Best Conditions for Cannabis Seedlings: Temperature, Humidity and Light

What are the best conditions for cannabis seedlings? A steady 23-25 °C (73-77 °F), humidity around 60% or higher, gentle light for about 18 hours a day, very little water and a light, airy medium with no added nutrients.

You can start a grow from a seed or a clone, but whichever route you take, your cannabis plant starts life as a seedling.

Starting from seed has some real perks. A pack of seeds can cost about the same as a single starter plant, so it's cheaper in the long run. Another plus of growing from seed is that you get more options for growing different strains. A clone comes from a mother plant with one specific genetic, while different seeds can give you different strains. Finally, a seed lets you start from scratch, so you know the seedling isn't carrying chemical residues or someone else's pests. Starting from seed gives you more quality control over your seedlings.

Plants are considered seedlings for roughly 2-3 weeks after germination. During that time, if you're growing outdoors, they need a spot with direct sun. If you're growing indoors, set your grow lights to stay on for at least 18 hours a day.

Caring for seedlings after germination isn't hard, but paying attention to things like humidity, nutrition, temperature, water, light and transplanting is what gets you strong, healthy seedlings.

Humidity and temperature for cannabis seedlings

Seedlings don't do well in extreme temperatures. Aim for a steady temperature of around 23-25 °C (73-77 °F) in your grow room. If it gets too hot, cannabis seedlings can struggle to pull enough water through their roots and may wilt. If it's too cold, growth slows down and eventually stalls.

Cannabis seedlings can handle high humidity of around 60% or more. They can take in moisture through their leaves, which helps if you're growing clones or seedlings with underdeveloped root systems.

Light schedule for seedlings

Lighting for cannabis seedlings doesn't need to be super powerful. The usual light schedule for cannabis seedlings is 18 hours of light a day, whether you're growing autoflowers, feminized photoperiod seeds or clones. Some growers run 20 or even 24 hours of light a day. That's fine too, as long as photoperiod plants get switched to the right light cycle when it's time to flower.

When to transplant seedlings

One good cue for when to transplant is how far your seedling's leaves have spread. Your plant should start out in a seedling container or starter pot, which is small by design. Once the leaves reach the same width as the container, it's a good time to transplant. Be gentle and careful when transplanting a seedling, since the plant is fragile at this stage.

Watering seedlings

Seedlings need very little water because they transpire very little. On top of that, the small, immature root system of a cannabis seedling can't take up large amounts of water.

Healthy vs. unhealthy cannabis seedlings

A sick cannabis seedling may have yellow or discolored leaves. You might also notice it just isn't growing. Your best bet is to compare it with your other seedlings: if it looks different and sad, it's probably not healthy. Problems can come from all sorts of environmental sources, including lighting, nutrition, temperature, pH, watering or pests.

A light, well-aerated growing medium is essential for good seedling growth, and at this stage you don't need to add nutrients, since the soil already has everything your little plant needs.