Boomseeds are green seeds that blow up. They have a range of about a phantom's range. You can find them in various adventures and they can help you get through your quest.
They originated in Meet Cosmo where some of Cosmo's scouts have gone missing and the alpha had to use a list of ingredients to make an extraordinary seed to defeat the Phantoms and blow up the boulders blocking many entrances. After the Jammer has collected all of the ingredients, the alpha stirs his pot and makes Boom Seeds.
Boomseeds can be used for various situations. They can be used to destroy a phantom, destroy a boulder, or destroy a phantom web.