Originally Posted by Unregistered

In my experience, bosses and gardens spawn completely independently of each other. There is no benefit to SG spawn rate from holding on to raid bosses...you'll just spawn nothing if the RNG determined that you were supposed to spawn another boss. Of course you may wish to avoid killing the boss for other reasons, but it won't get you any more SGs.

This is correct, you will often find you get no spawn when a raid boss is up, this can be because you have a boss and you were meant to spawn another.