Whether you dwarf English boxwoods are 10 years old or 100, there's one maintenance task you can do for them annually that will keep them healthy above all others - thinning them. That means removing individual branch tip sections using bypass pruners or by breaking off the stems by hand.
Now don't mistake this for the general shearing of the entire plant with hedge shears. Thinning involves taking off 6-inch branch tips so that the plant remains relatively unchanged, except for small voids in the canopy here and there. In the end, only about 10% of the plant’s branches are removed.
How it Works
So is this thinning out of so few branches once a year really a big deal? It IS a big deal when you consider that most of the plant’s green food-making capability is not producing anything and only the thin layers of green leaves on the outer rim are able to perform photosynthesis. If you part the branches and look down into the shrub, you will see that most of the branches are bare and appear to be dead. They are, in fact, alive, but will not produce useful green leaves because no sunlight can reach them through the dense outer foliage. By thinning out short branch sprigs, light does reach branches deeper in, producing green leaves on previously inactive branches. The thinning also allows better air circulation throughout the shrubs, which helps to discourage the breeding of damaging insects and fungal diseases.
Plants are under much less stress as more of the plant is contributing to food-making and there are fewer pests for the plant to fend off pests.
The end result of this annual thinning is a boxwood that's healthier throughout the year without much additional help from us. If you would like detailed information on how to do a proper thinning of your boxwood, check out this article by the U.S. National Arboretum.