source.
Make it Windy – plants also need a little stress to develop strong stems. Blow on your seedlings every day, or install an oscillating fan. Run it for 10-15 minutes per day to fortify stems against the wind.
Now let’s look at the other issue.
Sometimes seedlings stop growing or hardly grow at all from the get-go. While it can be a mystery as to why this happens, here are a few things to check:
Over Watering – Seed trays are often designed with a water reservoir that situates roots in direct contact with water as they grow. If we become overzealous with watering, plants become unhappy in this condition we call “wet feet”. Roots need oxygen in order to process nutrients and to grow. Allow seed trays to dry down a bit before adding more water. If your seedlings sit in a swamp (especially if you see moss start to grow) they will not perform well.
Feed the Babies – seed starting mix is almost always comprised of peat moss or coir and perlite, neither of which have any nutrient content. So you must feed your babies as they grow. Even if you add worm castings or compost to your seedling mix before starting seeds, the plants will use up those nutrients within a couple weeks of sprouting and will need more food before long. Water with kelp emulsion (and fish emulsion if you use it) to keep plants happy until you transplant them up to 4″ pots.
Feed them Again – when you transplant seedlings to 4″ pots to continue growing a few more weeks, mix compost and worm castings into the potting soil. This will help carry them along their next stage of growth. Water again with kelp emulsion two weeks later.
If none of these tips work, check your planting medium. We’ve had clients who purchased soil and seedlings mixes that tested excessively high in potassium and other elements that stunted growth. It was a fluke, but it does happen. Lab soil tests solve the mystery in most cases.
Don’t give up, gardenerds. Keep experimenting and keep trying. It will work eventually.