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Garden Math


Use these calculators to find your total garden bed area, how much mulch you need, how many plants to buy, and calculate watering. 
Rain garden size can be calculated finding the area of the roof and soil type to know how big to make the garden.

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. Length 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.


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. Deeper mulch is more effective at eliminating weeds but it can also prevent desirable plants from spreading and filling in the bed. If a bed is allowed to completely fill with native plants then you won't need to apply more mulch every year. 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.

Different species have all different types of spacing requirements. I put my plant spacing as 24 inch centers. 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.


One way to find your measurements is to go to the google maps webpage, zoom in on the address, switch to terrain view. Right click on the garden and choose measure distance.

Use the

Grow Native website

to look up how much space plants need.

Next, we calculate watering. Native gardens have a reputation for not needing to be watered. This can be true once a garden is mature with all plants situated in appropriate locations. Native gardens still need coddling during the establishment phase with regular watering until their roots have extended far enough to draw in the plant's moisture requirements. To encourage roots to expand water the entire root zone and not just the base of the plant. One inch of rain water each week helps natives thrive. Adjust this number based on the rainfall each week. If you've gotten 1″ of rain, you don't need to water at all. 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.

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. If the water drains away quickly, you have sandy soil. If the water takes several days to absorb you have clay soils and will need a larger basin.

If the new garden is a rain garden, you will want to calculate what size of basin you need to capture your roof water. The depth times the area of the rain basin needs to match the area of the roof. 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.

In St Louis we average about 3 inches of rain per month. But in March through May we average over 10 inches or rain so you may want to dig a little deeper for your rain garden.

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