This white Hydrangea plant is overgrown and taking up much of my backyard garden.
What can I do to stump the growth?



It is possible to divide your plant into 2 or more “new” plants.  For example, pull the bushy part of the plant gently apart into two halves, then examine the root ball or crown under the soil – there may be natural separations here where the plants can be easily divided – e.g., spots where there are no stems sticking out.  Separate the sections by pulling them apart or, if needed, using a sharp shovel.  Remove the “new” section and replant it in another spot.  As for the original site, fill in the existing hole with triple mix.  Water both plants. See SF Gate’s Caring for & splitting hydrangea

Alternatively, prune the hydrangea to make it more compact and less leggy.  Do so in late winter (after the worst winter frosts) or early spring– this will thin out the plant’s growth for the next season.  You can remove up to one-third of the branches each year.  See Ask a Master Gardener – Pruning hydrangeas  and Hydrangea pruning

From your photo, your hydrangea is a panicled hydrangea (Hydrangea paniculata) – the blooms resemble a cone – and its flowers develop on new wood, so the shrub can be pruned any time except summer.

Please note that your question reached the Toronto Master Gardeners, not Paul.