I'm planting some Kumquats along the side of the drive. However, I notice they grow to 3-4 metres (10-13 feet) high and wide. How can I keep them smaller? I considered putting the pot into the ground to stunt their growth but am concerned the plants will become weak.
You can't, really, not and still have them fruit anyway. Putting them in pots and planting them in those into the ground won't work either - restricted in a pot (unless the pots are extremely large) they will eventually die; but it's highly likely the roots will break through the holes in the bottom and root into the surrounding soil anyway, which rather defeats the object. The only pruning these trees require is removal of any sucker growth and a fairly light prune to reshape immediately after harvesting the fruit and well before they flower in spring. If you're not worried about flowers and fruiting, then you might be able to prune them back more often. More info on growing these trees here: Kumquat Tree Care: Tips For Growing Kumquat Trees.
Orchardists regularly tip the leaders of their trees to keep them smaller so they are able to restrict their growth. The backlash is this sends all that energy back to the branches to encourage lateral buds, suckers to grow. That is why they create dwarf varieties. Those apical or terminal buds have the most energy within the entire tree/shrub. When those are cut, that energy is then distributed among the lateral buds. Do not put them in pots to bury in the ground. I'd rather have vigorous too high trees than do that...root pruning does not necessarily dwarf a plant. It does reduce the vigor and ability to pull chemicals out of the soil that it needs with which to do photosynthesis. I would nip the leaders, prune the suckers and make sure I did not give my trees too much nitrogen...which promotes vegetative growth not reproductive growth. And buy a ladder or two, grins.!e