Gardening Sowing Seeds

Tips and Tricks for Top-Quality Seedlings

If you’re like me, you grow dozens of plants from seed indoors every year: annuals, vegetables, herbs, perennials, etc. Yet you can’t help but be a bit disappointed by the results. By planting out time, they’re usually a bit spindly and pale, looking little like the lush, healthy plants you can pick up in your local nursery. Of course, you’ll discover your wimpy seedlings do quickly pick up once they’re planted out, but … isn’t there a way of making your babies look happier while they’re indoors? 

Sure there is, and in fact, there are several things you can do. But first, you have to understand why homegrown seedlings usually don’t compare well to those grow in garden centers. There are in fact 2 main reasons: they get less light and receive warmer temperatures. 

Full Sun, Cool Nights

Recreating commercial greenhouse conditions indoors is nigh to impossible. Photo:

It’s hard to equal greenhouse conditions when it comes to light. In a greenhouse, light comes from above and all four sides. If you grow on a windowsill, even if the window faces south, that’s still only 5 or 6 hours of full sun. And many of us are using east or west windows, with even less light.

Also, professional 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 it also happens that the majority of seedlings do best with moderate daytime temperatures and considerably cooler night ones. Combine full sun, moderately warm days and cool nights and you’ll have short, dense, very green plants. 

Seedlings grown on a windowsill tend to look a bit weak compared to greenhouse-grown ones. Photo: Karen,

Compare those conditions to homegrown seedlings. They’re usually on or near a windowsill or under grow lights. Neither is as bright as a greenhouse. Also, we tend to heat our homes to what is comfortable to us: rarely less than 65 °F (18 °C), even at night. But most seedlings prefer a drop to 50 °F or 55 °F (10 °C or 13 °C) at night, even less for perennials. So, what can you do to give your seedlings the conditions they need?

Chillin’ With the Family

Seedlings like cooler temperatures than you do! Ill.: &

If you’re persuasive, you may be able to convince your family of the advantages of a cold night’s sleep. Get out the sleeping bags and set the thermostat at 40 °F (5 °C). Move any seedlings that like warmer temperatures (tomatoes and peppers, for example) well into the room where it will be warmer, but the others will love the cool of the windowsill (and no, it won’t really drop to 40 °F/5 °C in your bedroom). Your family may complain but your plants will be happy. And do remember that the family dog makes an excellent foot warmer.

Light Up Their Life

One of my mini-greenhouses, used in spring to give my seedlings the brightest light possible. Photo:

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 a cold frame (much less expensive). Or install a temporary greenhouse: there are many models on the market or you can make your own. Continue starting the seedlings indoors (they need warmth to germinate), then move them out after they have 4 to 6 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 will look! 

If that isn’t possible, at least grow your seedlings right up against a window in the brightest room you have. 

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. 

Keep your seedlings close to the lamp, although without touching. These could use a bit of a boost! Photo:

Generations of gardeners have learned that seedlings do best, staying much more compact, when they’re only 1 to 3 inches (3 to 5 cm) below the tubes. The problem is, they keep growing, so you have to adjust every two or three days. I keep on hand a pile of pots of different heights: turned 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 the seedlings touch the lights or their leaves may dry up.

Also, buy a cheap timer and set the days at 14 to 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 these rare examples is the African marigold (Tagetes erecta) that needs short days (less than 12 hours) to initiate bloom. However, their seedlings still grow and look best under long days. The solution? Grow them under long days under lights, then about 3 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

Use a sowing mix containing beneficial fungi. Ill.: Claire Tourigny, from the book Les idées du jardinier paresseux: Semis

Use a growing mix containing beneficial fungi (mychorrhizae) or add them at sowing time. They help encourage better growth and disease repression.

For green, healthy seedlings, you’ll need to start fertilizing after the plant has 4 or so true leaves. I like to use seaweed fertilizer, diluted according to the label, as it is unlikely to burn seedlings. 

Domes are great for germination, but once seedlings sprout, remove them to provide adequate air circulation. Photo:

Keep the humidity up: dry air can slow growth and result in dry leaf edges. A room humidifier can help, or set up a humidity try (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 notleave plastic domes over seedlings more than a few days after germination. Yes, the seedlings will love the humid air, but the lack of air circulation under a dome can lead to diseases like rot and damping off.

Brush or shake seedlings regularly to give them a shorter but thicker stalk. Photo:

Give your seedling containers a 60-second shake every day or brush over them back and forth with your hand or a soft brush. This movement 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.

A quarter turn once a week will keep your seedlings standing straight. Ill.: Claire Tourigny, from the book Les idées du jardinier paresseux: Semis

Turn the trays every few days if you’re growing them in front of a window; otherwise they’ll 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 veryquickly.

Switch to Cool White tubes. Horticultural tubes purport to do wonders, but were designed to induce flowering, not foliage growth, and at the seedling stage, it’s foliage you’re concerned with. Horticultural tubes are less intense than Cool Whites, far more expensive, and simply don’t do a good job on seedlings. Save them for plants that really need them, like orchids or cacti.

Harden off seedlings before you plant them out. Photo:

Finally, always harden off (acclimatize) your seedlings to outdoor conditions before planting them out. I give mine 2 or 3 days of shade, 2 or 3 days of partial shade and 2 or 3 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 near your goal!

Yes, you too can grow professional quality seedlings: you just need to know how to do so! 

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, will continue its mission of demystifying gardening and making it more accessible to all.

1 comment on “Tips and Tricks for Top-Quality Seedlings

  1. Pingback: Tips and Tricks for Top-Quality Seedlings | Gardening Partners

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