Using Osmocote to Grow Cannabis Indoors: No Brainer or Major Hassle?
* All ppm numbers on this page use the U.S. 500 scale.
In our Case Study on Cannabis Root Zone pH Monitoring article we talked about using Osmocote and some of the extra things to consider when growing cannabis with a time release fertilizer. Here we continue the conversation and point out some of the issues when using time release fertilizers.
There is a misconception that using a time release fertilizer for growing plants is a no brainer. You add Osmocote to the growing medium and then use plain water for the duration of the growing season, what could be simpler. Outside this usually works out ok. But indoors things can get tricky. For example, lets say you are monitoring the pH and ppm values of the runoff water every few days. Let’s also say that your plant(s) are in 2 gallon pots and each has 2 1/2 tablespoons of the time released fertilizer mixed into the top layer of an inert growing medium like peat or coco and perlite and you are watering your plants with RO water of 10ppm tds. Then you test the runoff water on Monday, Wednesday, and Friday after watering the plants and you record the data. On each of the three days the pH values are about the same hitting your target area but you notice that your ppm tds values are kinda erratic and not what you might expect since you have added nothing to the plants other than RO water. So what could be happening here? One thought is that over time the Osmocote fertilizer sheds less or more nutrient causing the fluctuations, but your recorded numbers are 250ppm on Monday, 540ppm on Wednesday, and 300ppm on Friday. So the shedding theory doesn’t really work.
Then you realize that when you water your plants you are not measuring the exact amount of water being applied each time. So on Monday you maybe applied 1/2 gallon of water, on Wednesday it was more like 1/4 gallon, and on Friday maybe around 1/3 gallon. So each of the three waterings will have a different concentration of nutrients in the runoff water. This is an important finding to be mindful of indoors. And maybe where you were pouring the water had the most Osmocote, further distorting your runoff test data.
Consider if you were to apply 1 tablespoon of Osmocote to a small plant in a one gallon pot and then water it with 1 cup of water, the runoff would likely be around 260ppm tds since the soil is fresh and loose and the water runs straight through. However, if you have a larger plant that has been growing in a one gallon pot in a peat based medium (which tends to compact over time) for a few months, that same 1 tablespoon + 1 cup of water scenario could produce a higher ppm runoff because it takes longer for that water to penetrate the soil and get from the top down and out the drain holes. Because the Osmocote has more time to sit in the water it can shed slightly more nutrients and increase the nutrient ppm concentration.
Another potential issue is that the popular Osmocote pictured above is a 15-9-12. A few weeks after you switch to a flowering cycle the plants Nitrogen needs diminish. Not being able to reduce the amount of Nitrogen can cause a delay in flowering. A 15-9-12 fertilizer is fine for the vegetative cycle but not so much for flowering. Osmocote only lasts about 4 months, but growing indoors you switch to flowering long before that. Click here to see the average nutrient levels of each element that are needed for growing cannabis.
So there are things to consider when using time release fertilizers indoors. Outside none of this problematic because nature is harsh and plants are tough. And with the longer growing season everything works out fine. When using an inert medium like peat/perlite/dolomite lime outside with Osmocote mixed in you can have great success. In addition, using a bloom booster fertilizer every 2 weeks will result in large, healthy, and very productive plants. If you can dial these things down a bit for the less harsh indoor conditions, it could be a winner.