Larry Hodgson has published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This piece was originally published in Gardens Central in March 2012.
If you’re like me, you grow dozens of plants from seed indoors every year — annuals, vegetables, herbs, and perennials – yet you can’t help but be slightly disappointed by the results. By planting out time they’re usually spindly and pale, little resembling the lush healthy plants you can pick up at your local nursery. Of course, you’ll discover your wimpy seedlings quickly pick up once they’re planted out, but… isn’t there a way of keeping your babies happier while they’re indoors?
Sure there is. In fact, there are several things you can do. But first you need to understand why our homegrown seedlings don’t compare well to those grown in garden centers. There are in fact two main reasons – ours get less light and experience warmer temperatures.
Full Sun, Cool Nights
It’s hard to equal greenhouse conditions when it comes to light. In a greenhouse, light comes from all four sides and above. If you grow your seedlings on a windowsill, even if the window faces south, that’s still only five or six hours of full sun. And many of us are using east or west windows with even less light.
Also, greenhouse growers start their seeds in special climate-controlled areas that are exceptionally warm and where daytime and nighttime temperatures are equal, but after germination, they move the seedlings to cooler areas. The main purpose for the move is to save energy.
In the spring, a cool greenhouse requires little to no heat. But in fact the majority of seedlings do best with moderate daytime temperatures and considerably cooler night temperatures. Combine full sun, moderate days and cool nights and you’ll have short, dense, very green plants.
Compare those conditions to homegrown seedlings. They’re usually on or near a windowsill or under grow lights. Neither of these places are as bright as a greenhouse. Also, we tend to heat our homes to temperatures comfortable to us – rarely cooler than 18°C (64°F), even at night. But most seedlings prefer a drop to 10-12°C (50-54°F) at night, even less for perennials.
So, What Can You Do to Give Your Seedlings the Conditions They Need?
It’s not so easy to fix light problems. A south-facing sunroom would give you greenhouse-light intensities, but that can be an expensive addition. Try adding on a cold frame (much cheaper) or install a temporary greenhouse. There are lots of models on the market or you can make your own.
Keep starting your seedlings inside the house, as almost all seeds germinate better under constant warmth, and then move them out after they have four to six true leaves.
By the time the snow is gone, most cold frames and temporary greenhouses will not even need heating. The heat they save up in the day will carry them through the night. You’ll be amazed at how great your plants look!
If that isn’t possible, at least grow your seedlings right up against a window in the brightest room you have.
Growing Under Lamps
If you grow under lights, two things can be done to increase the light – move the plants closer to the tubes and extend the lighting period. Generations of gardeners have learned that seedlings do best, staying much more compact, when they’re only 2-5cm (0.8-2″) below fluorescent tubes.
The problem is they keep growing, so you have to adjust every two or three days. I keep a pile of potsthat are different heights on hand – tured upside down, they make great supports for my seed trays and I just keep using smaller and smaller support pots as the seedlings grow. You can also have grow lights on chains so you can move them upwards. Just don’t let your seedlings touch the lights or their leaves may dry up.
Also, buy a cheap timer and set the days as 14-18 hours. That will give your seedlings much more light. Just be aware that a very few seedlings need shorter days to grow well.
One of the rare examples is the African marigold (Tagetes erecta) that needs short days (less titan 12 hours) to initiate bloom. However, their seedlings still look and grow best under 14-18 hour days. The solution? Grow them under lights using a 14-18 hour cycle, then about three weeks before planting them out, adjust their timer to 11-hour days. That will give you great-looking plants with abundant bloom.
Other Helpful Hints
For healthy green seedlings, you’ll need to start fertilizing after the plant has four or so true leaves. I like to use seaweed fertilizer, diluted according to the label, as it is unlikely to burn seedlings.
Humidity, Sunlight and Watering
Keep the humidity up. Dry air can result in dry leaf edges. A room humidifier can help, or set up a humidity tray (a tray of gravel that you occasionally pour water over, so the water evaporates and humidifies the air). Or simply grow many seedling trays in the same room, as they are natural humidifiers in and of themselves. However, do not leave plastic domes over seedlings for more than a few days after germination. Yes, the seedlings will love the humid air, but the lack of air circulation beneath a dome can lead to diseases like rot.
Give your seedling containers a 60-second shake every day. This imitates the action of wind outdoors and helps the plant develop a thicker stalk. Turning a fan (at the “breeze” setting) towards your plants will give a similar result.
If you’re growing your plants in front of a window, turn the trays every few days otherwise they tend to grow sideways! (This isn’t necessary if you grow them under lights or in a greenhouse.)
Don’t forget to water. Seedlings are very delicate and even the slightest touch of drought will hold them back. You’ll need to check often, daily, if possible, as seed trays dry out very quickly.
Finally, always acclimate your seedlings to outdoor conditions before planting them out. I give mine two or three days of shade, two or three days of partial shade and two or three days of sun before doing so. And if there’s a cold snap, I bring them all back indoors or into a cold frame or greenhouse overnight. Why risk losing your plants when you’re so close to your goal?