Do you like this tree? It may seem utterly unthinkable at the moment, but you might eventually want to remove it and find something better suited to the location -- unless you love it the way it is. The roots are overly aggressive and it appears the tree was shaped to allow a view from the house, but it's awkward. One of these days it's going to bust out of that brickwork.

Mulch, I would use plain compost.

That's a very tough growing condition.

Succulents and Bromeliads come to mind as plants that could possibly make it in this condition, but the leaf litter is really going to cause a major maintenance issue.

Ordinarily I would be among the first people to recommend bromeliads as a large scale ground cover in a shady, surface rooting overhead tree situation, but large leaves such as your Carrotwood trees(?) will tend to accumulate in the rosettes. Other choices mentioned by Michelle/deviant-duhziner, such as succulents might include particular succulents such as Crassula multicava, Aeconiums, variegated Spider Plant/Chlorophytum comosum or other grassy foliaged shade tolerant and Eucalyptus understory survivors such as Dianealla intermedia of D. tasmanica, or some of the larger growing Liriope species. If the trees were thinned out to allow more light, I might also suggest Oxalis spiralis var volcanicola or Asparagus densiflorus var sprengeri compacta, which both appreciate dappled shade rather than dense shade, but also compete well with surface tree roots. Many bromeliads would also work, but they tend to accumulate dead leaves from trees between their foliage, and look messy unless groomed periodically.

Anything you might plant in this area will greatly benefit from some new topsoil laid over the existing exposed roots of at least 4 to 6 inches in depth to allow for new plants to get established. I would also suggest that you convert your existing spray irrigation system to drip soaker hoses or emitter lines with emitters at 12 inches on center and spaced about 18 inches cross wise to the slope to give more uniform coverage to new ground cover plants, at a slower rate of application. You will also need to fertilize and water more frequently to allow these new plants to establish. All of the plants I mentioned are pretty good at absorbing leaf litter without needing to rake it up.

Kim's suggestion to consider removal of the trees before they can damage your brick retaining walls may be the most logical choice in the long term, as I agree with her that it is just a matter of time before they do start to damage the walls.

I would embark now on a continual renewal program for that group of trees, which I imagine have an important role in keeping patio and house a bit cool.

The graduated size of the trees suggests that that may be exactly what a previous owner has been doing. It would now be time to remove that largest tree, and plant a new one beside the stump. It's actually already a bit late to do it, because I don't think the next biggest one has more than 5-6 years before its removal becomes rather urgent too (depends on the growth rate of these trees), and your new one won't have put on much size by then. Ideally I think there would be about ten years between removals; what you don't want (if the shade is important) is to be doing more than one at once.

I'd say it should be done "now" because it is in the narrowest section of the brick and the roots are poised to do serious damage - there is no risk to acting early, and a very high risk to acting late. Another indicator that it is now time is that the surface of the bed, the enjoyment of which is integral to the success of the design, has absolutely ceased to provide any pleasure at all. Unless, that is, you wish to dedicate your life to the nurturing of plants in such an inhospitable environment, or really enjoy the look of mulch, up close. If you want to grow a ground cover, then the expertise of Bahia and DD in doing so will equip you with a fighting chance (me, I know my limitations), but the thing is, you have a better option, and one that is more conducive to the long-term viability of your walls.

It would be interesting to know whether these trees were in fact planted around the same time. If that's the case, there is something different about the growing conditions the two trees have, which would affect the scheduling of your tree renewal program.

KarinL

I just thought of one more thing to consider about tree removal: the actual process of removal. Heavy and large branches will hit the bricks as they fall if the removal is not done carefully and professionally - using pulleys and such. Getting the trunk out of there - over the edge of the wall - will require it to be cut into lots of smaller pieces so it does not damage the wall. The larger the tree gets, the more challenging, and the more potentially damaging, the process of removal will be.

You may love the tree and not want to contemplate removal. This is not irrational, because it is a cool tree with a fabulous trunk. But I think it's important to realize that the brickwork and the tree cannot co-exist in the long term - one of them will have to go. If you don't make a choice, the tree will!

By the way, does anyone know whether such a lot of surface roots are normal for this tree? In other cases where I've seen so many of them, it has been an indicator that the tree cannot get enough of something - moisture? air? - underground and so is coming to the surface to get it. You should probably figure out what the problem is if you plan to keep the tree. It would be a shame to sacrifice your brickwork and pool to the tree, and then have it die for lack of nourishment.

Covering the roots with topsoil will help in the short term, but the topsoil will quickly become invaded with roots too and the problem will reappear.

KarinL

If or when you do remove the tree(s), I would consider choosing replacement trees of a different species that wouldn't grow as large. A native Western redbud, for example, could probably fit nicely in those spaces.

Travoc, I liked your idea of moss. I could totally imagine moss growing around those roots, with the tops of some roots showing, and maybe some boulders added in. I've seen shady places like that when I go out hiking. But I'm not sure how to start moss there, or if it would look good all year. Maybe water it with a drip irrigation mist sprayer?

Some plants that can compete with roots:

Nandina, heavenly bamboo
Bergenia crassifolia, heartleaf bergeina
Rosemary
Asparagus fern
Leatherleaf fern?

Plants in pots around there would add color too, and you wouldn't have to deal with the roots. A really colorful plant for shade is the annual, impatiens, also begonias.

Trovoc, Most of your original post is gone, or i just can't find it.

It sounds like your talking about Carrot Wood or Carob trees with many be a brick walk way near by.

These trees are always shallow rooted with little hope of mitigating short of having to score the roots to keep them in check and away from any hardscape that might be affected. San Deigo tends to be heavy in sun most of the time, unless you live within the 1-2 mile strip near the ocean where it's over case until 11am-1pm.

You can mow over these, once or twice a year, on a high setting to help reduce the volume of the thick carbon leaves. Keeping up with these tree can be a real chore.

Well, after 11 years many out comes come to mind.

I hope you have had good luck with them, and not planted the asparagus fern, lol.

There's a plant nursery in Pacific Beach, "Green Gardens Nursery" on Cass St, Paul Backus, owner is very knowledgeable and most defiantly would be helpful.