Growing cannabis using hydroponics can be tricky, but very rewarding. It is the fastest way to grow large yield plants in a short period of time, but there are many things that can go wrong (which will be discussed in this article). If you are new to hydroponics and you are unsure where to start, look at Deep Water Culture.
The key to ensuring your plant is getting the correct amount of nutrients is to track the PPMs of your plant’s reservoir, and identify whether they are increasing or decreasing over time. If the PPMs decrease, this means the plant is consuming more nutrients than water and the concentration is becoming more dilute. If the PPMs are increasing, this means the plant is consuming more water than nutrients, and the concentration needs to be decreased. If the concentration is allowed to continually increase the pH will drop and the plant will likely go into nutrient lockout and stop eating, therefore it’s always better to see your plant’s PPMs decreasing over time. Because cannabis plants are so sensitive to nutrients, its best to utilize a feeding schedule that has been used by other people for specifically growing cannabis in hydroponics. A good starting place is to read about the lucas formula.
In general, it’s a very good idea to track water level, nutrient concentration and PH at least daily with your hydroponic grow. Use the chart below to learn about what to do about certain problematic (or benign) trends you may identify.
|Static||Static||Static||Plant not feeding/drinking, change EC, check meters. Usually, lowering the EC a little should get the plant feeding again.|
|Static||Static||Rising||Ph buffers probably raising ph. This is usual. Having a static water level is not though, so again, a slight reduction in EC or a res change should resolve this.|
|Static||Static||Falling||Usual cause of this is when media has been rinsed at a lower ph than you require. The other possibility is that too much CO2 has been pumped into the water. See Note 1.Change your res and look at the volume of air pumped plus look at your air source.|
|Static||Rising||Static||Plant is leeching nutrition, raise EC. Note 2|
|Static||Rising||Rising||Plant leeching nutrition, Raise EC. An unusual state. The rising ph is probably caused by what nutrient leeching back. If these are alkaline, it will lead to the rise in ph. Could also be ph buffers.|
|Static||Rising||Falling||As above but be aware of the acid rain effect mentioned in note 1. Res change, plus increase in EC.|
|Static||Falling||Static||Plant eating but not drinking. Not ideal. Lower EC or res change|
|Static||Falling||Rising||As above but rising ph is a better sign. Lower EC slightly or res change.|
|Static||Falling||Falling||Falling ph along with falling EC but no drop in water level suggests a res change. Could also be an acid rain effect as per note 1. Depending on other symptoms, lowering EC after res change.|
|Falling||Static||Static||Perfect conditions. EC and ph are at the correct level.|
|Falling||Static||Rising||Normal state most people encounter. Nothing to worry about, carry on doing what you are doing unless other plant symptoms.|
|Falling||Static||Falling||Res change plus a change of EC. Lower EC if over 1.4, raise EC if lower than 1.0|
|Falling||Rising||Static||Plant is drinking more than eating, lower EC.|
|Falling||Rising||Rising||Plant is drinking more than eating, lower EC|
|Falling||Rising||Falling||Plant is drinking more than eating, lower EC. Also, res change due to possible acid rain problem.|
|Falling||Falling||Static||Hungry plant, raise EC. Very good situation to be in. Nute buffers are working and plant is taking a balance of nutrients.|
|Falling||Falling||Rising||Almost as above, usually considered almost perfect, raise EC slightly.|
|Falling||Falling||Falling||Res change. Potential acid rain issue but plant is still eating & drinking. Raise EC on new res.|
Nutrient deficiencies while growing cannabis are typically rare and usually caused by bacterial issues, but nutrient burn on the other hand is very common. A common pitfall for new growers is using “full strength” nutrients per manufacturer recommendations - but Marijuana plants have special nutrient needs and using full strength nutrients is a surefire way to cause nutrient burn.
Typical symptoms of nutrient burn include:
- Brown, crispy, “Burnt tips” on fan leaves
- Dark green fan leaves (Nitrogen Abundance)
Mobile Elements (Nitrogen, Phosphorus, Potassium, Magnesium, and Zinc) typically develop visual deficiencies in older leaves. Immobile elements (Sulphur, Calcium, Iron, Manganese, Chlorine, Boron, Copper, Molybdenum, sodium, silicon and cobalt) on the other hand develop deficiencies in younger leaves first and progress down the plant.
Bacteria that inhibits plant growth in hydroponics are surprisingly common (root rot). Even if res temperatures are in the correct range, bacteria could have started growth at some point before. There are many symptoms of a bacterial infection. The most common symptoms of root rot are discolored roots, that are slimy or fuzzy. The plant itself might show symptoms of many different deficiencies at once - this is because the bacteria is obstructing and/or consuming nutrients that the plant needs. Additional symptoms may include pH dropping or rising more than 1 within 12 hours.
Solving a bacteria issue (root rot) Before attempting to solve a bacteria issue, you need to try to correct the underlying issue. Bacteria growth is often related to your res temperature being too high. If that is the case, consider buying a chiller, or using some other method for lowering res temperature. Some people rotate blocks of ice through their res to keep the temperature now. If your res temperature is in the correct zone, try the following solutions for a bacteria problem.
- H2O2 flush. This method is not universally recommended, many people will advise against this. BUT, there are numerous success stories, and there is even a case of a Advanced Nutrients customer service rep suggesting this method. The customer service rep suggested 150 ml/Gal of 3% H2O2 for 12 hours. Here is a notable instance where a H2O2 flush solved the bacteria issue.
- Compost Tea homebrew
This is probably the most popular form of hydroponics for cannabis growers, as well as the cheapest.
- Growing Medium
- Net Pots
- Plastic bin or 5 gallon paint bucket (common)
- Air pump with airstone
- Res Chiller (optional)
The basic idea for DWC is to suspend the plant’s roots in a nutrient and oxygen rich solution. Typically the plant is grown in a medium like Rockwool or Hydroton, and put within a net pot. The roots will grow through the holes in the net pot and be suspended in the nutrient solution.
The Air pump with airstone is a critical component to this setup because it oxygenates the water. Roots require oxygen to grow, and if there isn’t oxygen dissolved throughout the nutrient solution your plant can actually die. Typically the larger the pump the better, especially if the res temperatures are high. If the res temp is high, oxygen will have trouble staying in solution and many people will buy a res chiller to decrease the res temperature in order to ensure enough oxygen stays dissolved for the roots.
The Lucas Formula is a common method for feeding cannabis plants when growing in a DWC setup.
Q: My pH keeps dropping
A: First check whether your PPMs are increasing - if so, add plain water to dilute your solution. If the pH is dropping more than 1 within 12 hours, you likely have a minor bacterial infection. See the bacteria section for advice.
Here is an example of someone with a pH issue and no other symptoms, but ended up having a bacterial problem.