Every autumn, garden centres sell bags of “shade lawn” seed to people standing beneath a big tree, staring at a patch of bare dirt and moss. Some of that seed does well. Much of it struggles through one season and gives up. The reason is not usually the seed itself but the combination of conditions under a mature canopy, which is far tougher than shade alone.
Here is a realistic look at what grass can and cannot do under trees, and what to grow instead when lawn is a losing battle.
Why grass fails under trees
Shade is the obvious problem, but it is only one of four. Under a dense evergreen such as a puriri, a magnolia or a large camellia, light levels at ground level can drop to a small fraction of open sky. Most lawn grasses need at least four hours of direct or bright dappled light a day to thrive.
The second problem is water. A tree canopy intercepts a surprising amount of rain, and the roots below take up most of what gets through. Soil under a big tree can be dust-dry in February while the open lawn a few metres away stays green.
Third is root competition for nutrients. Fertilising the lawn often feeds the tree more than the grass, because tree roots are concentrated in the top 30 centimetres of soil, exactly where turf roots live.
Finally there is leaf litter. Fallen leaves smother grass in autumn, and evergreen litter keeps arriving all year.
Grass options that sometimes work
Where there is moderate, dappled shade from a deciduous or open-canopied tree, a lawn can be maintained with the right species and some extra care.
- Fine fescues such as chewings and creeping red fescue tolerate shade and dry soil better than most. They are the backbone of most shade mixes sold in New Zealand.
- Shade-tolerant tall fescue copes with moderate shade and has deep roots, though it is coarser in texture.
- Poa trivialis handles damp shade but tends to die out in dry summers.
Ryegrass and kikuyu, the two staples of many North Island lawns, both perform poorly in real shade. Kikuyu in particular needs full sun and thins out quickly beneath a canopy.
To give a shade lawn its best chance, mow higher, at around 50 to 70 millimetres, so each blade captures more light. Rake leaves promptly, water deeply during dry spells, and oversow thin patches each autumn. Even then, expect a softer, less uniform lawn than in open sun.
When to stop fighting and plant something else
If the ground under a tree has been bare for two years despite reseeding, it is probably time to accept the site for what it is. Woodland-style planting often looks better than struggling lawn anyway, and it protects the tree’s roots from mower damage and compaction.
Good shade groundcovers for New Zealand gardens include:
- Native options such as rengarenga lily, hen and chicken fern, Libertia, and the low-growing Acaena species.
- Mondo grass and liriope for tidy, tough edging.
- Ajuga and Lamium for colour in damp shade.
- Clivia under large evergreens in frost-free areas.
The Department of Conservation website is a useful place to check whether a plant is considered an environmental weed before introducing it. Several groundcovers once sold for shade, including tradescantia and some ivy, now cause serious problems in native bush.
Mulch as a middle ground
A circle of coarse wood chip mulch, around 75 to 100 millimetres deep and extending to the dripline if possible, is one of the best things for a tree’s health. It holds moisture, feeds soil life and removes the need to mow over surface roots. Keep it pulled back from the trunk to avoid collar rot.
Slopes, surface roots and tricky spots
Lawns under trees on sloping ground lose even more water to runoff, and exposed roots make mowing dangerous. On banks, groundcovers and terraced planting usually do a better job of holding soil than grass. For ideas on combining planting with structure on steep ground, see this piece on slope planting and retaining walls around Wellington.
Avoid the temptation to cover surface roots with a thick layer of new topsoil to level the lawn. Adding more than a few centimetres of soil over roots can suffocate them and slowly decline the tree.
Frequently asked questions
Can I raise the canopy to let more light through? Yes, careful canopy lifting and thinning can improve light for lawn. Keep removals modest and spread them over several years to protect the tree.
Does artificial turf work under trees? It can, but leaf litter, sap and bird droppings still need cleaning off, and the impermeable backing may reduce water reaching the roots.
Is moss under trees a problem? Not really. Moss is a sign of shade, moisture and low fertility. It is harmless to the tree and can make an attractive groundcover in its own right.