Nutrient Matters is a recurring series focusing on plant nutrition by university researchers. There are many to consider when germinating cannabis seeds — not only the germination method you choose but also the conditions your seeds require to thrive.
New Research Results: Optimal pH for Cannabis
Nutrient Matters is a recurring series focusing on plant nutrition by university researchers.
The recommended substrate pH for cannabis (Cannabis sativa) varies widely. A recommended substrate pH of 6.0 to 6.5 often is cited as the standard. However, we have observed pH ranges between 4.0 and 5.0 or greater than 7.0 during our visits to commercial greenhouses growing cannabis.
Our observations from those greenhouses indicated that plants grown at acidic (lower) pH appear to have normal growth and lack the symptomology of lower leaf blackening or bronzing typically seen with other greenhouse species at lower pH levels. In crops grown at higher pH, only in rare instances did we observe interveinal chlorosis of the upper (youngest) leaves (a common symptom of greenhouse crops grown at high pH). These observations suggest that cannabis is not prone to iron deficiencies when the substrate pH nears 7.0.
Returning to our labs at North Carolina State University (NCSU), we began our work to understand the underlying factors affecting pH in substrates and how those fluctuations impact the Cannabis sativa plant in order to determine the optimal pH range for cannabis.
This is what we discovered.
pH on the Range
Substrate pH is important to plant nutrition because it directly impacts plant nutrient availability.
Substrate pH levels below 5.0 result in increased micronutrient availability that can lead to iron (Fe) toxicity or manganese (Mn) toxicity, or both. Symptoms of Fe toxicity and Mn toxicity may appear as black speckling on older leaves (observed with zinnias or gerbera) or as bronzing (observed with marigolds or seed geraniums). While some crops display low pH symptoms on lower leaves, others may only display low pH through stunted growth, as is the case with poinsettias.
In our initial experiments at NCSU, we did not observe symptomology of lower leaf blackening or bronzing on Cannabis sativa plants, only stunted growth. These preliminary results suggested that cannabis can regulate micronutrient uptake under low substrate pH conditions similar to a poinsettia.
When the substrate pH becomes too high, many species develop interveinal chlorosis (yellowing) on the youngest leaves. This is a common situation that occurs with many greenhouse-grown species (such as petunias) when the elevated pH makes micronutrients, such as Fe, unavailable to the plant, even if adequate levels are provided in the fertilizer.
We did not observe symptomology of interveinal chlorosis in cannabis plants grown at pH 7.0 in our initial experiments at NCSU, either. Iron uptake can also be hindered by over-irrigation, cold substrate temperatures or root disease such as Pythium. In other experiments at NCSU, we did observe the development of interveinal chlorosis on plants with a pH of 7.8. This observation provides more refinement of the upper pH limit and would suggest that the pH should be lower than 7.5.
The preliminary results of NCSU’s research imply that vegetative stock plants of cannabis have a wide substrate pH range in which the plants will optimally grow. That range appears to be as wide as pH 5.0 to 7.0. Based on experience with other species, a narrower range of 5.5 to 6.5 may be more appropriate to target, as this will allow growers to more easily adjust as pH approaches the ends of the targeted range.
For growers, utilizing these pH values in a monitoring system such as the PourThru method implies that the safe pH zone to target would be a narrower 5.8 to 6.2. If pH drops below 5.8 or above 6.2, corrective procedures should begin to adjust the pH back into the 5.8 to 6.2 range.
(Note: These values are for plants grown in greenhouse conditions with a soilless substrate (growing medium). Optimal ranges for field-based growing conditions typically would move up the lower and the upper range by 0.5 pH units to 6.2 to 6.7—as field soils latch on to more nutrients than a soilless substrate such as peat or coir, and risk of nutrient lock-up is lessened.)
How to Correct pH
Again, when the pH drifts into unwanted territory, adjustments must be made. Below are standard corrective procedures used to modify substrate pH for plants grown in soilless substrates in greenhouses and adapted for cannabis.
1. Low Substrate pH Correction When Fe toxicity and Mn toxicity become a problem, raise substrate pH to the recommended pH range. Corrective procedures to raise low pH levels are listed on p. 52 (Table 1). Switching to an alkaline fertilizer when substrate pH is nearing the lower limit will help stabilize the pH.
To make small adjustments of roughly 0.5 pH units, mix 1 to 2 quarts of flowable lime per 100 gallons of water. If using an injector, avoid using higher concentrations of flowable lime as it will damage it. If higher rates are needed, then split your applications to avoid damaging your injector. To avoid leaf burn, it is best to rinse the foliage after treatment if any flowable lime comes in contact with the leaves.
For more stubborn low-pH problems, use a hydrated lime mixture. Mix 1 pound of hydrated lime in 3 to 5 gallons of warm water. Mix it twice and let it settle after each mixing. Once the sediment collects in the bottom of the container, pour the liquid through your injector system set at a 1:15 ratio. This product is corrosive, so rinse the foliage as soon as possible to avoid leaf burn, and avoid skin contact.
Potassium Bicarbonate (KHCO3)
Potassium bicarbonate should be handled with care. It can throw off your substrate chemistry as it provides 993 ppm of potassium (K) in mixtures of 2 pounds per 100 gallons of water. Rinse the foliage immediately after application via injectors and leach heavily the following day with a complete fertilizer to reduce substrate electrical conductivity (EC) and restore nutrient balance. The 2-pounds rate will increase the substrate pH by roughly 0.8 pH units. Rates greater than 2 pounds per 100 gallons of water can cause phytotoxicity.
As always, remember to recheck your substrate pH to determine if reapplications are needed.
2. High Substrate pH Correction The target pH for cannabis is between 5.8 and 6.2. Higher pH values may result in Fe deficiency and create interveinal chlorosis on upper leaves. Check the substrate pH to determine if it is too high. Be careful when lowering the substrate pH, as low pH can be more problematic and difficult to manage.
Cannabis plant growth is less vigorous at low substrate pH levels.
If the substrate pH is beginning to increase, consider switching to an acidic-based fertilizer. These ammoniacal nitrogen (N)-based fertilizers are naturally acidic, and plant nitrogen uptake will help moderate the substrate pH over a week or two.
Acid Water Drench
Some growers use this intermediate correction when pH levels are not excessively high and a quick lowering of substrate pH is desired. Sulfuric acid is recommended to acidify irrigation water to a pH 4.0 to 4.5. Apply this acid water as a substrate drench, providing 5 percent to 10 percent excessive leaching of the substrate. Rinse the foliage to avoid phytotoxicity. Results should be visible within five days. Retest the substrate pH and repeat if needed.
If the levels are excessively high, then a Fe chelate application can be made to the substrate.
- Iron-EDDHA can be mixed at a ratio of 5 ounces per 100 gallons of water.
- Iron-DTPA can be mixed at a ratio of 5 ounces per 100 gallons of water.
- Iron sulfate can be mixed at a ratio of 4 ounces to 8 ounces per 100 gallons of water.
Apply the iron as a substrate drench with enough volume to leach the pot, and rinse the foliage immediately after application.
Based on grower observations and initial experiments at NCSU, it is possible to grow cannabis with a wider soilless substrate pH range than most other species. Cannabis plants do not appear prone to develop leaf symptomology when substrate pH is too low or too high compared to the current general greenhouse standards—only plant stunting occurs at sub-optimal conditions.
Therefore, based on research and experience with other species, a wider range of 5.5 to 6.5 may be used. When adapting these values to a monitoring system, the recommended pH zone to target would be 5.8 to 6.2. By monitoring the substrate pH over time, one can assure that plants are within the optimal range.
Brian Whipker, Turner Smith and Paul Cockson are from Department of Horticultural Science, North Carolina State University, Raleigh, N.C. Hunter Landis is from North Carolina Dept. of Agriculture and Consumer Services, Raleigh, N.C.
Care and Caution When Germinating Cannabis Seeds
For growers looking to produce top-shelf cannabis, it all starts with the seeds. Rich Hamilton explains there’s far more involved than simply burying a seed in a pot of dirt.
There are many to consider when germinating cannabis seeds — not only the germination method you choose but also the environmental conditions your seeds require to thrive. These conditions include temperature, light exposure, moisture, and oxygen levels. Get any of these wrong and it could spell the end for your grow before it even begins.
Cannabis seeds can be very delicate and temperamental even when germinating in the perfect conditions. You can understand why you should approach this task with best-practice knowledge and caution.
To try and give your seeds the best chance, we are going to look at some tips and methods to help you get better results, first time, from your germination efforts.
Firstly, what is germination? Well, germinate means “to bring to life.” Germination is the first part of a plant’s life, the growth or shooting of a seed into a seedling and then hopefully a plant.
Cannabis seeds germinate via epigeal germination which sees the cotyledons (seed leaves) pushed upward and out of the soil as the plant begins to grow. For germination to occur, several natural elements and conditions must be present in the correct ratios. Let’s have a look at them.
Water — This is needed to start the germination process as the seed is dry, holding only five percent water and needs to absorb a lot more to begin the germination process. Seeds store starch, oils, and proteins and when germination begins, hydrolytic enzymes are activated, releasing the stored supplies and giving the plant just enough of the right chemicals to get it started. Water is also needed to expand the seed’s shell and soften it, allowing it to split for the beginnings of the plant to emerge. The hardness of a cannabis seed shell can vary quite a lot. If your seed is ripe and mature enough for germination, then you shouldn’t have any problems. A more immature cannabis seed, however, will have a more rigid shell and may fail to germinate.
Oxygen — Seeds need to breathe, but without oxygen the seed may drown or suffocate. Overwatering can suffocate your cannabis seeds preventing oxygen from getting to them, so keep them moist but not soaked. Don’t overwater your seeds — use a spray mist rather than pouring water directly onto them.
Temperature — Ideally, your cannabis seeds will germinate best at their “sweet spot” between 70-75°F. If you germinate a cannabis seed outside this range, then it can negatively affect the plant’s growth and health in the future. When temperatures are too cold, it can ultimately stunt germination so keep seeds away from any drafts, open windows, or fans.
Light conditions — Cannabis seeds germinate best in dark conditions. During germination, your cannabis seed is working to develop its first root (the radicle), and roots (unsurprisingly as they exist underground) do not like light! It is therefore vital to keep your cannabis seeds somewhere dark until transplant. But as soon as germination occurs and your new seedling develops leaves, it will need to be exposed to light to start the vital task of photosynthesis. Now that we understand what your cannabis seeds need, let’s explore the most popular methods used to germinate them.
The Glass of Water Germination Method
You can leave your cannabis seeds in a glass of mineral water in a dark environment until the radicle/tip of the root shows.
Depending on the seed strain, you should generally see something happening within one to five days. Why mineral water? Well, depending on where you live, your tap water can vary immensely in terms of chemical composition and pH. The ideal pH for water when germinating is pH 7. Anything outside of this can have adverse effects on your seed. Using mineral water is a cheap and effective way of keeping a stable pH environment. If you use tap water, however, leave it to stand for 24 hours first. The tap water needs a chance to settle and dissolve any chemicals that are present and may be harmful to the seeds.
You can use an air stone and pump to oxygenate the water whilst it stands for 24 hours. By doing so, you will reduce the chance of the seeds drowning and helping to dissolve any unwanted chemicals at a faster rate.
The Paper Towel Germination Method
Using a paper towel to germinate your cannabis seeds is both reliable and straightforward. You should find that 90 percent or more of your cannabis seeds will germinate when following this method correctly.
Take a sheet of premium paper towel and fold it in half, then place the seeds to the left of center. If you are germinating more than one seed, then you can use the same piece of kitchen towel but do not let the cannabis seeds touch each other. Now fold the kitchen towel in half again so all the seeds are covered up and then saturate it with water.
Mineral water is excellent to use, or if you are going to use tap water, then follow the recommended process for the glass of water method as previously discussed. Now leave the kitchen towel on a plate in a dark place as the roots will hopefully begin to develop soon and they do not like the light.
Humidity levels are equally important and keeping the seeds in the paper towel will keep the humidity levels where they should be. Keep the kitchen towel moist at all times and do not let it dry out. If you have been successful, it should take about two to four days for you to see until the radicle begins to emerge.
It is best practice to place the moist kitchen towels between two clean plates. Doing so helps to stabilize temperature and humidity and block out any light. The plates should be face-to-face, with the germination towel in between. Once a cannabis seed starts showing signs of successfully germinating, you can easily remove them whilst leaving the others, which may not yet have germinated, undisturbed.
Direct Germination Method
Germinating “directly” is where you place the cannabis seed directly within a pellet or block of a medium of your choosing. You can germinate your seed within a soil or coco pellet, however, I would recommend choosing a one-inch rooting sponge or stonewool cube.
You can use a soil pellet, but the risk here is that soil already contains a certain level of minerals and elements that could cause an imbalance in your seed, affecting its development later on in life.
For seeds in their natural environment, there can be double, triple, or a hundred times more plants growing in the soil at any one time. From those plants, thousands of seeds are dropped for germination and of those thousands, only half may germinate, even under natural conditions.
We are trying to grow our seeds under controlled conditions and in much smaller numbers. The margin for error, therefore, is much smaller. Using a rooting sponge is an excellent alternative for germination as it eliminates any risk of deficiencies or toxicities you may otherwise experience when using a natural medium such as soil at this early stage.
The direct method of germination can be hard to control as you must wait until the seedling first shows itself out of the chosen medium to know whether it has been successful or not.
As the cannabis seed is buried within its medium, you cannot check on it during the process to see if it has begun the germination process. As a result, this process can take much longer than the methods already discussed.
If you are germinating “directly” then firstly you should moisten your medium by misting it with a spray bottle. The cannabis seed will then need to be carefully inserted into the chosen medium and be surrounded by its encasing environment to keep any light out.
Now that you have chosen your method and you have successfully germinated some seeds. What now? How do you know when you should transplant? If everything goes according to plan with your germination efforts, you will see the radicle/root tip of the cannabis seed breaking through the shell. This new root is the first sign germination has occurred. It is best to transplant each seedling when the radicle is around three times the length of the seed.
A rooting sponge is a perfect choice for transplanting as it is compatible with whatever medium or system you use later. Rooting sponges are also an excellent choice for those who are looking to sell their seedlings.
Rooting sponges and stone wool blocks make for more robust root systems. It can take longer for the roots to break through, but when they do, they will be stronger.
When you are transplanting the seed, it must be placed radicle first into its new home, quickly, gently, and precisely as it only has a minimal amount of energy to survive and develop into a seedling. Once your seeds are transplanted, they should go into a propagator, ready for the start of the growth phase of their lives. Your cannabis seed holds all the potential to give you the vibrant, robust, healthy plants, and bountiful yields you desire.
Taking the time and care to get your seeds off to a good start should be a no-brainer. Knowledge is power if you want to be a better grower, so think of the learning process just like a seed itself. Nurture it, cultivate it, and you will soon harvest success.