Some of your fruit trees may have started to blossom already, and you want as many bees pollinating your flowers as possible for a great harvest of fruit later on.
Borage is perfect for bringing in the bees as is Phacelia and red or white clover.
I plant comfrey round each tree, so it brings up nutrients such as selenium from the subsoil while itself is high in potassium. The comfrey also provides cover for the soil around the tree and prevents it drying out during the hot summer months.
Other plants which mine the subsoil are the spring bulbs such as daffodils, Erlicheer and jonquils and they look great as the first sign of spring.
Yarrow has flowers which provide food for predatory wasps and other beneficial insects,
while nasturtiums are great for deterring woolly aphids on apple trees.
The more diverse variety of companion plants you have in the orchard, the more you mimic nature and the less problem you will have with pests and diseases.