Thank you for contacting Ask an Expert for help with your manzanita.

There are quite a number of Arctostaphylos varieties available in nurseries, from ground covers to shrubs to small trees. An excellent description of many of these can be found in the Oregon Landscape Plants database. I am going to assume that the plant you want is the native manzanita A. columbiana or hairy manzanita. Though you may possibly be thinking of A. manzanita, which is native to northern California

In either case, you are correct that this plant needs near perfect drainage. Planting this tree at the bottom of the slope may not be the best location. Planting farther up the slope, to take advantage of the better drainage, would be preferable.

Please do not add sand to your soil. Sand combined with the natural clay soil of the Clackamas County area will not increase drainage and may actually exacerbate the problem. If drainage is an issue in the planting area, consider incorporating volcanic pumice or gravel (1/4-10 gravel is widely used) or installing drainage tile. There are no precise ratios because each site has a different level of aeration.

When amending the area before planting, work any material into the soil over a wide area, not just in the planting hole. We want roots to move out from the planting hole. Also, avoid adding composts or manures, because these plants do not do well in high organic matter conditions.

You are taking up quite a challenge, remember drainage/aeration is key.

Good luck and Have a good gardening year,