I know this comes little late but it can be relevant to slow growth of a plant and pups
Your landscape containers can be heating up too much also. That will slow things up a bit. It does for me.
If you pull the plant out of the container and notice no roots on the sunny side of the pot the it is time to give the container more shade.
Roots will not fill a hot pot.
Here is one of my containers as an example ....the sunny side has no roots.
A temperature check revealed 130+ degrees at times
Heat damage on the left
by
Hostafarian,
on Flickr