Hi David - Welcome to the forums! Growing box hedges in containers is a great way to create some privacy and add some warmth to your patio. Here are some tips I have to offer for growing good sized, healthy hedges in containers:
1 - Containers should be fairly large. I would recommend that the pot you pick out be about the same size (or slightly smaller) than the size of the hedge you want to grow.
2- Deep pots are better than shallow wide pots. The deep pots let your your roots grow deep and allow the plant to be sturdy.
3 - Box hedges can grow in an array of light settings. Full sun or partial shade is recommended though.
4 - Make sure your soil is well-draining so that you do not experience root rot. Box hedges prefer a peat/peat plus loam based soil over a normal potting soil.
5 - Water only when fairly dry. Do not over water!
6 - When box hedges are young and growing in pots, they cannot handle winter weather very well. Make sure to cover up your plants during cold weather over the next two years until your plants are mature and sturdy.
7 - Nutrients! Box hedges need to be fertilized regularly to ensure good growth. Unfertilized hedges will remain small. Try adding time-release pellets mixed into a top dressing of compost every few months during growing season.
I hope these tips help! Let us know if you have any other questions. We're always to help ! :)
Laurel
Happy Gardening!
The Get into Gardening Team