Yes! Using a water-soluble plant food, such as Peter's, is a good idea because the leaves can absorb the nutrients. I suggest doing this on a cloudy day and in the evening, as the sun and plant food mix could cause leaf damage. My other suggestion is to use only half a teaspoon, so that the product is diluted by half when used to spray the leaves.  Keep in mind that most trees and shrubs do not need extra fertiliser this time of year. Annuals such as hanging baskets and those in containers are the most likely plants to benefit from additional fertilizer in the summer. Keep growing, Marianne Binetti