Light Penetration and Optimal Distance of Light from Your Cannabis Plants

What Distance Should I Keep the Light from my Plants?

The general rule of thumb is that if you can keep your hand comfortably just above the tops of your plants and keep it there for a minute without it getting excessively hot then they should be fine.

As your plants grow closer to the lights you simply raise the lights on the yoyo strings provided with most grow tent kits.

If your seedlings start stretching and looking flimsy like they could fall over, then your light is too far away and your plants are stretching for the light. This is not ideal and the light should be lowered.

If they have stretched a lot, you can cut up some small pieces of a drinking straw and then slice one side open so that you can open the straw up, place it around the seedling and it will act as a support when it wraps around the stalk by reverting to its original shape.

We recommend that if your grow tent kit didn’t come with one, you acquire a small clip on fan, these can easily be attached to one of the side tent poles and it’s a good idea to keep your young seedlings ‘’dancing’’ in the breeze, this will strengthen their stalks and make them more robust while at the same time provide them with air circulation and cool them down.

If you have chosen a grow tent kits with either the 400w, 600w or the 1000w HPS light then you may want to consider looking at purchasing either a T5 fluorescent light or a metal halide light for the early stages of vegetation and the seedling stage as these HPS lights are somewhat overkill for such small plants and it’s an unnecessary use of power.

It’s something to consider if you intend to be a long term grower or if you plan on perpetual grows. The savings will add up over time.

Light Penetration of a Surface Area

250W – covers 2′ x 2′ (0.6m x 0.6m) area up to 2.5′ x 2.5′ (0.8m x 0.8m)

400W – covers 3′ x 3′ (0.9m x 0.9m) area up to 3.5′ x 3.5′ (1m x 1m)

600W – covers 3.5′ x 3.5′ (1m x 1m) area up to 4′ x 4′ (1.2m x 1.2m)

1000W – covers 4′ x 4′ (1.2m x 1.2m) area  up to 5′ x 5′ (1.5m x 1.5m)


Optimal Distance Range between HPS Light and Plants


Grow Light Strength (Watts) Nearest Distance to Plants Furthest Distance From Plants
150W 5″ (10cm) 11″ (28cm)
250W 6″ (13cm) 13″ (33cm)
400W 8″ (15cm) 19″ (48cm)
600W 9″ (20cm) 25″ (64cm)
1000W 11″ (26cm) 31″ (79cm)

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