Adding plenty of sharp sand or horticultural grit to heavy soils will help improve drainage and aeration.
Hardy perennials, shrubs, climbers and trees can be planted at any time, provided the soil is not frozen or waterlogged. However, on well-drained soils planting in autumn is preferable because the plants have a chance to get established before the summer.
On heavy clay plant in spring once the soil becomes workable.
Always mulch the soil after planting with a thick layer of well-rotted organic matter to suppress weeds and lock-in moisture.
Plants in containers are particularly vulnerable to winter cold, so insulate the roots by wrapping the pot with bubble wrap. Then insulate the top growth with a double layer of garden fleece.
The key to successful watering is to apply it only where and when it is needed and in sufficient quantity: soak the whole rootball each time you water.