The number-one mistake people make under mature trees is tilling or digging deeply. This damages the feeder roots that trees depend on. Instead, work only the top 4 to 6 inches, then build up rather than down.

Add a layer of coarse compost and a porous soil mix — one-third native soil, one-third pumice or perlite, one-third compost — to create a raised planting layer that drains freely and encourages shallow-rooted, drought-adapted plants.

Rock selection is where many berms go wrong aesthetically. Mixing rock types — river pebbles next to angular granite next to red lava rock — reads as chaotic. Choose one or two rocks from the same geological family, in similar tones, and use them consistently throughout.

For dry shade, I recommend decomposed granite as a base and path material, with fieldstone or flagstone in complementary tones for structure and edging. The rocks should look like they might have always been there.

Under a canopy, plant in odd-numbered clusters — three or five of the same variety — rather than single specimens. The canopy creates visual complexity; groupings cut through it and create intentional, restful rhythm.

• Carex (sedge — graceful in dry shade)
• Erigeron ‘Wayne Roderick’ (fleabane daisy, self-seeding)
• Salvia sonomensis (creeping sage — a personal favourite)
• Arctostaphylos cultivars (low-growing manzanita)
• Ribes (flowering currant — shade-tolerant)

Water under a tree canopy is trickier than it looks — the canopy intercepts rainfall, so your plants may receive far less moisture than you think. But overhead sprinklers waste water and promote fungal disease on the foliage.

Drip irrigation — a simple soaker line on a timer — solves both problems. Run it briefly twice a week during the first summer (establishment), then once a week or less once plants are settled in.

A 3-inch layer of arborist wood chip mulch — not beauty bark, not dyed mulch, the real stuff — is the single best thing you can do for a rock berm. It suppresses weeds, holds moisture, feeds soil biology as it breaks down, and gives the whole planting a naturalistic, finished look.

Resist the urge to refresh it every season. Let it decompose fully — that decomposition is feeding your plants’ roots. Top-dress every second year at most.