Cannabis is an interesting plant: it’s both extremely hardy and extremely demanding of growers. It’s a weed that grows in ditches all over the world, but it also requires a lot of time, effort, and attention from growers in order to grow to its full potential.
This applies to every aspect of growing – the soil you use, the environment in the grow room, the lighting, the blend of nutrients you feed your plants, and more. The nutrients in particular can be complicated because over the course of your plant’s life, its dietary needs will shift. The biggest shift happens during the flowering phase, but how do you know exactly when to switch to flowering fertilizer.
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When to Switch to Flowering Nutrients
The growth of a cannabis plant can be broken down into two basic stages: the vegetative stage and the flowering stage. You will need to pay close attention to your plants to know when to switch to flowering fertilizer at the beginning of the flowering stage.
Plants will have a short growth spurt then stop growing taller when the flower period begins. At that point, they’ll be focusing all of their energy on growing bigger, juicier buds. Typically, this happens around three weeks into the flowering stage.
Indoor – When you switch lights to 12 hours of darkness a day the plant will naturally go to a flowering state. A week after the switch is when you should start feeding plants Bloom fertilizer.
Outdoor – Plants will normally begin to flower a few weeks after the summer solstice (June 20) as the days begin to get shorter. Look to the formation of flowers as a sign to switch to Bloom fertilizer.
Autoflowers – These types of strains will flower automatically no matter how much light they’re getting daily. Autos typically start flowering after 3-4 weeks. Keep an eye out for new flowers and switch to Bloom fertilizer.
Click the image above to see a close up of the very early stages of flowering.
Those small white “hairs” are called pistils. The pistils emerge from a little pod called a calyx. The cannabis flower is made up of a cluster of these calyxes.
Ideal Flowering Fertilizer for Cannabis
During the flowering stage, your plants will need more phosphorus and potassium and less nitrogen. At Reefertilizer, we use an NPK mix of 20-5-10 while our plants are growing, and switch to a 5-30-20 blend at the appropriate time. It’s always a good idea to research the specific nutrient requirements of your current strain before you start fertilizing! Some strains require less nutrients than others, and will get nutrient burn more easily if you overdo it. For example we recommend going with slightly lower nutrient doses when growing auto flowering strains.
Finally, remember to stop feeding your plants nutrients for the last week before harvest to flush them out. This should help get rid of any excess nutrients in the soil so the plant has time to metabolize the nutrients stored in the plant. This helps the final flavor to be as smooth as possible. Happy growing!
This guide will answer many questions about growing cannabis, like the following...
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Mike Drouin is the co-founder of Reefertilizer. He’s an experienced craft cannabis grower and a writer of many articles regarding the process. Mike enjoys cycling and camping and will sometimes combine the two.