Growing cannabis is part art, part science. In some ways, gardeners are very artistic; like a conductor in front of an orchestra, they guide their plants from start to harvest, listening for any off notes signaling something isn't right. On the more scientific side, there are ways to pick notes out of the right key for a more harmonious garden. The more critically important success is, the less room there is for artistic exploration, and more reliance on quantifiable results is called for, especially when tallying cannabis yields.

For a basic idea of success in the garden, one could simply measure how much was spent divided by how much was harvested. For example, if a gardener spent $100 towards materials, nutrients, and labor on an outdoor container plant on the back patio and it produced a little under a kilogram (kg), then each half kg cost $50 to produce. Even a calculation as simple as this can be useful when comparing operating costs between grows or to find a minimum required profit margin.

To improve the resolution of the overall picture, more exacting data can be added. Simply adding how many of which variety was grown can help. This allows for an average to be taken by adding up the total harvest for each variety and dividing by the number of each variety. Then the varieties can be compared against each other to determine which produced more.

If two Purple Lady plants produced a total of 170 grams (g), then they averaged 85g apiece (170g/two plants=85g).

This information can be used the following planting season when selecting how many seeds to plant. If the goal is to have half a kilogram of smoke at harvest, and your garden tends to produce 70g per plant, then that would be about four plants taken to harvest. If the average is a 160g yield per plant, then two to three plants should cover it.

While production numbers should still be mitigated with other factors such as quality, it can be useful when making judgment calls. A middle-producing variety with okay smoke should generally lose space to a high-producing great smoke. If the numbers show a high-quality variety doesn’t produce well, then an informed decision can be made. A commercial grower may decide to only grow that variety for their own use or to charge more to make up the difference if the market will bear it, or they may not grow that variety at all anymore in favor of better-producing strains.

Outdoor gardens that only produce a single harvest a year can be calculated by the year, but with light deprivation or indoor growing, seasons are a bit more arbitrary and at the whim of the grower. Indoor growing seasons may be as short as nine to 10 weeks or extend to several months depending on how long the plants are kept in growth, and how long the variety requires to flower.

Calculating Grams Per Day

To account for these differences in growing seasons, a time factor can be included in the calculations. Grams per day (GPD) allows for plants of different growing periods to be compared. Take the planting date and subtract the harvest date to find the number of days between. If plants were started on May 23, 2018 and harvested on Sept. 29, the season would be 130 days.

If the Purple Lady example were used, then its average weight of 85g per plant could be divided by the 130 days it took to grow them, to show a GPD of 0.653 (85g/ 130 days = 0.653).

If it instead didn't ripen until October 5, then the 85g would have taken 146 days. That would give a value of 0.582 GPD (85g/146 days = 0.582).

The GPD for the whole garden or each individual plant can be calculated and compared. Individual plants with high production rates are often particularly good candidates for breeding. Although production should not be the only factor in selecting parent plants—as long as there is not a reduction in quality—quantity is often considered a desirable trait.

To further examine and compare costs, each expense can be separated into individual categories such as rent, utilities, lighting, equipment, nutrients, etc. If these costs are calculated separately, then they too can be compared for improvements or lack thereof. For example, if an additional $100 in nutrient expenses improves production or quality by more than $100 in value, then it is generally worth it to continue the practice. Costs on durable goods (those that aren’t used up during the grow) can be spread out over their expected lifetime. If the lamps for the lights are used for four runs, then each run bears 25 percent of the cost of the lamps. If the lamps are changed every other grow, then each grow must support 50 percent of the cost.

Getting Value from Nutrients

A little math comes in handy when making nutrient selections as well. The NPK listing on nutrients indicates the percentage by weight of nitrogen (N), phosphorous (P) and potassium (K).

What this means is nutrient solution A, made with a fertilizer with a value of 5-0-0 that is applied at 15 milliliters (ml) per liter, should have the same nitrogen content as nutrient solution B with a value of 15-0-0 applied at 5ml per liter. It is the final amount of nitrogen in the nutrient solution that counts, not how concentrated the original nutrient was. If 15ml of nutrient A is 20 cents cheaper than 5ml of nutrient B, then, all other things being equal, it should be the better choice. When deciding which to use in the future, it will come down to whether nutrient B is worth an additional 20 cents per treated liter. To find out for sure, test a few plants using each nutrient. Record the nutrient expenses used to grow both sets and calculate the GPD at harvest. Not only should the more expensive nutrient produce more product, but enough above and beyond to cover the additional expense to be worthwhile.

Calculating Lighting Costs

The same can be calculated for electricity and lighting costs. An examination of your electric bill should tell you at what rate you are charged for electricity, and that can be used to calculate how much it costs per day to run lights.

To calculate lighting watts per square foot, calculate the square footage of the garden area. If 8x1,000W lights are used in a 10x 20-foot area, then 8,000W is used to cover 200 square feet. So, 8,000W divided by 200 square feet is 40W per square foot. A change in lights or an increase of wattage per square foot should also show a commensurate increase in production to be cost effective.

Once such numbers are recorded and calculated, then decisions can be made based on results. If a change in gardening techniques improves results, then consider keeping the change. If it doesn't, consider discarding it and returning to previous methods. Improvements to the garden should result in documentable improvements in production or quality to be cost-effective.

However, there is more to growing quality cannabis than just quantity and cost. There is also the matter of how good it is. For the scientific minded, there are lab tests that can report on tetrahydrocannabinol (THC) and cannabidiol (CBD) levels along with more information that can be had from such tests. While someday all aspects of quality may be possible to test for, the technology hasn’t yet gotten to the point of being able to replace a human tester for matters of taste, appearance, effect, and personal preference. A printout of the terpenes and flavonoids may be a fair place to get suggestions from, but it still takes a human to report on the human experience of it.

Although the finer points of rating a plant are largely artistic and subjective, responses don't have to be.

To set up a simple double-blind test, first, take identical opaque containers for each variety to be tested. Write the name of the variety on a piece of paper and seal in an unmarked envelope. Put the sealed envelope into the container with the matching cleaned weed. Have a friend mix up the containers while you are out of the room and can’t see them. When you return, mix up the containers again with your friend out of the room so neither of you have any idea which container has which cannabis.

Mark each container with an arbitrary number or a letter. While smoking from each, keep track of reactions from the weed. Questions should include things like “Which do you like the best: A, B, C, or Can't Tell?” and “Describe the flavor of each.” Record and tally the results from each participating tester. Since the envelopes contain the real identity of each container, after the tests have been completed, the envelopes can be opened in turn, and associated with their lettered responses.

Why go to the trouble of a blind (or in this case double-blind) test? Because humans are very suggestible, and some variety names sound better than others. Cannabis labeled “top shelf” or “mids” tends to influence the reactions of the people who smoke it. Relabeling the smoke into something innocuous helps minimize the influence. If practical, even the person handing out the weed shouldn't know what types are being sampled, as their behavior can have an influence as well.

Although it involves some basic math and effort, by quantifying the results of a harvest, a gardener can use the data collected to improve the efficiency and quality of their harvest. In this current time of reduced prices and lower profit margins, expense versus return is particularly important for commercial growers since it effects their bottom line. Home gardeners can be less efficient since they only have to beat retail pricing to come out ahead and don’t have to make a profit against wholesale pricing. Crunching the numbers may not be as fun as some aspects of growing cannabis, but it can be a useful tool.