How many plants do you actually need? The arithmetic nobody does

Most disappointing garden beds are not a plant problem or a soil problem. They are an arithmetic problem that nobody did.

Someone stands in a garden centre, looks at a bed they have in their head, and buys what looks about right. Sometimes that is forty plants for a space that wanted twenty-one, and the bed is a thicket in year three. More often it is twelve for a space that wanted twenty-one, and the bed looks thin and weedy for two seasons while everything fills in.

Both outcomes are avoidable with two pieces of arithmetic that take about a minute each.

How many plants fit in a bed

For plants set out on a square grid, the whole thing is one line:

plants = bed area in square feet ÷ (spacing in feet × spacing in feet)

The only trick is converting the spacing on the tag into feet before you square it.

Worked through: a bed twelve feet by four feet is 48 square feet. At 18 inch spacing, 18 inches is 1.5 feet, and 1.5 × 1.5 = 2.25. So 48 ÷ 2.25 = 21.3, which is 21 plants.

Not thirty-five, which is what most people would buy for that bed standing in the aisle. Not twelve, which is what most people buy when they look at the price of thirty-five.

Staggering the rows buys you about fifteen per cent

If you offset alternate rows so the plants sit in a triangular pattern rather than a square one, you fit roughly fifteen per cent more into the same area at the same spacing. Multiply the square-grid answer by about 1.15.

Our 21 plants becomes about 24. More usefully, it stops the bed reading like a car park, which a square grid always does for the first two years until the plants touch.

Use the spacing on the tag, not the spacing that looks right on planting day

This is the hard part, and it is not about arithmetic.

A correctly spaced bed looks sparse on the day you plant it. That is what correct looks like. The tag spacing is a statement about the mature width of the plant, and the plant is going to get there whether or not you left it room.

A bed that looks full and satisfying on planting day is a bed you will be pulling plants out of in year three, usually the ones that were doing best. Shrubs in particular are unforgiving about this: crowd them and you get leggy growth, poor air movement, and a hedge you did not intend to plant.

The honest way to handle the thin look in year one is annuals or mulch in the gaps, not more shrubs.

The second piece of arithmetic: how much soil, mulch or compost

This is where money actually gets wasted, in both directions — people order three yards for a job that wanted one, or make four trips with bags for a job that wanted a delivery.

The number worth memorising is 324.

A cubic yard is 27 cubic feet. Spread one inch deep, 27 cubic feet covers 324 square feet, because 27 cubic feet × 12 inches per foot gives 324 square-foot-inches. That is not a rule of thumb; it is a unit conversion, and it is published by Clemson Cooperative Extension's Home & Garden Information Center among many others.

So:

cubic yards = (square feet × depth in inches) ÷ 324

Worked through: 480 square feet of bed at three inches deep. 480 × 3 = 1,440. 1,440 ÷ 324 = 4.44 cubic yards. Order four and a half and you have a little spare for settling.

Bagged material is sold in cubic feet rather than yards, so multiply your cubic yards by 27 to get cubic feet, then divide by the bag size. Two cubic foot bags are common, which makes 4.44 yards about 120 cubic feet, or sixty bags. Working that out is usually what convinces people to take a bulk delivery instead.

One thing worth knowing before you amend heavy soil

While we are doing quantities: if you garden on clay, the amount of organic matter needed to actually change anything is larger than most people assume. Research-based guidance from NC State Extension for amending clay soils is to bring organic matter to roughly 25 per cent by volume in the amended zone, which in practice means incorporating around two inches of compost into the top six inches. An inch scattered on the surface is a mulch, not an amendment, and it does very little in the first season.

And the thing that gets repeated constantly and is wrong: do not add a bit of sand to clay. NC State Extension is explicit that you would need on the order of four to five times the existing soil volume in sand to change the texture, and anything short of that produces something closer to a hardpan than to loam. In a garden bed you cannot add that much. Add organic matter, or build up rather than dig in.

There is also a point at which the cheapest honest answer is a raised bed. If the ground holds water for days after rain, if the area sits at the bottom of a slope, or if you are working over compacted construction fill, amending six inches buys you very little. Standing water is a drainage question rather than a soil question, and I would not try to fix it with compost — see standing water after every rain for what is usually going on underneath.

Do the arithmetic on paper, before the garden centre

The reason to write it down rather than do it in your head in the aisle is that both formulas need the bed dimensions, and nobody remembers bed dimensions accurately. Measure once, draw it to scale on a grid, count the squares, and go shopping with a number.

I keep this in a Garden Layout and Planting Planner — six quarter-inch grid pages where you set the scale yourself, bed plan sheets with a count-needed column, both formulas worked out on a reference page, and the logs for sowing, watering and harvest. It is a fillable PDF and it is undated, because it keys off your own frost dates rather than a printed year.

Graph paper and a tape measure do the same job. What matters is doing the multiplication before you are standing in front of a trolley.

Who wrote this

Alex Purdy owns Carolina Terrain LLC in Waxhaw, North Carolina, and holds NC Landscape Contractor Licence CL.1872. He is not a Professional Engineer and not a Registered Landscape Architect. Plant selection and spacing depend on your own site and your own species; your state extension service is the right source for both, and this article does not replace it.

Sources

• The 324 square-foot-inch relationship (27 cubic feet per cubic yard × 12 inches per foot): Clemson Cooperative Extension, Home & Garden Information Center.

• Amending clay soils to roughly 25 per cent organic matter by volume, and the warning against adding sand: NC State Extension.