Healthy stock blooming when purchased would normally be expected to keep blooming after purchase. Growers use particular fertilization and watering regimes planned specifically to produce attractive specimens as quickly as possible. Reductions in tree and shrub growth (and flowering) after purchase and planting by the end consumer are probably very often due to this same diligent fertilization and watering not being continued. Top and root growth schedules, plant nutrient requirements do not change because a specimen has been planted out. New shoots and roots are produced at the same basic time every year, the same mineral requirements are present every year - if the soil on a planting site is deficient it will need to be supplemented if optimum results are to be had. Whether the planting is new or not. This is why potted stock needs to be kept fertilized and potted on, or it will deteriorate - the plants do not turn their growth (top or root) and their mineral needs on and off with changes in horticultural circumstances.

Maybe a picture would help??? Do you mean magnolia grandiflora? John

Give it another year. Some individual perennials are slow to mature. Some of my columbines are still small after 2 years and some from the same cell pack are large. And 3 of my baptisia sulked for 3 years before exploding in growth (and bloom) this year where 2 siblings are still waiting to develop. That's part of the fun of perennials. Some of them just sit and sit and sit, then BLAM, they explode on you for years. By the same token. 3/4 of my dependable Black-Eyed Susans just didn't come up at all this year. You never know what is going to happen.

Unless they have put more than one seedling in a pot, which isn't likely, you are dealing with one tree that has been cut back to create multiple trunks, as Alabamatreehugger has said. Even if there were 5-7 trees together, by this time, the roots would be well intertwined, and would NOT be happy to be disentangled from each other. Dividing can work for perennials, or if you had planted a bunch of tree seeds in a pot and then dealt with the babies at a few inches tall, but most semi-mature trees won't do it. Suckering shrubs maybe, but that's not what you are dealing with here. I can appreciate your wanting the shape that you want and the number of trees that you want, but I don't think this is how you will get it. Keep looking..... HOWEVER if they aren't too much money, so you wouldn't mind too much if it/they died, it MIGHT be worth buying one and trying. I think I would wait until fall or early winter to "operate", when the tree won't be actively growing and will be as dormant as they get, so you may need to pot this one on to a larger pot over the summer. Carefully removing the soil from the root ball (try using a water spray), at least at the top, would let you see what the trunk/root system really looked like, and then you could make up your mind if this is at all feasible. Since you do have experience dividing plants, I think it would be up to you to decide. They MIGHT all die, and they MIGHT live, I wouldn't want to say for sure. If it were me, and I really wanted this, and couldn't find what I did want, AND it weren't too much money, I think I would have a go - experimentation is how you learn more after all.