I have a very large horse chestnut tree at the bottom of my garden. It is obviously very dry around the base of it and I was just wondering if there was anything that I could plant there which would survive? My wife suggested lavender, as the base does get quite a lot of sun in the afternoon/evening.
I think that lavender might struggle a bit. Not sure where you live so a bit dificult to suggest something but epimediums are hard to kill and have pretty spring flowers in either red or yellow. They also make a nest site for the local pheasants every year.
Hardy geraniums do well in dry shade. Spring flowering bulbs will usually do well under large deciduous trees. Foxgloves are also ok.
Also Vinca (periwinkle) will grow in almost any conditions if you need any evergreen ground cover. It grows here on a steep bank in total shade underneath Yew trees.