This is a dwarfing rootstock so should be OK in a pot, however the pot needs to be as large as possible in order for the tree to thrive and fruit. Put the pot in position before you fill it with compost or else it will be very hard to move when full. Use John Innes no. 3 (this is a soil based compost mix and heavy as it is made with loam (soil). The tree needs to be positioned in sunshine and not overshadowed by other trees. It is self-fertile so won't need another plum for pollination. It should be planted at the same level as it is in it's pot at the moment. Water well and then only if no rain falls for a couple of weeks (haha!). In spring start to water in dry weather and include some general plant food, either watered in or scattered onto the surface of the compost.