Virginia creeper is a native vine that does OK in shade and would climb over that cable house in a jiffy. Virginia creeper is sometimes mistaken for poison ivy, but it is much prettier, turns red in the fall, and has a whorl of FIVE leaves instead of three like poison ivy. Sometimes the bottom leaves only get three, hence why it gets confused for PI. On a further note, the partial shade conditons in your yard are perfect for growing poison ivy, and since birds aren't allergic to the plant, they spread the seeds through their droppings. I'd learn to identify it and keep a lookout for it showing up and then pull it right away. I pull it using rubber gloves to protect my hands and then additionally covering them with plastic bags from the grocery. I put the bag over my hand and then pull the ivy, and then turn the bag inside out and cover the ivy with it, it ends up a neat little package and then you can put it in the trash.

Do check out the irrigation system -- it would make more sense to irrigate the top of the slope and leave the bottom to live on runoff. I do that with several of my beds on our very sloped property (not only front to back, but side to side!) and it works quite well. I watered in everything once a week for the first few months, and after that let the lower beds survive on their own. They have thrived, even through the recent 2-week heat wave. I did improve the soil in most beds,and I have found that the beds I did NOT do this, the plants have not done as well. The clay has been exhaused by trees, weeds and unwanted shrubs for decades, and is no longer good quality soil unless very heavily amended. Replacement was actually easier than amending, BTW. I mulch heavily. I water (mostly soaker hoses, some hand watering) once every 2-3 weeks. If it's extremely, viciously hot, then once a week. Most of my beds are partial shade beds and it is surprising how much will grow in CA under these conditions. Our light is so bright that even under trees (or at least at the edges of the canopy) one can grow all kinds of things. And you're in a warmer zone than I am; I have had some trouble with winter chill damaging some frost-tender plants. I'm way north of you in the SF Bay Area, but in our sloped and rather shady backyard (a huge walnut and an enthusiastic silver maple that threatens to engulf fully half our large backyard), we still get enough bright shade overall to grow a surprising number of plants that are supposedly full sun. These include: -erysimum -canna -oleander -helichrysum -senecio -lantana -Meyer lemon -variegated euonymus -pelargoniums -shrub roses, rugosas, and miniature roses -New Zealand flax (phormium) I even got snapdragons to flower and return in one of the shadiest spots right at the foot of the silver maple. Although it probably helped to have a soaker hose in the bed -- the maple sucks up so much water that even weeds won't grow next to it. And of course, star jasmine grows like a weed, but it needs so much room I'm thinking of taking both of mine out. They are really uncontrollable! I'm not big on natives. They're nice, but spend half the year dormant or ratty. I want flowers, interesting foliage, and variety - all year round, even in January. OK, I'm a cottage gardener, I admit, LOL. Anyway, here's some pix of our backyard, and also the narrow north-facing sideyard, which is divided into two parts: the top half irrigated with soaker, the bottom lives on runoff. Looking upwards towards the house from the very bottom of our lot, from the backyard. Walnut to the right, silver maple dead ahead. Date: May 2006 for these pix Looking sideways at the bed around the walnut, towards my neighbor's yard Looking at the bottom of that bed, with the fence marking our lot boundary on your RH side. This side gets east-facing sun for about three hours before the walnut shades it all. Now the north-facing side yard: Top (Front) of the bed looking downwards - this is irrigated with soaker hose. The 'Emperor One' Japanese maple is a new variety and vastly superior to the old 'Bloodgood'. It grows in sun or shade; even windy conditions don't bother it. The bottom of the top bed, looking upwards (reverse of the photo above) The lower bed that lives on runoff. Contains star jasmine growing up a pillar, variegated aucuba, hellebores, bearded iris, liriope, and ferns.

If you don't want anything to grow taller than 10', you are looking at shrubs, not trees. And there are a fair number of shrubs that will easily exceed 10' as well, so you might need to prepare yourself for some pruning down the road :-) CA has an amazing array of native plants that could work, including a bunch of appealing flowering shrubs - ceanothus, manzanitas, flannelbush, Carpenteria, salvias, etc. A visit to a good local independent nursery would expand your choices significantly. The planting bed depth will need to be sufficient to accommodate the full mature spread of the plants being used plus some wiggle room. If the shrub grows to be 8' wide, than you need at least 8' plus.....so 9-10'

It looks like a fence and the neighbor has a hedge. Who has the arborvitae? But yes, you can stick a bush in it. You will have to move the pots, the rhubarb, the green hosta and give up some lawn to do it. Now a close city location is needed for appropriate plant suggestions. Your snow picture says the right selection of Yew might be what you need.