Good Lighting for Seedlings

For many gardeners, March is the month for sowing seeds. Since we can’t garden outside yet, we garden indoors, preparing the annuals and vegetables that will adorn our summer gardens. And it’s so easy to do… when the lighting is right.

Lighting is indeed the weak point of seedlings grown indoors. If there is not enough light, the seedlings become spindly: the stems grow excessively long and remain pale, often leaning rather than standing upright. The leaves are also pale and weak. And the young plants become very susceptible to damping-off, a devastating disease that causes them to fall to the ground, as if they were pinched at the base, and then die. Therefore, a fair amount of light is needed to produce beautiful seedlings.

Lighting during germination

However, at the very beginning, the quality and intensity of light are not very important. The first stage in the life of a seedling is germination, the period when the seed prepares to grow. It absorbs water, begins to swell, and then a small root descends into the soil while the first leaves, usually two, the cotyledons, unfold. This is germination.

During this period, which can take as little as 3 or 4 days for some seeds or 2 months or more for others (7 to 14 days is more typical), light is generally unnecessary for most seeds to germinate. Seedlings can therefore be placed on a windowsill, under a fluorescent lamp, or elsewhere.

At this stage, cover the seed tray with a clear plastic dome or a piece of glass to increase humidity and moderate temperature changes. But be careful not to overheat the seeds! If the seedlings are covered with plastic or glass and the sun is beating down, the temperature can rise to lethal levels. At this stage, therefore, avoid locations in direct sunlight.

However, be aware that a very small minority of seeds germinate better when covered with substrate and protected from light (pansies, delphiniums, etc.). The seed packet will highlight this detail. Cover the tray of seeds with a few sheets of newspaper during germination or place them in a wardrobe. However, as soon as germination is complete and small seedlings appear, they will need light. You will therefore need to check on them every day.

The window or the lamp?

Change the conditions once the seedlings have sprouted: remove the dome or plastic covering and expose them to much more light. From this point on, the quality of light becomes crucial.

If you are growing your seedlings on a windowsill, choose the sunniest spot available. Full sun is not too much. Intense sunlight will produce more compact and greener plants. As seedlings tend to lean toward the light source, rotate the seed tray a quarter turn every 3 or 4 days: this will expose them to sunlight on all sides and help them grow straighter.

But in most homes, the natural light in March and April is insufficient to produce sturdy seedlings. The simplest solution is therefore to use artificial lighting.

Today, full-spectrum LED lamps have become the gold standard for seedlings. They consume very little electricity, generate hardly any heat, and produce light that is perfectly suited to the growth of young plants. Above all, they provide constant lighting, which is impossible with the spring sun. Most gardeners set a timer for 14 to 16 hours of light per day to mimic the long summer days.

To be effective, the lamps must be placed very close to the plants: about 15 to 20 cm (6-8 inches)above the leaves. As seedlings grow quickly, the lamp must be raised regularly to prevent the plants from touching it.

When seedlings grow

Small seedlings will grow… and take up more space. They need to be transplanted into larger pots… and new, well-lit locations found. From the beginning of May, it is possible to place seedlings outside, but under a temporary greenhouse. You can build one with a few pieces of wood or PVC pipes, a roll of transparent plastic, and a stapler, or buy a small, ready-to-assemble greenhouse.

The advantage of a greenhouse at this stage is that the light is more intense (it comes from all sides) and temperatures naturally drop at night, which helps seedlings acclimate to garden conditions. Starting in May, the risk of frost decreases significantly, but does not disappear completely.

The greenhouse stage is not mandatory—you could continue to care for your seedlings in front of a window or under a lamp—but it is very useful for gardeners who lack space.

Acclimatization

At the end of May or beginning of June, when nighttime temperatures exceed 10°C (50°F), you can finally expose your seedlings to full sun by planting them in the ground. But be careful! Don’t rush it! There is still the acclimatization stage to go through.

Plants that have spent their entire lives indoors have not yet been exposed to ultraviolet rays. These rays are largely filtered out by greenhouse glass and plastic, and indoor lamps produce very little of them. These rays can easily burn seedlings that are still fragile.

So, place your seedlings outside in a shady spot that gets a little morning sun for 2 or 3 days, then in partial shade for 2 or 3 days, and then in full sun for 2 or 3 days. They will then be ready to plant, either in the ground or in a container.

And there you have it: providing your seedlings with the right amount of light isn’t rocket science. You just need to know how to adapt to their needs.

Editor’s note: This text has been revised and updated to reflect current horticultural knowledge and practices.