I would suggest a water-soluble type of fertilizer with a balanced NPK formula. The brand is not important, however, the formulation should contain at least 10% nitrogen (N) and near equal amounts of phosphorus (P) and potassium (K). A general purpose garden formulation would be good. Apply the fertilizer at two- to four-week intervals at rates suggested on the product label.

 

The plants should respond with improved growth in 14 to 21 days.