What kind of trees are you thinking about? The obvious ones are acers, and miniature fruit trees designed to go in pots.
Yew can do well in pots, as can holly - they lend themselves to being clipped and kept to a manageable size and give formality and structure to your garden - just remember that they will need good quality compost - I find John Innes No 3 loam is the best one to use for trees - and this makes the pots heavy so if you want to move them around, plan how you're going to do this well in advance.
You'll have difficulty getting a lot of height as there's a to what a pot can support - bigger trees need more water than you can get from a pot. How about corkscrew willow - that can get to about six foot?
I have grown a Silver Birch in a pot and it is doing well as you can see. I have also got Lilacs and they flower beautifully every year. Anything can be grown in a pot if you water and feed well.
Ive just bought a corkscrew willow at a local fete for 2 pounds (cant be bad eh). Its going to be grown in a pot. I think it will look grand!
I've got a eucalyptus in a pot. The small leaved variety, very dainty. I've kept it like a round lollypop shape on one tall stem. I did not want to plant in the garden and have a 50ft monster like a neighbour of mine! I've also done the same with a bay tree which stands about 5ft tall. How high were you hoping to go? Remember that the taller the tree, the deeper the roots want to go, which may be restrictive if they're in pots.
As I have said many, many times, to everyones boredom I imagine - you can grow anything in a pot as long as you are prepared to put the work into watering, feeding and caring for it all year round I have an oak tree, several acers, and quite a few other trees and shrubs in pots, including a golden catalpa and some Japanese flowering cherries, most there for several years, all doing well. The only one which was sulking this year was the magnolia stellata, which probably wants taking out if its pot, root trimmed and fresh compost - will do it when I can find a spare pair of hands - hopefully attached to someone else.
The downside is that you really do have to keep a very close eye on them and give them plenty of atttention and care - which is why we don't go away in the summer - but with 400 plus pots in the garden I can't ask my neighbour to pop in for half an hour every two or three days to water!!! We get on well, but that might just put an end to it. I'm not suggesting everyone, or anyone, should be as daft about pots as I am, the point is that you can grow anything in a pot if it is big enough, the pot that is, and you are willing to act as mother earth for the plant.
Goodness me! I thought I had a lot of pots (about 100), but your 400 plus pots really takes my breath away. You must be a real slave to the watering. Even with seven waterbutts on our house and two on our greenhouse I find it hard to keep any water in them. And I find I need to water the pots regularly, even when it is raining sometimes.
Well, some of the pots are so large and have such mature plants in them that in fact it is extremley rare that they get any watering at all. Some of the trees are now immobile as the roots have almost certainly gone throughthe bottom of the pot and are happily meandering around the world!! The bulb pots get put out of sight by the shed in summer and can be ignored till they start again in Spring, and provided you give things like clematis and smaller shrubs a really good soaking when you do water, they don't need watering as often as you might think. Having said that, the potted summer bedding, the baskets and bowls and so on do of course need alot of watering, but it is a self inflicted problem, so I just get on with it. It is rare that we have lost anything from not being able to water enough, though it does get tight sometimes and then hard decisions have to be made. Many of the pots sit in beds between ground planted things, and get some ground water as well as rain or my efforts. You are right about watering in the rain, some things jsut need it as the leaves are so large that the water runs off them onto the ground not the pot - but as my neighbours are alrady convinced I'm pretty mad, they don't notice any more!