Yard Drainage & French Drains on Mercer Island, WA

Sometimes, with a modified approach, and it needs establishing before the alignment is fixed. Most of the roots keeping a large tree alive sit in the top foot or so of soil and spread well beyond the branch line — the same zone a drainage trench occupies. Cutting straight through that severs a substantial share of them, and the tree may take three or four years to show it. The workable options are shifting the alignment outside the critical root area, hand excavation with root pruning kept to a minimum, or in some cases a shallower system. The city's tree regulations may also bear on significant excavation within a root zone.

Both, and the balance is worth understanding before removing anything. A mature conifer moves a substantial volume of water out of the ground through the growing season, so it is doing real drainage work for free, and its roots bind the soil around it. Against that, a closed canopy keeps sun and wind off the ground so it dries very slowly, and heavy needle litter mats down and sheds water rather than absorbing it. Clearing litter and thinning a crown usually improves matters. Removing a large tree on a sloped island lot can leave you with a wetter garden than you started with.

Yes, in the same way it is on any sensitive slope. The edges of the island carry looser material than the level crown and are more responsive both to loading and to water, so a system that collects a garden's worth of water and delivers it as a concentrated stream onto the slope is creating a problem rather than solving one. The discharge has to be controlled and designed rather than simply daylighted at the most convenient point. On the flat ground through Island Crest none of that applies and the work is straightforward, which is why we look at where on the island you are first.

Woodland planting, and it is genuinely the better answer than fighting the conditions. Under an established canopy you have low light, ground that stays damp for long stretches in winter and dries out in summer because the trees take the water, and acidic needle litter. Sword fern, salal, evergreen huckleberry, hellebores and woodland groundcover thrive in exactly that combination and need no drainage intervention at all. Where drainage is worth installing is around the areas you actually use — a patio, a path, the ground against the house — rather than across a whole wooded lot.