Snow drops can be planted at different times of the year.
In the autumn Snowdrops bulbs are on sale ready for planting, which is the cheapest method to acquire Snowdrops, but not always the most successful. Snowdrops bulbs seem to be harder to get going than other bulbs. Few suppliers will give an assurance that the bulbs are freshly lifted. Bulbs which have been in garden centres and hanging around for some time can dry out, which make them more difficult to grow. Buying Snowdrops in the green, avoids this problem and can be more successful, although more expensive.
Snowdrops sold in pots during the winter can be planted out into the garden and this is a good way of getting them going. Growing Snowdrops in containers permanently is not ideal because as with all container-grown plants, they will be more prone to drying out which is fatal for Snowdrops.
Probably the best time to plant Snowdrops is in the spring, when they are planted "in the green" which is usually after flowering but whilst still in leaf around March time. This is not as cheap as bulbs, but less expensive than buying Snowdrops ready to bloom in containers and is a good way of getting Snowdrops established.
If planting either as established plants or in the green, plant to the same depth. This means planting so that the white part of the stem is underground. Given that they flower during January -February, the best time to buy Snowdrop plants is in March and April and plant out straight away.
Dormant bulbs can be planted in the summer, bulbs are always cheaper than plants but less reliable. The risk with bulbs, and especially so for Snowdrops, is drying out. A moist soil and a mulch of leaf mould is a good growing medium. In addition, squirrels like the Snowdrop bulbs as well.