Gardening Math

Planting a garden requires some math. Finding the area of all the weirdly shaped garden beds can be an unwanted flash back to geometry. This might be why there are so many rectangular beds, to avoid the math. However, in my experience native plants don’t like straight lines. To calculate the bed area, I measure my beds as if they were a lot of smaller rectangles.  Height times width of every rectangle gives me their area. When I add them all up, I come up with a close enough measure of the total area.

After the area is found, then the number of plants needed still must be calculated. Different species have all different types of spacing requirements. I put my plant spacing as two-foot centers in an offset pattern that looks like a triangle between plants. Not all plants are two-foot centers but a lot of them are. If I’m planting a group of larger plants, like asters, I can calculate that bed separately to give them a larger spacing. With my plant spacing and bed area the calculator gives me the number of plants needed. I divide up this number by the number of species and find out how many I need to order.

The calculation of the bed area can also be used to figure out how much mulch is needed. I prefer to mulch lightly with 2 inches of mulch. Bed area times height of the mulch will tell you how much mulch you need. Converting from feet and inches into cubic yards can be done using conversion tables.

Next, we calculate watering. Tree and shrub watering in the first year is five gallons of water per inch of trunk, spread around the root zone once a week. Herbaceous plants should also be watered once a week to wet the soil to a depth of one inch. Calculate the amount of water by multiplying one inch by the bed area and then convert that to gallons. Or I usually just water and then check that the soil is wet to one inch down. If the soil is already wet from rain, then there is no need to provide supplementary water. After the first full year of watering, plants do not need to be watered regularly. In the summer and winter there may be times of drought and supplementary water on the garden will help plants stay healthy until the rains return.

If the new garden is a rain garden, you will want to calculate what size of basin you need to capture your roof water. For that you will need to know the area of the roof connected to the diverted downspout, and how much the average rainfall event is. The depth times the area of the rain basin needs to match the area of the roof. To find out how fast the ground can absorb all the water, do a percolation test by digging a hole in the garden plot, fill it with water and time how fast the water disappears.

To help with all these calculations I have added a few online calculators to this website. They can help to calculate bed area, mulch, plants, and water needs. As well as how big to make a raingarden.

Geometry, who knew it would be so important. Another math problem to solve is always how much garden can be made within a certain budget but that will have to wait for another day. However, a few of the calculations here will get us started on an estimate.