Bonsai Bullets

Bonsai bullets consist of a sphere of the kind of biodegradable fibrous material used to make some pots for seedlings.
Contained within each bullet is a concoction of damp soil, nutrients and a maximum of 3 tree seeds (Something small is preferable - e.g. apple, or seeds of a similar size).
The bullets can then be propelled towards unreachable places, such as cliff walls / the sides of houses.
A habitat such as a cliff wall is the perfect environment for a tree to naturally grow in miniature, since it will have to make do with what few nutrients it can get its roots on - the brew contained inside the sphere is a booster, similar to the one which would naturally get a tree into this position (i.e. Bird poo).