most size estimates are at ten years..

no 'fast growing' tree will stop growing magically at some given height ...

google pix.. will probably show you that most mulberry have a much greater potential than 20 feet ...

my un-named native is 40 feet tall ... the birds love it.. i hate it ... the kids love it.. it stays.. this good daddy stuff sux at times ...

ken

I figured the fast growing might be a problem but some mulberry descriptions say 'grows aggressively to X height and then slows down. Also will the limitation of the root space by the retaining wall and impenetrable surrounding clay limit the size?

Pakistan will work, but the roots of any mulberry will likely be just as invasive as your ficus.

I'd go with Pakistan or one of the high quality mulberries that I can't grow in my Z6. Less vigorous should be a virtue- believe me. It will take a lot of pruning to keep an IE to the size you desire if it grows there at all as they grow here in SE NY.

If staining is a major concern, white mulberries don't stain. But they also have zero acid in the flavor. Have you tasted nigra vs non-nigra? Huge difference, a different fruit entirely.

I'd put in a nigra with zero hesitation over an IE.

I have 2 KOKUSO MULBERRIES grown in 3ft. of poor limesone soil over bedrock for the last 6 years. One is 15ft. the other 20 ft. , neither one has a cristmas tree shape, no central leader but rather a bush shape -I topped them a 3ft. and the side branches took off without the forma
tion of a new central leader - as a pear tree would do.
Non staining ---large 1''-1.5''--- tasty--but do not overpower the other fruit in say pies and such, and in one stage of ccoking the fruit the seeds float and can easily be spooned away. HARVEST tarp harvestable. In my expierience a real fruit producing machine with minimal care . Hope this helps.

Surely kokuso, and any other Morus alba with black fruit, is in fact staining, just as much as nigra ones are. the choices are always: either white fruit and bland flavor, or black fruit and cope with birds and staining.

We cut several mulberry trees down at my grandparents place because of the birds and all the staining. They would cover the tractors with purple mess from the birds roosting in the shed.