Trees in a mixed border

Hi, I have been a serious gardener for 10 years now in CA, zone 8 to 9. Small yard and I have raised beds in the front yard and back yard because the native soil won't grow much mostly because the site was originally bulldozed to level it. My garden is purely decorative with roses, clematis, hydrangeas, cranesbill and lavender, For groundcover campanula and a verbena hybrid.

Initially everything I planted took off although I had to abandon many plants because they flopped or grew too large, finally settling on the list above. I have a couple of 20' Crepe Mrytle's and a Japanese maple in the borders.

After 5 years or so I noticed many of the shallow rooted plants were languishing. Eventually I realized they were being choked out by the tree roots. Tree roots are as dense as steel wool and close to the surface so they get the water first. I have since started cutting the surface roots with a steel bar which definitely helps but this is not advice you typically see in gardening books. Also there are a few 100' pines on the other side of my fence. Luckily I can access this area and cut the roots which I have to do every few years. Basically I dig a 50', 10" deep trench along my fence to cut all of the roots entering my yard.

The roses seem able to compete with the trees just fine but they are about the only ones. I have a love/hate relationship with clematis as I have probably bought 30 clematis but only have 3 vigorous ones in ground, and they are mostly the ones far from trees. IMO its a myth that clematis can out compete other trees and plants as I have not found this to be the case.

Anyhow I was wondering if there are some rules landscape designers use on mixed borders with trees. I have several books on border designs and don't recall any advice on how far trees should be from the front of border. Admittedly, I should not have trees in 3-5' wide borders but someday I will move and hopefully have wider borders.

Here are some things I have learned the hard way-

1. Water the tree. Some tree roots can go 50' in search of water and my japanese maple completely choked a rose bush 10' feet away with a mass of roots i could not believe.

2. Root barriers. The plastic root barriers actually work. I found this out when I replaced some sidewalk as the original builder put some in to shield the sidewalk from tree roots, only reason it failed is roots spilled over the barrier and made its way between barrier and sidewalk because it was too low.

3. Cutting surface roots. Highly disruptive and probably has to be done frequently. Labor intensive.