This is the projects major challenge, and may require heavy adaptation to avoid this scenario. Please discuss your suggestions to avoid non-native seeds getting into jet streams and landing where they are no-longer native species
My first thoughts here are that we could attach a sponge underneath with the most needed region specific seeds - which will be identified through public debate at a later time.
The sponge would fill with moisture on any long trip, become heavy enough to bring the balloon down. I wonder if some natural physics discussions forums might be able to help us with whether that is a practical/robust solution
My second thought is that we make all balloons fairly heavy. There’s no reason they need to be as light as today’s standard balloons. Then we arrange people to release them from high, either tall buildings or hot air balloons.