Spacing is a decision about the tree in thirty years
The spacing that looks right on planting day is always too tight, because a two-year whip is a stick and a mature tree is a room. The only useful input is mature spread, and the discipline is to lay out to that number and then live with a plantation that looks bare for a decade.
The base rule is simple. Two trees whose crowns just touch at maturity need the average of their two mature spreads between their trunks — each tree needs half of its own spread, and the two halves add up. A pair of 30 foot trees goes 30 feet apart. A 30 foot tree next to a 50 foot tree goes 40 feet apart, not 50 and not 30. Getting that average right is most of the job when you are mixing species.
From there the style factor is a design choice rather than a rule. Open-grown at 1.3 times the spread gives every tree light on all sides and a full, low-branched form. Touching at 1.0 gives a continuous canopy with each tree still recognisable. Closed at 0.75 makes the group behave as a woodland, drawing each other up with clean straight stems and little low branching, which is what forestry planting deliberately does. A screen at around half the spread produces solid cover fast and commits you to thinning later.
Square or triangular
A square grid is easy to lay out with a tape and a couple of stakes and easy to mow between. A triangular or offset grid — every other row shifted half a space, with rows placed 0.866 of the spacing apart — fits about 15 percent more trees into the same ground at the same trunk-to-trunk distance, because it is the denser way to pack circles. On an acre at 20 foot spacing that is 109 trees square against 126 triangular.
| Spacing | Square, per acre | Triangular, per acre | Typical use |
|---|---|---|---|
| 8 ft | 681 | 786 | Screens, short-rotation, hedgerow |
| 12 ft | 303 | 349 | Dense forestry planting, small fruit |
| 20 ft | 109 | 126 | Standard orchard, closed woodland |
| 30 ft | 48 | 56 | Large shade trees, avenue planting |
| 40 ft | 27 | 31 | Specimen trees, parkland |
The counts fall with the square of the spacing, which is the number that catches people out on a budget. Doubling the spacing does not halve the tree count, it quarters it. Laying the grid out square on the ground is a right-angle problem, which the site layout squaring calculator solves with a tape and no instrument.
The distances people actually get wrong
Spacing between trees is the easy half. The distances that cause trouble later are the ones to things that are not trees.
Buildings. The naive setback is half the mature spread, so the canopy edge stops at the wall. That keeps branches off the render and out of the gutters, and it is a floor rather than a target. Roots do not respect it — they extend well past the canopy — and on shrinkable clay soils the interaction between tree roots, soil moisture and shallow foundations is a real and locally specific engineering question that a spacing rule does not answer. If the building is old, the soil is clay, or the tree is large and thirsty, that is a conversation with someone qualified rather than a number off a page.
Drives, paths and paving. Surfaces lift long before canopies are a problem, because roots thicken under the slab where the soil is moist and uncompacted. Half the mature spread is again a starting point, and more is better. If the paving is already there and the tree is going in beside it, root barriers and structural soil exist and are worth asking about before planting rather than after the first crack.
Anything overhead. A tree planted under a power line is a tree condemned to be cut into an awkward shape forever, and the cutting is not work you may do. Utility clearance is restricted to trained line-clearance crews almost everywhere, and the reason is that electricity will cross a gap to a wet branch or a metal ladder without contact. Ask the utility what is permitted under their line, and take the answer as the constraint.
Underground services. Get utilities located before digging a planting hole, every time, including on your own land. Water, gas, telecoms and electrical services run shallower than people assume and often not where the map says.
The other constraints on the layout
Trees on or near a boundary are a legal question before they are a horticultural one. Who owns a trunk that straddles the line, what you may do to branches or roots that cross it, and what you owe a neighbour if your cutting kills their tree are all answered by local rules and, sometimes, by where the line actually runs rather than where the fence does. Cutting a neighbour's roots or branches has consequences that can run well past the cost of the tree. If the line matters, get a survey and read the local rules before anyone starts.
Beyond the boundary, check what the site allows before ordering stock. Street trees, conservation areas, subdivision covenants and species restrictions all exist and vary between jurisdictions, and in some places planting is regulated as tightly as removal. This page cannot tell you what applies where you are.
Once the spacing is settled, the next questions are the ones that decide whether the trees survive the first few years, which is watering — covered in the tree watering and establishment calculator — and, if anything is being built nearby, the root protection zone calculator. For the bed preparation around each planting spot, the mulch calculator and the compost amendment calculator cover materials, and the fence post calculator handles the stock fencing if this is a plantation that needs protecting from browsing.
Questions people ask
How far apart should I plant trees for a privacy screen?
Around half to sixty percent of the mature spread, which is deliberately tighter than the crowns want and produces solid cover in a reasonable number of years. The trade you are making is explicit: the screen closes faster and the individual trees never develop full form, they compete for light and water, and at some point the row needs thinning or it thins itself by losing the weaker trees. A staggered double row at wider spacing gives denser cover than a single tight row and ages better, at the cost of more ground and more trees. Whichever you pick, plan the thinning now.
How far from the house should a tree go?
Half the mature spread is the minimum that keeps branches off the building, and it is a floor rather than a recommendation. It says nothing about roots, which extend well beyond the canopy, and nothing about the specific interaction between a large tree, a shrinkable clay soil and a shallow foundation, which is a genuine engineering matter with locally specific guidance. For a small ornamental beside a modern slab it is a non-question. For a large forest species near an old house on clay it is a question for a structural engineer or an arboriculturist who knows the local soils, and a spacing calculator is not the right tool.
What is triangular spacing and is it worth the trouble?
Every other row is offset by half a space and the rows are moved closer together by a factor of 0.866, so each tree sits at the centre of a hexagon of six neighbours rather than a square of four. It packs about 15 percent more trees into the same ground at the same trunk-to-trunk distance, and it gives more uniform light and root competition. It is worth it on a plantation where density matters and the layout gets pegged out once. On a small garden with five trees it is arithmetic for its own sake, and on ground you have to mow in straight lines it is a nuisance.
Can I plant closer and thin later?
Yes, and forestry does exactly that on purpose — dense early planting draws the stems up straight and clean, and successive thinnings select the best. It works if the thinning actually happens. The failure mode is planting at forestry density with garden intentions, never thinning, and ending up in year twenty with a stand of drawn-up, weak-stemmed trees that all need work at once. If you plant close, write down which trees are the keepers before you plant them, because in fifteen years you will not remember and every tree will look like it deserves to stay.
The nursery tag says 15 feet wide. Should I trust it?
Treat it as a lower bound rather than a specification. Tags often describe size at ten years rather than at maturity, they usually describe an average site rather than a good one, and cultivar claims about compactness are marketing as much as measurement. A tree on deep moist soil with room and light routinely exceeds its tag. The safer approach is to look up the mature dimensions the species reaches in your region, ask what similar trees nearby actually look like at forty years, and lay out to that. Spacing errors on the generous side cost you a sparse decade; errors on the tight side cost you trees.