It is best to make seed balls in spring or fall as the weather will still be cool as they are drying and you will have time to get the balls distributed while there is plenty of rain.
With seed bombs you can make a beautiful variety of gardens! When you know the type of soil you are planting on etc you can adjust your seed mix accordingly. I have tested out several mixtures of seeds, the one that worked the best on hard, depleted, stony and packed urban soils, was a mixture of crimson clover with diakon and wildflowers. Any kind of clover is great for nourishing the soil. Diakon radish is added because it is such a powerful plant, I have seen images of a huge diakon growing out of asphalt in Japan -- diakon are great at helping to break up earth that is packed and rocky so that roots can grow deeper. Wildflowers add some diversity and I usually like to put in a couple sunflowers.
Whenever I make and/or distribute these seed balls I send thanks to Masanobu Fukuoka for developing this method and think of the words of Masanobu Fukuoka:
Masanobu Fukuoka was my inspiration for getting into guerrilla gardening. His book The One Straw Revolution describes such a beautiful way of life. Check out this video to see Masanobu Fukuoka demonstrating how to make seed balls.
Whenever I am out watering my garden I look at the large intersection and gas stations that are all around it and just imagine a day when the whole area is covered in wildflowers and trees. Sometimes it is hard to do. Every day there is new trash to pull out of my garden (I know it looks like I am just cultivating weeds, but I love those kind of plants.) Sometimes my flowers get stolen :( Sometimes they get mowed down, and that is when I cry the hardest.
This little garden is going to take over this whole intersection someday. In the meantime the bees just love it.