You don't rescue the the gardener until about halfway through Khrysalis, towards the end of part 1. Until the gardener returns to Bastion, the only way to get rid of lvl 7 pests is to Summon Pixie.
Barley in Khrysalis has the highest level gardening spells like Ant Lion (which kills any level pests). In order to access the spells you have to complete a series of side quests that list completion of the Eclipse Towers as a prerequisite. You also have to be at least level 15 in gardening.
First you need to become a rank 15 gardener, then you will get a gardening quest in Bastion, Khrysalis. When you finish the quest, you will be able to buy the rank 7 pest spell from the gardener in Silent Market, Khrysalis.
