There seems to be inconsistencies in the world with the plants, I go out and pick the plants, in some places they spawn back in 14 days in other places they do not regrow at all and I have not been to that area it takes about thirty days for those to respawn maybe a few days extra.

Then I wait for about a month and I go back and plants have regrown along a path I have been and then in an area where I have not been nothing will be there. I go back and wait some more and the plants will be back.

Then in a major city if you pick the plants once in that city they grow back yes but if you do not touch any of them and go into a castle or a house you own the plants in the city will not be there when you come out. This has happened to me in Solitude, with the property and castle there and Whiterun with the castle there.

This is on the 360 and I am waiting inside a building away from the holds I am testing.

My guess is the respawn timer is effecting this in some way.