Companion Planting Under Plum Tree?

I don't have experience as my plum tree sits in the middle of a lawn but it does have a clear area around the base where the roots can be watered and fed. So I think you should be aware that the tree will also need attention and not to crowd its roots with large veg that may compete for nutrients and water. Maybe crops that are quick growers with small root systems might work, such as radish and lettuce?

Some comfrey?

Thanks Mark - where I was visualising the plum tree is actually the apple tree, I now realise lol

So... On one side there are roses, onions (for the roses) and then potatoes behind...
This is the thing about companion planting? It's like those slide puzzles I can never solve lol you get couple of pieces to fit and then it doesn't match with the other? lol
And how close or far away do you keep the ones to be matched of the ones to be avoided?

Yes, you're right the tree's needs come first, so best to go with short root plants underneath it, or those that can be moved easily if needed?

This plum tree seems to be doing okay though? I'm still eating the jam I made from its fruits last October lol It's self propogated a 'teenager' tree about 5ft and I've just dug up about 6 baby plum trees that it was root propogated for me this year! ( I know this should really be done in dormant season, but you can still do in Early Spring - and yeh I guess it's not early Spring any more? lol) But also found this:
"
Because plum trees have lots of roots near the surface, they will often produce lots of of suckers (mini-trees) for 3 metres (9 foot) around the trunk. Regular cutting of these suckers will keep them under control. A more permanent way is to gently dig away the soil to the point where the sucker joins the main root system. Simply pull the sucker away from the root. This will slow down the production of suckers considerably."

I'm turning the area around my plum trees into a little wildlife haven. I've not had much luck growing anything around them, other than self wandering strawberries. (They've made themselves a very nice and full bed with very little help from me)

Yes Feral - comfrey would also be beneficial to the apple tree and the potatoes and can use as fertiliser (comfrey tea) for lots more plants.
Would have to be just that though - a fertiliser plant - as while it has lots of uses, I don't know what else I, personally would use it for...

di - that sounds great - my whole garden is a wildlife preserve - not really designed around humans LOL - don't have smart decking or patio or 'that' side of gardening lol - it would be a bit wild for some tastes I guess lol

hmmm - lemon balm, camomile - anyone grown these under fruit trees? At least then they would be useful and could make into teas?...

You can mulch all the trees with the leaves of the comfrey plant as well as make comfrey tea for fertilising Garden Faery. It's good stuff to have. I have two little plants I'm hoping to turn into huge plants like the local community garden has - I covet that bush!

Additional advantage of keeping a bare soil : additional protection against late spring frost (will make a difference of 1-2°C)
I'm planting wild raspberries under my plants (covers the ground really well and gives nice fruit)

Yeh Sugar - since Di's post have read a lot about using short fruit crops - strawberries, wild raspberries etc - just thinking bout water conservation too though if there is to be hosepipe ban (you might be lucky if you're in Belgium not to have one?) as the plum uses a fair bit of water and strawberries always like it too..