I am thinking you may not need to fertilize the hedge at all. Lavender is a very drought tolerant perennial that does well in rather pool soil as long as it gets good drainage. In the wild lavender thrives on rocky hillsides and will rot in rich, moist soil. To keep your lavender compact and producing blooms shear it by cutting it back by one third right after it blooms. Because you have a long hedge, I suggest you try a little experiment. Add some all-purpose Osmocote at half the recommended rate to the soil around just one section of the hedge. Now see if the plants respond better or it they grow tall and floppy with the added nutrients. Then you'll have your answer for years to come. Keep growing, Marianne Binetti