A guide to plant growth: two basic rules

By Kerrie & Kurt Badertscher

Nutrients make everything possible, especially profit. They allow cultivators to produce crops with consistent yields on consistent schedules.

While nutrient science can seem daunting to some, there are only two lessons cultivators need to understand about the relationship between plants and their food. The first is: When nutrients are available, plants grow. When nutrients aren’t available, they don’t.

Now let’s apply that lesson to cannabis plants, specifically.

Nutrient management begins the moment the grower puts a mother plant into production. The nutrients in that mother’s leaves are used by cuttings to build new roots. Mother plant tissue (or petiole sap) analysis can provide an objective assessment of nutrient levels; but subjectively, if the mother looks healthy, the cuttings will likely be healthy, too. It is ultimately the propagation team lead’s responsibility to keep the moms healthy, so those lead propagators should use the method of analysis with which they are most comfortable.

The next decision to be made is which shoots should be taken for a cutting. Even the healthiest cutting won’t produce good roots if there isn’t enough leaf area. Trials of different cutting sizes, leaf counts and trimming techniques can quickly point to an ideal cutting that produces the most robust rooters.

Once roots appear, the next set of tests is to find the nutrient profile that produces the best root mass. We know that whatever else that profile includes, calcium and magnesium are the nutrients that experience suggests have the most influence over cutting health.

Nutrient x Time = Growth

Nutrient levels and time determine how much plant growth occurs. The road from cutting to flower is approximately an eight-week journey, during which the grower needs to grow plants of a certain size and yield potential. With time set at eight weeks, nutrients are the variables with which growers get to play.

Veg is often considered the “boring” part of cannabis cultivation, but it is a critical period where a lot must be accomplished in a short time. To see just how much nutrients control growth, all it takes is to feed a veg plant at half the normal veg strength, and another at twice the veg strength. After a week or two, there will be no doubt that a grower can dramatically control his or her plants’ growth.

All growers need to pay attention to nutrients and grow schedules, but outdoor growers need to pay special attention. The year-round grower has roughly four weeks to veg their plants, while the outdoor grower’s veg window is basically any time after Mother’s Day through mid-July.

To avoid needing a hedge trimmer, outdoor farmers should plant the crops in the field on a date that will provide ample time to grow the plants to the desired size. Then they can monitor their progress, adjusting nutrients as the season progresses, to reach the perfect size to enter flower, just as declining daylight triggers transition.

Photo: Adobe Stock

Finding the sweet spot

In traditional Horticulture 101 nutrient labs taught at the university level, plants are grown with various nutrient deficiencies to show students how those symptoms look. Without proper levels of all nutrients, plants do not perform. First and foremost, be sure nutrient solutions are delivering everything plants need (and maybe just a touch more).

Cultivators can spend the time and lab dollars searching for the exact parts per million (ppm) of every nutrient that gives them the best results, but we suggest that they first try turning the volume of their current nutrients up and down, while watching for signs of stress in the plants. This exercise defines the nutrient sweet spot, which may turn out to be a much wider range than many would expect.

Growth speedometers

Knowing how big we want plants to be for flowering is a matter of monitoring new plants against their ideal size after one, two, three and four weeks of veg and then transition. The grower can establish that ideal growth profile by measuring the plants over time and using that history to compare against new plant growth.

We suggest using a Growth Progress Index (GPI) to assess plant growth rate. In a GPI, the ideal plant size is given an index of 1. Plants lagging in height have a GPI less than 1 and likely need more nutrients in the next week to bring them up to size. An oversized plant’s GPI is above 1 and indicates growers can reduce nutrients for a while.

The grower can establish that ideal growth profile by measuring the plants over time and using that history to compare against new plant growth.

Another growth rate indicator is in the distance between two nodes on a stem or branch. Plants lay down nodes on a schedule, so the distance between them tells us how fast the plant is growing. (The greater the distance between nodes, the faster the plant growth.) Just like plant height, capturing a history of these internode lengths provides a road map that can be replicated by manipulating nutrient levels to achieve consistent plant growth every time.

Not a spectator sport

Outdoor cultivators are faced with more variation in growing conditions, so this matching technique — where growers compare the internode lengths of their current crops to the lengths on their predetermined road map — offers a simple tool that allows them to steer their nutrient levels to keep their plants on schedule.

While simple manipulation of nutrient levels allows plants to be grown fast or slow in veg, this is also the key to managing plants during transition. Many growers we work with refer to transition as the “stretch phase.” In reality, it is nothing more than the plants responding to the higher photosynthetically active radiation (PAR) levels of the flower room, which drive higher rates of photosynthesis and longer internodes. It’s not “stretch,” per se — it’s growth driven by nutrients and light levels. Outdoor growers can expect to see this same “stretch” when veg plants are planted under the sun.

Growers can withhold nutrients even to the point of giving no nutrients if plants become too big. By cutting the nutrient supply, growers will find they can essentially stop the plant from growing, a useful trick to know if veg plants start to get away from them. This process is similar to flushing: the use of pure water for irrigation, with no nutrients added.

Nutrients can also be used to control how fast plants bud. Seed potato growers have shared that they withhold nitrogen entering the flowering phase, and this stress results in faster flower set. Cultivators can test this technique on a select few of their plants by withholding nutrients in the first week or two of flower to see if transition proceeds any faster because of lower nitrogen levels. When this is done, however, the grower needs to be aware of how much shorter the plants will be if the “stretch” is eliminated. (Expedited flowering comes at the expense of plant height.) If growers like their current canopy heights for scouting and maintenance, they likely will want to adjust how big they grow the plants in veg after reducing stretch.

Once transitioned to flower from veg, the growing points on the plants are all flower buds. On examination, these are indeed small versions of the larger plant, and the same point about internode length in veg applies in flower. The nutrient levels given will influence whether the buds are loose and airy (larfy) with long internodes, or tight and dense with short internodes. Again, this is easy to demonstrate, and we encourage cultivators to give it a try.

Queen P and King K

Nitrogen is necessary for growth, but once the nitrogen level is set, research points at phosphorus (P) as having the most influence on plant growth rate according to the article “Control Plant Growth and Height for Potted Herbs,” published in e-GRO’s newsletter in May 2017. That fits with what growers already know about P and potassium (K) being the nutrients that deliver yield.

If growers can manipulate the levels of P and K relative to one another, they are only a few trials away from knowing exactly what relative levels of P and K deliver the biggest yields. This can be done easily with water-soluble fertilizers, while growers using pre-mixed products can modify the quantity of those products that supply P and K to achieve the same results.

Different product, different growing

Finally, as the industry becomes more focused on extraction, it is reasonable to ask if we should be growing plants differently than when growing smokable buds, and the answer is: Yep! For example, it may be (though not yet proven) that super dense buds are harder to process into homogenate for extraction. If so, growers may need to do the unthinkable and grow larfier buds to improve extraction results and nutrient level control. As the industry matures, we will all learn that we need to grow to the needs of downstream processing and not just to yield demands — and nutrients are a key to being able to meet those needs.

Finally, remember the opening lesson (no nutrients means no growth) and then ask if flushing the plants for two weeks is necessary when the boss or investor is demanding higher yields.

This brings us to our second lesson about understanding nutrient management: The “right” nutrients aren’t found in feed schedules intended to grow football-sized colas; they are the ones that fit the situation at hand.

Kerrie & Kurt are co-owners of Otoké Horticulture, LLC and authors of “Cannabis for Capitalists.” info@otokehort.com