Touching is not covering
Plants are usually spaced as if their mature width were a circle and the job were to make the circles meet. Make them meet and look at the ground: circles packed on a square grid cover pi over four of the area, which is 78.5 percent. Nearly a quarter of the soil is still bare, in the four-pointed gaps between neighbours.
Offset rows do better because the packing is tighter — the cell is 0.866 times the square one — but the same effect is there. To actually cover the soil, the canopy has to grow past the spacing, by a factor of 1.128 on a square grid and 1.050 on offset rows. On a 3.6 ft spacing that is 4.06 ft and 3.78 ft of width respectively, and at half a foot of spread a year those extra inches are another season of open ground.
This is why beds that look finished still need mulch, and why the second and third year top-ups are the ones people forget to budget for.
The trade the spacing percentage is really making
Spacing enters the plant count as a square, so it moves fast. Tightening from 90 to 70 percent of mature width raises the count by 65 percent. Loosening from 90 to 100 percent cuts it by a fifth. Go past 100 percent and the plants never meet at all, because the spacing is then wider than the plant ever gets.
Closure time moves differently, and not in proportion, because what the canopy has to cover is the gap between its width at planting and the spacing. On the numbers the form starts with — a 4 ft mature width, one foot out of the pot, half a foot of spread a year — the years come out like this.
| Spacing as % of mature width | Spacing | Plants in 600 sq ft | Years to touch |
|---|---|---|---|
| 70% | 2.8 ft | 89 | 3.0 years |
| 80% | 3.2 ft | 68 | 3.7 years |
| 90% | 3.6 ft | 54 | 4.3 years |
| 100% | 4.0 ft | 44 | 5.0 years |
| 110% | 4.4 ft | 36 | never |
Which side wins is a price question and it is not always the same answer. Where plants are cheap and mulch is delivered by the yard at a distance, tight spacing pays. Where plants are expensive specimens and mulch is a Saturday with a trailer, wide spacing pays. The cost lines in the calculator are there so the comparison is made with your own two prices rather than with a rule of thumb.
The spread rate is the weakest input
Everything about the timing rests on a figure for how much width a plant gains in a year, and that figure is not a property of the species alone. Water, soil, exposure, root disturbance at planting and the first winter all move it, and the same plant in two parts of one garden will not match.
Treat it as a bracket rather than a number. Run the calculation at half your estimate and at your estimate, and look at what the difference does to the mulch total. If the answer between the two is a couple of cubic yards, the uncertainty does not matter. If it is ten, the mulch budget is really a bet on growth rate, and it is better to know that.
Why the count is only a starting point
Dividing area by cell size assumes the bed is large relative to the plant and that edges do not exist. Real beds have edges everywhere, and a bed six feet wide planted at three foot spacing is almost all edge. At the boundary you are choosing between a plant that will overhang a path and a gap that will need covering permanently, and neither shows up in the division.
The other thing the count ignores is that beds are rarely one species. A mixed planting has several mature widths and several growth rates in the same ground, and the honest approach is to run each group over the area it actually occupies rather than averaging them. The zone-by-zone approach in the hydrozone water budget calculator is the same idea applied to water.
Questions people ask
Should I space at the mature width or tighter?
Both are defensible and the calculator will price either. Spacing at the full mature width gives each plant the room it will eventually want and leaves the ground open for a long time. Spacing tighter closes faster, costs more plants, and accepts that the planting will be crowded at maturity and may need thinning. The number that usually decides it is not horticultural: it is what a plant costs against what a cubic yard of mulch costs, applied over however many years the gap stays open, which is exactly what the cost section works out.
Why does the calculator say the ground is only 79 percent covered when the plants are touching?
Because circles do not tile. Pack circles on a square grid so that each one touches its four neighbours and they cover pi over four of the plane, which is 78.5 percent; the rest is the concave gaps at the corners. On offset rows the same packing covers about 90.7 percent at touch. Neither arrangement reaches full cover until the canopies grow past each other and overlap, which is a genuinely later date than the one the bed starts looking full.
How do I handle a bed with several different plants in it?
Run the calculation once per group over the square footage that group actually occupies, then add the results. Averaging mature widths across a mixed planting gives a spacing that is wrong for everything in it, and the closure date it produces is not the date any part of the bed closes. If the groups are interplanted rather than in blocks, the ground cover is set by whichever grows fastest and the water demand by whichever wants most, which are usually not the same plant.
Does the mulch top-up have to go on every year?
The schedule assumes it does, because organic mulch breaks down and the layer thins, but the depth and the interval are yours to set. Setting the top-up to zero removes the schedule and leaves only the mulch at planting. What the yearly rows are really showing is the shape of the cost rather than a prescription: it falls as the canopy closes, and the total is dominated by the first two or three years, which is when most people have already stopped budgeting for it.
Can I use this for lawn substitutes and low ground covers?
Yes, and the closure arithmetic is if anything more relevant there, because a ground cover is bought specifically to cover ground. The one thing to watch is that spreading ground covers often run rather than mound, so a mature width from a label can describe an indefinite spread rather than a stable one. Where that is the case the touching date is meaningful and the mature width is not really a limit, so read the never-covers branch of the result with that in mind.