Classroom Window Box Plant Density Experiment

This project sets up a classroom experiment allowing for hands on activities growing plants to observe planting densities/plant spacing and it’s effect on yields over a number of species.  This project has a number possible mathematics, statistics, and geometry problems as well as potential for volume and measuring related activities.  The number of variables that could change the growth across boxes of each type of plant should be reduced by making water, light and nutrient amounts equal or rather not lacking in any way in comparison to any other box so that the only random variable will be the spacing between plants.

Window boxes are created from 14 gallon totes.  Holes are drilled at about one inch from the base regularly around the outside ever 3-4 inches in a regular pattern.  The totes are filled with approximately 1-2 inches of wet perlite in the bottom and then filled to approximately 10 gallons of soil mix containing no weed seeds.  Each bag of perlite holds approximately 30 gallons we need approximately 50 gallons as about 10 gallons are spread throughout the bottom of the totes for aeration and 4o gallons are used to create a soil mix.  In this case we used a 2:1 mixture of worm castings to perlite for the soil mix.  The worm castings contained approximately 300 ppm of nitrogen(N), 150 ppm of phosphorus(P) and 75 ppm of potassium(K), the relative fertility of the worm castings are 4-2-1 NPK and with the addition of the perlite the mixture becomes approximately 1.33-0.66-0.33 NPK. which should be sufficient for smaller leafy crops within a 40-60 day growing period. During the mixing is a good time to discuss plant nutrient needs. What does a plant need, water, nutrients, air, carbon dioxide, warmth, light etc.

Materials list:

These are then planted with seeds at a 2 inch spaced planting and at a 4 inch spaced planting. Would be a good idea to discuss how the geometry of the planting might affect the experiment. We planted Sugar Daddy Peas, Royal Burgundy Bush beans, lettuce, spinach, and turnips across the 8 totes.  With the four spacing only 15 holes are possible at this spacing so the center hole is not used if you are splitting the planting so that there are an even number of plants in the planter as at this spacing there should be no limiting factors due to space between plants anyway.  It is possible to get 40 seeds per planter in the 2 inch spaced seeds and this is typically below the density recommended for square foot gardening and should produce some stress on the plants for light and possibly for nutrients depending on the seed type and variety.

Depending on Humidity and temperature the soil is to be watered enough to keep it moist without drying out and will require checking and likely daily watering.  The squeeze test is the standard for water retention.  which is to take a small amount of soil after watering you should be able to squeeze out moisture but the soil should not be able to drip.

The plants are to be checked for germination over the next two weeks and record when germination starts and when either all seeds have germinated which for most seeds will be when the majority of seeds have germinated and an additional  3-6 days have passed with no additional germinations.

Notes should be recorded on plant appearance each week.   Color, size and notable characteristics of plants should be recorded as well as differences between the same types of plants in each of the boxes with differing spacing.  The general quality and health of the plants are to be noted as well as the edible appeal of the plants as they reach a harvestable state which should take between 40 and 60 days depending again on type and variety.  Throughout this process it is good to draw speculation as to what the results might be and frame them as testable hypotheses.

Upon the decision to harvest plants the edible plant material should be cut and weighed or measured as appropriate to the type and variety.  A discussion of what to measure that would be representative of the plant should be discussed;  i.e. should leaf area be measure, just leaf length and or leaf width or just harvested material per plant weight measurement.  The total weight, average weight,  maximum length , minimum length and mean length should be recorded for each species by the spacing it was planted under.  

A general discussion of what these numbers reflect in the real world and how we can use them to draw comparisons and possibly inferences about the effects of spacing on plant health vigor and  yield.   This would also be a good time to discuss the possible nutrition issues related to stressed plants.  How could this type of experiment be expanded and used to find the ideal spacing and geometry for maximizing plant yield. When discussing yield it is important to ask where did the carbon the plant used to build its edible portions come from and discuss how the air supplies this and how that could be a variable in this experiment.

Ray Bush,
PCF Farm Manager