Planning fundamentals
What does "on center" plant spacing mean?
The measurement runs between the points plants grow from — not between their edges, and not between their nursery pots.
The short answer
If a plant tag says "space 24 inches on center," it means the center of one plant should be approximately 24 inches from the center of the next. Not 24 inches between the edges of the plants. Not 24 inches between nursery pots. Center to center.
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Why spacing is measured this way
Plants change size.
A one-gallon perennial and a five-gallon perennial may look dramatically different at the garden center even though they are the same variety.
Their centers, however, remain fixed.
Measuring from the center gives you a consistent way to position plants based on how large they are expected to become.
An example
Suppose a perennial matures to approximately 24 inches wide.
If you plant two of them 24 inches on center, each plant extends roughly 12 inches toward the other.
At maturity, their canopies should approximately meet.
If you measured 24 inches from the outside edge of one nursery pot to the next, the mature plants would end up much farther apart than intended.
How to measure on center in the garden
After deciding where the first plant goes:
- Mark its center.
- Measure the specified distance from that mark.
- Mark the center of the next plant.
- Dig each hole around its center mark.
Landscape contractors often mark planting centers with flags, paint or stakes before digging anything.
This allows the whole composition to be adjusted while changes are still easy.
What about triangular spacing?
Plants do not have to be arranged in straight rows.
For groundcovers and mass plantings, staggered or triangular spacing often produces more even coverage.
Instead of aligned rows:
you offset each row:
The spacing is still measured from center to center.
The only difference is the geometry of the arrangement.
What if two plants have different mature widths?
“On center” becomes slightly more nuanced in mixed beds.
Suppose one shrub matures to four feet wide and the neighboring perennial matures to two feet wide.
Their radii are two feet and one foot.
To allow their mature canopies to meet, their centers would be roughly three feet apart.
So while nursery instructions often give one standard on-center spacing for a mass of identical plants, mixed garden design works better when you think about the mature radius of each plant.
On-center spacing is a planning tool
The phrase sounds technical, but the concept is simple:
You are positioning where plants grow from, not measuring the empty space between them.
Once you understand that, plant spacing becomes much easier to draw, measure and adjust.
See how far apart should plants be for choosing between close, natural and open spacing.
Related principles
Using Klora
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The product guides for the tasks this article describes.