It would help to know where you are.

Two suggestions: variegated English Ivy and Veronica 'Georgia Blue'.

A pic or two of the tree and the pot would also help.

tj

Sorry I didn't put in the zone. I am on Vancouver Island, zone 8 or so. I am thinking annual plants that grow quickly. It is quite sunny, obviously, which is why the tree is struggling, but we don't get temperatures over 80 F. The pot is large, the tree is about 7 feet tall.

I may be out in left field on this one, but are you sure that your tree is getting enough food and water in a pot? It is surprising how much a growing tree needs. Does it need to be repotted?

First a disclaimer- The only time I have a tree in a pot is when I'm waiting to plant it in the ground. I tend to buy unusual native trees that are small (and therefore are affordable to ship). They come to me either bareroot or wrapped in something with very little soil. I pot them up immediately till they can be safely planted, sometimes within a week, sometimes after several months. Even after a couple of months, my little seedlings have sometimes outgrown their pots.

Just a thought.

What kez said. Could you perhaps build a large planter box for it?