Overplanting mosquito-repelling herbs like citronella grass, basil, lemon balm, peppermint, lavender, and catnip could potentially have negative effects on other garden plants. These herbs tend to spread quickly and can become invasive if not properly contained. They might compete for nutrients and space with other plant species in the garden leading to stunted growth or overcrowding. To prevent such issues, it's advisable to plant these herbs in separate containers or designated areas within the garden.