Leaves and more leaves.

This is a good time of year to contact some yard services who are doing yard clean ups. They have to pay to dump it, and will be glad to dump them in your yard for free. Have them dump 12+ inches of leaves onto the ground under your Doug Firs and where you want your garden. Leave the area around the trunks of the trees bare so that you don't cause rot to the trees.

The leaves form a smothering layer to kill all grass and weeds. The critters in the soil do all the work for you pulling the decaying matter down and breaking it down. A year later it's all crumbly humus. Do not turn it or spade it under as you will expose all the weed seeds to germinate.

It takes a year for it to turn into soil but it's dynamite. For the garden plot, you can grow spuds in the leaves next spring. The following year it will be ready for other veggies.

Refresh with new layers of leaves as needed in future years.

Cheers,

Jim

And it's all free.

Are you looking for one soil blend or additive to use in two separate parts of the yard for those needs, or will the veggies and stuff be near the Douglas firs?

M. D. Vaden of Oregon

I have added compost annually, both my own and purchased to my bed that is under neighbor's Doug firs for over twelve years and sadly, the bed still suffers from root competition and dryness.

It runs the width of my back yard and I really have no choice but to garden there if I want to garden....the grass won't grow either due to the roots and shade.

I don't think you can successfully grow veggies under those trees unless you grow in large planters like whiskey barrels.

If you make raised beds without a bottom the tree roots will grow up into them.

Raised beds of suitable topsoil for the vegetables, arborist wood chip mulch placed over rest of soil areas. Do not apply substantial layers of new soil over roots of existing trees unless you are planning to cut them down later.

There is no soil amendment that will eliminate the root competition from the doug firs. The only way to garden under them, is to either plant natives that are adapted to it, or to build a raised bed with a root barrier at the bottom - otherwise the tree roots will grow up into the raised bed.

And as bboy states, you can't change the soil level directly underneath existing trees without causing health problems for the trees. Ideally you should be out from under the branch canopy when you raise the grade.

The best use of the space where the doug firs are, is as native forest vegetation - things already adapted to growing under doug firs. Huckleberries, ocean spray, flowering currants, snowberry, ferns, mahonia, vine maples, Indian-plum, salal. They can take the poor soil, the root competition (two totally separate issues), the needle drop, and the summer dryness, with no attention from you once established.

for the veggies, in a raised bed? get any nice garden soil mix from a landscaping supply place, and add lots of compost. Free autumn leaves, or arborist's wood chips, will do just fine. If you're growing root crops in a raised bed be sure you put hardware mesh on the bottom to keep burrowing rodents out or you'll never get any potatoes or carrots out of it. If the veggies will be in the ground, mix in compost, leaves or tree chips and heap it up so you have a slight mound, for drainage and warmth.

If there were areas where the tree roots were not prolific yet, you could install tree root barrier panels around the planting area. That could do a fine job.

But my choice would not be to cut a bunch of existing roots in an area.

If you can't find them locally, Tree Tools in Lake Oswego, OR, shipped them last I heard.

M. D. Vaden of Oregon

Here is a link that might be useful: Tree Tools Website

Roots extend well beyond branch canopies. At their extreme ends tree roots are little delicate things, not as fragile as roots of some herbaceous plants but still tiny. These can be affected markedly by hostile conditions, same as with a small plant. For example, a Douglas fir fell through the roof of a house up the hill from me last winter. I could see it from the street. The lot had surely been developed decades ago (ours was the last lot to be cleared in our neighborhood, and that was in 1965). Presumably the now-failed tree pre-dated the development, although it is possible it grew up afterward. Anyway, after it failed the root flair could be seen to have ended abruptly on the same side and at the same point where a strip of lawn began - as though the tree's roots couldn't cope with the conditions under the turf at all and had either died or never grown into that part. It fell away from the rootless side onto the house.

Root barriers are another way to mess with tree roots, I'd think walling off much of an individual tree's root growth might become quite a problem for it eventually. I wouldn't want to do anything that might cause a native conifer of sufficient height to come loose and topple on my house. As it is any tree has the potential to fall at any time (floors of some native stands are criss-crossed with fallen trees), I wouldn't want to increase the chances of one coming down on me (engage a certified arborist to inspect and evaluate any that appear to be having problems, if concerned).