If your land is often wet and cannot easily be drained there are still plenty of options for planting trees. Firstly, there are species adapted to wet ground. Willows, Alders and Poplars are amongst the commonest but a good book (such as Hillier’s Manual of Trees and Shrubs – ISBN 0-7153-9942-X) will give you lists to choose from. It will also give you an idea of how high your trees may grow or other conditions, such as strong winds, which they may tolerate. Red Alder, River Birch, Liquidamber, Black Gum, Caucasian Wingnut, Scarlet Willow, Corkscrew Willow, Chinese Swamp Cypress, Dawn Redwood, Pond Cypress and Swamp Cypress will tolerate or even enjoy permanent dampness.
Many other trees may survive occasional flooding but their survival chances will be improved by planting them with their rootballs above ground level. They can be planted with more than half of their rootball above the surrounding ground level and soil can then be drawn up to cover the exposed roots. Better still they can be planted into a raised mound. As the roots grow into the mound they give long term stability to the tree. In addition fewer roots will be killed by the lack of air which flooding creates and the tree is more likely to thrive. This technique allows a greater range of trees to be tried – for instance the Common Silver Birch, Hawthorn, Pin Oak, Rowan and Sitka Spruce. Native Oak (Quercus robur) and Ash will also tolerate some periods of dampness.