I had two holly plants growing in pots with the intention of planting out in a hedge. I considered them just too small yet. In March this year, I noticed that one of them had lost its leaves. The other followed suit and both have since died. Neither were sitting directly on the ground, so couldn't have frozen. Can you shed any light as to why two year's work has just joined the compost heap?
Death of Holly Trees
Certified GKH Gardening Expert
Plants in pots actually suffer conditions as though you live one zone colder. The ground actually acts like insulation, trapping heat from the sun and protecting roots from cold fluctuations. Plants in pots do not have this protection. This winter was colder for many areas, so it may have been too cold for the plants.
In the future, consider heeling plants in that you want to baby a bit until they are ready for the garden proper. This article will explain more: