Question
I just purchased a rose mallow in a gallon container. Can this be left in the container over the winter or must it be planted in the ground? (LIsland)

Answer
Lavatera trimestris, 'Rose Mallow', is often confused with Hibiscus.  I am assuming you actually have a Hibiscus, which is perennial, and not the annual Lavatera.  If you know the Latin name of your purchase, and it is indeed Lavatera, I'd like to revisit this answer with you.

Potted shrubs -- and this goes for all hardy shrubs, not just Hibiscus -- can be left out all Winter on Long Island and SOMETIMES they will survive.  The problem is that roots are NOT as cold hardy as plant leaves and stems.  Where leaves and stems are hardened to sub-freezing and even sub-zero temps, roots don't have the same armor, and are far more susceptible to low temps.  Even VERY tough plants that will grow in Alaska have a high chance of damage, maybe enough to kill them, down here on Long Island IF YOU DO NOT PLANT THEM.

Now, if you want to chance it, try to tilt the odds in your favor by burying the plant in a layer of leaves and then covering it with something like a garbage container when the weather gets very cold, in mid-December.  Removing the container in early March.  This will shield the plant from the worst winds and the roots from the most frigid Winter temperatures.

For next Spring, purchase a container of 'Messenger' Harpin Protein treatment and be ready to use it when the plant is ready for transplanting next Spring.  The sooner you get this plant in the ground, the better.  If you use Messenger, you do not need ANY fertilizers.  Of course the Soil you use must be first rate.

Thanks for writing, neighbor.  Keep in touch.

THE LONG ISLAND GARDENER