The main and original function of the taproot is to give initial stability to the brand new plant when grown from seed and that is why the tap root grows deep down into the ground. The taproot then develops small fibrous roots from which it absorbs water and nutrients from the soil until the plant grows old enough. When it grows old enough, many secondary, tiny and fibrous roots start growing from the sides and from the crown of the oakleaf hydrangea. The hydrangea will then absorb water and nutrients from these new guys.

At this point, the need/usefulness for the taproot is over and the latest secondary and fibrous roots absorb water and nutrients from the soil. If you then cut a "no longer useful" taproot, nothing bad happens. The hydrangea will continue to grow as usual.

Interestingly, when beginners do bonsai, the taproot of a plant is cut so that a tall plant can be fitted in the small bonsai container. Also, "real" trees usually do not have a carrot-type taproot though, but they develop lateral roots that, in turn, develop vertical roots.

Thank you, @luis_pr

I will get it situated soon!

my other question was how long does it take for rooted cuttings to start growing additional limbs/branches? This cutting was about 4-5” high when I plopped it in that container. Now it’s about 2.5 ft high and had beautiful leaves last year. It’s budding out now. Does new growth come from the root or???

many thanks for your answers :)

It probably depends on the variety. The tallest one that I have, Snow Queen, has branches starting at a height of 1.5 to 2', while the more compact ones start at about 1 foot or more. The new growth will originate from around the root ball area.

Cut the pot away instead of the escaped roots. When roots escape through drain holes those replace the newer roots that made up the outer section of the root system when all of them were still confined to the container. So that cutting off escaped roots can be so traumatic that a given specimen may even die as a result. And for the plant shown here to have such a hugely disproportionate (relative to the pot size) top present it means there is a also a quite large root presence beneath the pot, that has been out of the pot for some time.