Guerrilla Gardener's Blog

Seed balls, simply put, are a method for distributing seeds by encasing them in a mixture of clay and compost. This protects the seeds by preventing them from drying out in the sun, getting eaten by birds, or from blowing away.

Seed balls are scattered directly on the ground, not planted. Self-sufficiency and sustainability website Path To Freedom says seed balls are useful for seeding dry, thin and compacted soils and for reclaiming derelict ground (which is why they are often used in guerilla gardening). Seed balls are particularly useful in dry and arid areas where rainfall is highly unpredictable.
You can “sow” your seed balls on a sunny day – and just leave them. When sufficient rain has permeated the clay, the seeds inside sprout and are aided by the nutrients and beneficial soil microbes surrounding them.

In fact, the seed ball method has been working for centuries. It seems that North American First Nations’ tribes used seed balls. More recently natural farming pioneer Masanobu Fukuoka has experimented with them. And in New York City, seed bombs were used in 1973’s revitalization of the Bowery neighbourhood and the development of the city’s first community garden.
(Taken by blog: http://heavypetal.ca/archives/2007/03/a-brief-history-of-the-seed-ball)

Here’s a picture of these first seedbombs, probably less “eco-friendly” than the new ones made with clay:

Click to enlarge