I think it's a good idea if it's a bag that doesn't tend to get escape roots. You can also prune mulberries to keep them small if you keep at them. They bear on this years wood so you can just keep stub cutting them and leaving only very small wood. You can also train them to a weep, but it is a tricky proposition.

Either way, expect to prune them at least twice during the growing season to keep them small.

Anyone with any experience with limiting tree size using a root bag?

I do have experience using the original Whitcomb bag, which is made from a fabric resembling a heavy landscape fabric.

It works, but you need to provide irrigation to get good growth- even in the east and escape roots can be a problem.

I have a lot more experience with Whitcomb's second bag which doesn't restrict growth much.

Are the bags of which you speak root pruning bags? Their purpose is to prune the roots and form many inner roots to prevent circling and to help the tree grow big and tall. I have two Pakistan Mulberries, both in their second year and full of unripe fruit. Nice to know they can be pruned as we wish to keep them to picking size.

I have a natural root pruning solution. It's called gophers! My trees are in-ground in gopher wire baskets, so if roots escape they will be pruned for sure! :-)

Suzi, I mean root control bags, to limit growth... If you limit the size of the root system, you will trigger the plant to focus on fruiting, rather than growth.

I'm planting a Morus nigra which doesn't have a problem with succering, but can become rather tall... Escape roots are not such a big problem, as the bag will have limited the growth enough and the tree is slow growing. So I hope the tree will slow its vigor over time...

My planting area is around 25' x 25' and can possibly have a tree that is 20'-30' tall, but anything taller will not be practical...

I hope also that my 5b -6a location at 60 degrees north in Oslo, Norway, will make the tree grow slower as dormancy over winter will last longer... This said a short growing season does not seem to bother the ash trees in the neighboring plot which is around 60'

Alan, why do I need to irrigate? is the bag not permeable ? I was also looking into using plastic sheeting and making a barrier that is about 2yards in diameter and digging in deep, so it will contain the spread and send the roots downwards after water.

I need to irrigate if I want the trees to be vigorous enough to grow and, if it was my intention, to produce fruit. I use these bags in my bearing age fruit tree nursery and the first bag I used was Whitcomb's original invention, which he does not own the patent for. They are popular with west coast nurseries and manufactured in Oregon, It think. They work fine with drip irrigation and have a very dwarfing affect on fruit trees the way pots do.

His second inground bag invention is completely different. It is constructed of a knitted fabric with visible holes that allow much more root to escape the bag. The roots do break easily when you dig up the trees and the roots on the inside of the bag are supposed to store a lot of extra carbohydrate because of being somewhat restricted by the bag and establish well with much less soil than BandB'd trees.

The original black bags are the ticket if you are trying to constrict growth. I use them for my fig trees so I can easily move them inside in the winter. They do require less fuss than above ground pots and allow you to use soil instead of potting soil so you can grow more tree in fewer square inches of medium. Of course, only volume is reduced- not weight.

Their are some small trees like the weeping mulberry (Morus alba pendula), the
contorted mulberry (Morus bombycis ‘Unryu’) which grows 6-8 feet. Black Beauty
that grows to 12 feet. The Geraldi dwarf mulberry grows to 6 feet. Also another called Dwarf everbearing. The Black Beauty is a Morus nigra and out of the nigra's some believe it is the best tasting. Which is saying a lot as Morus nigra trees are known to be great tasting trees.

Alan, thanks for your insight.

Drew, I do not want to plant a Morus alba, as this is moderately close to the neighbours yard, and I hear that suckering and bird dropping seed germination is high with these trees.

Living in Norway the availability of different spiecies and cultivars is limited. The nigra has in England got at least five or six cultivars, but getting it through customs requires a health certificate, but few nurseries wants to help.

I'be found one with a generic nigra said to taste good. I've also started a sowing from seed project, but have since learned that the plant is heterozygous and won't come true to seed.

But I'm exploring all avenues here...

In Denmark and Sweden, the alba is common, but I do not want it... I could settle for the rubra hybrid Illinois everbearing, but I hear conflicting stories about size and vigour... Finding a way to control growth, would then give me a choice.

I might have a bare root nigra on the way and hopefully, with a large bag it will halve the size of the tree for me....

I didn't realize you were in Norway. The Illinois everbearing is a good
tasting fruit, but it is a very large tree. Fast growing too. My concern with
any root limiting attempt is that it will fail. With time the bag will be
compromised and the tree will grow bigger. I think they are only meant for
temporary use, until the tree is sold/moved. I disagree that escaping roots are not a problem, they are and the tree will grow large. Also this species is very fast growing, these are near impossible to control. They can grow 10 feet in one year. If another species maybe escaping roots would not be a concern, but these trees grow faster than any other tree I know of except maybe some willows.

Mulberries aren't that long a lived tree anyway. They grow fast and die young.

An IE mulberry can reach 50 ft in about 10 years in the right soil with full sun if you don't prune it.

This tree well over 20 years old, more like 30. It's slow growing for a mulberry. On Russell Island. About 10 trees are on the island. They have small fruit, dark red to purple. Nice trees. Most are under 20 feet, but this one is at 50. They do grow quickly until they reach 20 feet then slow down. I have been foraging fruits from them for 10 years and they all look about the same. The mulberries are fantastic in jam with blackberries or currants. My best jam last year was red currant-mulberry jam. A classic acid sugar combo. That always works.

I put a tarp under the tree, climb into them and shake the tree best I can. It rains fruitful goodness. I can't harvest any from the tall tree. One branch comes down low, but you can only hand pick, it's too much work.

Alan, Drew, as I understand, the black mulberry grows to about 30-40 feet, slowly. The white to about 30 feet quickly and the red to around 50 feet moderately quick?

In my northern location trees only grow to around 2/3 size anyway... I aim for max 25 feet... but maybe topping it will ensure this?

It all depends. Black Beauty is a black and it only grows to 15 feet. Also some dwarfs are around too, as mentioned in this thread. But yeah in general that sounds right. I think the ones at my cottage are reds, but they get very dark for reds. IE mulberry has bigger fruit than at my cottage. Most cultivated types do have larger berries. It is just a native mulberry, not really cultivated so not really a cultivar.