On the scientific side, there are empirical ways to show how productive a garden was. The first and simplest bit of data to collect is simply what was grown. That’s because it’s helpful to know which varieties you’ve tried and which ones you like best when sitting down and making wish lists with seed catalogs.
(For example, I like the flavors of black krim and big rainbow tomatoes more than I do early girls, which is handy to know when I’m picking out seeds to plant next year.) When looking for new varieties to try, you can try those similar to known favorites or you can intentionally try something completely different from what you have in the past.
A somewhat more labor intensive record that should be made is how much was harvested in pounds. Note that plants with smaller harvests, such as saffron or lavender, might be easier to measure in smaller units like ounces.
Basic Gardening Math
To make the math easier to follow, the numbers used in the following examples are hypothetical. Let’s say two black krim tomato plants yielded 10 lb. of tomatoes, or three big rainbow tomato plants yielded 9 lbs of tomatoes. If individual plants aren’t tracked, calculating how much each plant averaged is simple: just divide the amount harvested by the number of plants.
If two black krims produced 10 lb., the average from each plant is 5 lb. (10 lb. ÷ 2 plants). Using this same calculation, each big rainbow plant produced on average 3 lb. This information can be used when selecting how many of which variety to plant for the next season.
If the goal is to have 30 lb. of tomatoes for the season evenly split between the two, the math would suggest planting three black krims and five big rainbow:
30 ÷ 2 varieties = 15 lb./variety.
15 lb. ÷ 5 lb./black krim = 3 plants
15 lb. ÷ 3 lb./big rainbow = 5 plants
I recommend planting a couple more of each as a safety net since extra harvest tends to be a smaller issue than not having enough. As your data set increases over the seasons, your estimates should improve as, over time, the good years and bad years will tend to cancel each other out.
Outdoor gardeners can easily compare growing seasons, spring and fall plantings, spring and fall harvests, and harvests year to year. Indoors, the seasons are a bit more arbitrary. Summer and fall are created with lighting timers and other environmental controls. A growing season can be as short as a few weeks, or extended to several months.
Indoor gardeners tend to refer to the number of days in each life stage, with the days spent in summer-growth variable dependent on the gardener and the number of days in the fall stage varying with the cultivar.
To account for these differences in season length, a time factor can be included into calculations in order to compare them. To do this, simply take the planting date and subtract the harvest date to find the number of days between.
For example, if our black krim tomato plants were started May 23, 2012 and harvested September 9, 2012, the season would be 110 days (nine days in May, 30 in June, 31 in July, 31 in August, and 9 in September).
Then, the average yield of 5 lb. per plant could be divided by the 110 days it took to grow them to show an average daily production rate of 0.045 lb. per day (5 lb. ÷ 110 days = 0.045 lb./day). If the big rainbow finished in 60 days, then the 3 lb. yield per plant would be divided by 60 to show a rate of 0.05 lb. per day.
Since the black krim produced 0.045 lb. per day and the big rainbow produced 0.05 lb. per day, the big rainbow is a better producer—a fact that might not be apparent before calculations. Even though the amount harvested per big rainbow plant is less, it took less time until harvest and the next set could be started sooner.
Individual plants with high production rates are particularly good candidates for saving seed. Although production should not be the only factor in selecting parent plants (preference and quality should also be taken into account), quantity is often considered a desirable trait.
Once the base rate of production is calculated, it can be used in conjunction with other pieces of data for a variety of useful purposes: nutrients, growing media, and other expenses can be taken into account.
For example, if nutrient A comes in a 1 gal. bottle and is applied at a rate of 1 fl. oz. per gallon, then there are 128-gallon treatments in the bottle. So, if that bottle costs $20, then each treated gallon costs just under 16¢ (128/$20.00). If nutrient B comes in a 4-gallon jug and is applied at 8 fl. oz. per gallon, then there are 64 treatments in a bottle. If the jug costs $40, then each treatment costs 62.5¢.
When deciding which of these to use in the future, it will come down to whether nutrient B is worth an additional 46.5¢ per treated gallon. To find out for sure, test a few plants using each nutrient.
Record the nutrient expenses used to grow both sets, and calculate the differences in production rates at harvest. In order to be worthwhile, the more expensive nutrient should produce enough to cover the additional expense.
The same can be calculated for electricity and lighting costs. Each additional 1,000 W light adds 1 kWh in electrical expense, not to mention the cost of the equipment and bulb replacements. If a change in gardening techniques is shown to improve, only then consider keeping the change instead of returning to previous methods. Improvements to the garden should result in documentable improvements in production or quality to be cost-effective.
While only the most avid gardening geeks will calculate every plant in the garden, this type of information can be valuable when comparing successes and determining the value of a change in the garden.
By comparing production rates before and after, improvement can be compared to cost and future plans fine-tuned accordingly. The benefits of homegrown vegetables include peace of mind and knowledge of the conditions that the food was grown under, but that doesn’t mean that they can’t be grown with a nod to efficiency and expense concerns as well.