Cannabis seedlings: ideal temperatures, humidity, and light

Knowing the right amount of light to expose your seedlings to, plus the perfect temperatures and humidity is vital for growing healthy cannabis plants.

Cannabis seedlings thrive in bright, sunny conditions (21°C to 23°C). They also like humidity and a gentle breeze.

Growing cannabis seedlings inside under lights

If you’re growing inside, the seedlings can be placed under artificial light as soon as they’ve sprouted from the soil. These distances will vary depending on wattage, but as a rough guide:

• If you’re using HID lighting, keep the seedlings at least 50 cm from the bulb.
• If using a compact fluorescent lamp, the distance should be about 15 cm; with normal fluorescent tubes, the distance should be reduced to just a few centimeters.
• If you are using LED lights, the distance should be about 30 cm.

Your seedlings will also need a gentle breeze to strengthen their stems; this means they’ll be less likely to fall over as they mature. A small household fan on its lowest setting works fine. It should oscillate so that the plants don’t receive airflow from only one direction. If you can see the plants moving gently, they are getting enough breeze to strengthen them.

Growing cannabis seeds outside – natural daylight

If your seedlings are going to be planted outside, you should move them near a sunny window for the first week or two, just after they emerge from their growing medium (avoid long periods of direct sunlight).

When they’ve developed their second or third growth of serrated leaves, they’re strong enough to thrive in direct sunlight. Give them increasing amounts of exposure to the sun, to get them used to outdoor conditions.

Propagators

This plastic tray has a see-through top, and traps moisture and warm air in with your seedlings. However, as air can’t circulate inside, you shouldn’t keep your plants in a propagator for too long.

When your seedlings have their second or third set of serrated leaves (or if they’re taller than seven centimeters), the cover should be taken off. Use a small household fan on the lowest setting to create some air movement.

If you’re planning to grow your plants outside, place the covered propagator next to a window. This ensures your seedlings get the light they need. Once the plants are big enough to thrive without the propagator cover, you can start acclimatizing them to being outdoors.

When to put your seedlings outdoors

Firstly, make sure it’s warm enough outside. Temperatures should consistently stay above 15° centigrade, also at night. To start with, leave the seedlings outside for a few hours, when it’s as sunny as possible. Increase their exposure by an hour or so each day. After two weeks, they should be ready to be left outside permanently.

Maintaining humidity for your cannabis seedlings

It’s important that your seedlings don’t dry out. In a propagator, this is less of a problem, but outdoors and under lights you should check on them regularly. Seedlings will benefit from being regularly misted with a plant spray. However, take care that they are not in direct sunlight with drops of water on their leaves, as this can act as a magnifying glass and burn them!