I'd stick my fingers in the potting mix against the pot and see if there is a developed root system holding the potting soil together- if so, you can transplant it at any time, if not, it is probably already in shock from being transplanted into a tiny pot after having most of its roots cut at the nursery you bought it or by their supplier. When flower buds show, the tree is just beginning to throw out new root- if it is already established in the pot it would benefit from moving now into a larger one. If not, it might benefit from staying put.

It is almost surely a transplant issue and also may have something to do with the state of the tree when you bought it. BB's are hard to store once dug up and are best planted shortly after digging before they begin to leaf out. The tree may have been weakened well before your purchase just by a single drying out of the ball. The BB soil is probably of a different texture than yours and watering can be tricky if this is the case- you have to be sure that the ball is thoroughly watered as well as the soil the roots need to grow into. That soil outside the ball often needs to be broken up a ways to insure easy penetration of new roots. BB'd trees are always a risk because you don't know how much root you're buying- if the roots are very spread out at time of digging the tree may not be worth much. Best trees usually come from nurseries with soil that has high clay content so roots are condensed.

I bought a foreclosure in January that had been owned by a landscaper. There was a crab apple tree in a large (18-24") plastic nursery pot sitting in the dirt in a corner of the yard, so come springtime, I dug a nice hole and planted it. ===>>> to sum it up.. this thing could have been sitting in that pot for 2 years.. during the foreclosure.. and have never been tended nor watered .. and that aint good ... no amount of love.. from you.. will overcome the abuse .. yes its speculation ... if you are an experimenter .. have fun with it.. but dont let it get you down ... if you are a worrier.. get rid of it ken

How many cultivars - Many apple cultivars are self fertile. However, even the self fertile cultivars will set more fruit if cross pollinated by another cultivar. Wild apples/crabapples in the area can also work as pollenizer. _______________________________________________________ Same cultivar - Using two trees of the same cultivar will not be beneficial. That would just be like having a bigger tree. You need another cultivar to get cross pollination. _______________________________________________________ Rootball size - The figure you give is too small, especially for smaller trees. Below is my generic rootball guide. Soil type, tree species, and other factors are not taken into account, but it will give you an idea: For trunk diameter below 1/2", multiply the diameter by 20 to get approximate rootball diameter. For trunk diameter between 1/2" and 1", multiply the diameter by 18 to get approximate rootball diameter. For trunk diameter between 1" and 1-1/2", multiply the diameter by 16 to get approximate rootball diameter. For trunk diameter between 1-1/2" and 2-1/2", multiply the diameter by 14 to get approximate rootball diameter. For trunk diameter between 2-1/2" and 4", multiply the diameter by 12 to get approximate rootball diameter. Here are guidelines for rootball depth: For a rootball with a diameter of 1', depth should be approximately 8". For a rootball with a diameter of 2', depth should be approximately 1'. For a rootball with a diameter of 3', depth should be approximately 15". For a rootball with a diameter of 4', depth should be approximately 18". _______________________________________________________ Leaving the rootball in place - Leaving the rootball in place for a few weeks is unlikely to help chances of survival. Root pruning can be used, but is generally done at least a few months in advance. If you dig a proper rootball, you won't need to worry about this for an apple tree. _______________________________________________________ Linked below is a guide to transplanting trees. I think it may be helpful to your task. Here is a link that might be useful: Planting a Tree or Shrub