Why won't grass grow under my tree?

Three things are happening under a tree at once — no light, no water and no nutrients — and only one of them is fixable. What to try, and when to stop trying.

Difficulty: beginner
Time: 45 min
Read: 5 min

Short answer

The tree takes the light, the rain and the nutrients, in that order of importance. Raise the mowing height, thin the canopy if you lawfully can, and use a shade mixture. If the area gets under about three hours of direct sun, no grass will hold and a mulched bed or shade planting is the honest answer.

Under a tree is the hardest place in a garden to grow grass, and people usually blame only the shade. In fact a mature tree's fine roots occupy the whole of the topsoil in that zone and take the water first, the canopy sheds rain sideways so the ground beneath gets less than the open lawn even in wet weather, and grass with too little light cannot make enough energy to compete for what is left. Fixing one of the three does not fix the problem.

What you'll need

Shade-tolerant grass seed
Sandy loam or leaf mould (optional)
Bark mulch (optional)

Step by step

Measure the light before doing anything.Check the area at 9am, 1pm and 5pm across a summer week. Over four hours of direct sun and grass has a chance. Under three, it does not, and everything else you do is postponing that conclusion.
Check whether the tree can lawfully be pruned.Before touching a canopy, check with the council whether the tree has a Tree Preservation Order or stands in a Conservation Area, and check for planning conditions on newer estates. Doing protected tree work without consent carries serious fines. If it is a neighbour's tree, the rules are different again.
Consider crown lifting or thinning by an arborist.Removing the lowest branches — crown lifting — lets low morning and evening sun under the canopy and often makes more difference than thinning. Both are arborist jobs on any tree of size, not ladder-and-bowsaw jobs.
Raise the mowing height dramatically.45 to 50mm, not 30mm. Shaded grass has less light to work with, so it needs every millimetre of leaf it has. This one change fixes more shaded lawns than any product.
Sow a fescue-based shade mixture in autumn.Early September, so seedlings establish in autumn light and moisture, before the tree comes into full leaf and takes it all. Spring sowing under a deciduous tree is a race the grass loses in May.
Add organic matter, but only a little at a time.A thin dressing of leaf mould or sandy loam raked in improves the moisture-holding of an exhausted root zone. Never add depth of soil over a tree's roots — more than 50mm or so of soil over a root plate suffocates roots and is a genuine way to kill a mature tree.
Water the area separately, and generously.It is competing with a tree that will always win a shallow watering. Deep, occasional soaking in dry spells rather than a light sprinkle.
Clear leaf fall promptly in autumn.Grass under a tree is already short of light. Three weeks under a mat of wet leaves finishes it.
Know when to give up gracefully.Under a mature beech, conifer, or anything with a dense evergreen canopy, grass will not work. A mulched circle with shade planting — hardy geraniums, epimedium, ferns, wood spurge, spring bulbs — is easier, looks deliberate, and stops the annual reseeding.

Tips

Deciduous is easier than evergreen. Grass under a birch or an ash can work; grass under a leylandii or a mature holm oak will not.
The bare ring right at the base of a trunk is usually mower damage and root flare, not shade. Mulch it and stop mowing up to the bark, which also protects the tree.
Never strim around a tree trunk. Repeated strimmer damage to the bark at ground level girdles and kills trees, and it is one of the commonest ways garden trees die.

Common mistakes

Reseeding the same spot every spring — Sowing into deepening shade in April guarantees failure. If it must be tried, sow in early September into autumn light and moisture.
Spreading topsoil over the tree's roots to make a seedbed — Tree roots are shallow and need air. A layer of soil over the root plate can suffocate and kill a mature tree, which is a very expensive way to fix a lawn.
Cutting the tree back without checking for protections — A Tree Preservation Order or Conservation Area status makes unauthorised work an offence with substantial penalties. The check is a phone call or a look at the council's online map.

If it doesn't work

Grass thin but present in spring, gone by July

Cause: Deciduous canopy closing plus root competition for water — Fix: Raise the mowing height, water deeply in dry spells, and sow in autumn rather than spring.

Bare soil year round under an evergreen

Cause: Permanent deep shade and dry root zone — Fix: Stop trying to grow grass. Mulch and plant shade-tolerant perennials, or gravel it.

Moss instead of grass under the tree

Cause: Low light and damp — Fix: This is moss telling you the truth. Either accept the moss, which is fine to look at, or change the planting.

Bare ring at the trunk base

Cause: Mower and strimmer damage, plus root flare — Fix: Mulch a circle around the trunk, keeping the mulch off the bark itself, and never mow or strim to the trunk.

Questions people ask

Is there a grass that grows in deep shade?

No. Shade mixtures based on fine and hard fescues tolerate more shade than ryegrass, but every lawn grass needs a few hours of direct light. Below about three hours, tolerance runs out.

Can I just cut the tree down?

Check first — a Tree Preservation Order, Conservation Area status or a planning condition can make felling an offence, and a tree of any size is arborist work regardless. Ask the council before you commit to the idea.

What grows there instead?

Dry shade planting: epimedium, hardy geraniums, euphorbia amygdaloides var. robbiae, ferns, hellebores, spring bulbs, and a bark or leaf mould mulch. It looks intentional and needs a fraction of the effort.