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height of lights when germination of marijuana seed

Cannabis seedlings are super fragile and can easily “burn” in nutrient-rich soil. In general, we don’t recommend feeding during the seedling phase. Instead, keep your seedlings chilling in their Easy Start pots until they’re ready to be transplanted and start vegging.

These small, slender, winged insects leave irregular snail-trail-like spots on healthy leaves.

Remember to be very gentle when handling your seedlings and transplanting them. Any minute damage to their roots can result in a ton of stress that, for such young and fragile plants, can take a while to recover from.

HOW TO PREVENT DAMPING OFF

Cannabis seedlings like daytime temperatures of 20–25°C and nighttime temperatures that are roughly 4–5°C cooler. High temperatures will stress your seedlings and stunt their growth, which, at such an early stage, can prove fatal. Dry leaves with curled up edges are a telltale sign of heat stress. With time, your seedlings might also develop other symptoms, including pale foliage and red or purple stems. Heat stress can also cause weak, wilting leaves with downward folding tips.

Cannabis plants constantly lose moisture through their leaves in a process known as transpiration (which plays an essential role in a plant’s ability to transport water from its roots up through its stem). Hence, it’s super important they always have access to water from their soil. When a plant goes too long without water, a lot of its vital functions start to slow down. Any roots that dry out completely die off, stunting the plant’s growth or possibly killing it all together (if its root system is underdeveloped).

Luckily, unlike some of the other seedling issues we’ve mentioned in this post, it is possible to remedy nutrient burn. Simply lay off the nutrients for at least one week and water your plants with plain, pH-balanced water. Once your plant starts to grow more healthy, green foliage, slowly dial the fertiliser back in.

There’s no universal schedule on how to water your cannabis seedlings. Instead, you’ll need to pay close attention to your plants and their medium. We recommend sticking your finger roughly 2.5cm (1 inch) into the soil and watering only when the soil is completely dry. Also, remember to water your plants close to the stem where you know their roots are. Finally, remember that your pots need to have drainage holes in the bottom so excess water can drain out.

Get many colas close to the light for the best yields!

Closeup of yellow leaf from light stress

The reason this can be such a problem with LEDs is it can take a few weeks before your plant really starts to show symptoms. Usually the upper leaves are affected first (yellowing or turning brown or spots) by too-close LEDs, but symptoms can also affect other parts of the plant.

Measuring Optimal Cannabis Light Levels

Sometimes the tips of leaves will turn yellow in response to the grow light being too close. This is different from nutrient burn which produces brown tips, or nutrient deficiencies which typically appear all over the plant and are often caused by incorrect pH at the roots.

LEC grow lights are surprisingly powerful and must be used with caution to prevent light burn, especially with seedlings and strains that tend to be more sensitive to light. This 315 LEC is being kept 2.5 feet (75 cm) away from the tops of these young plants until they get bigger. Some CMH models need to be kept 3 feet away from the tops of plants!

< 15,000 lux – sparse or “stretchy” growth – plant isn’t getting enough light
15,000 – 50,000 lux – good amount of light for healthy vegetative growth
45,000 – 70,000 lux – optimal amount of light for cannabis plants in the flowering (budding) stage
70,000 – 85,000 lux – a lot of light, some strains thrive at this light level, but some plants (especially auto-flowering and indica strains) lose their top leaves early under this light intensity and need lights to be kept further away for healthy growth
> 85,000 lux – at this light intensity, you’ve hit the plant’s “saturation point” which means your plant can’t use all the light (be careful of light bleaching!)

If you’re using older MH/HPS bulbs, you should replace them if you can, but if that’s not possible you can keep your bulbs closer to help make sure your plants are still getting good light levels. This is why it’s beneficial to be able to test your current light levels with a lux meter.