The wonderful thing about dill is that you can harvest it anytime after it's about 12" high. It will keep on growing, it will branch and thicken, much like any plant that you pinch and it sprouts new branches. You don't really have to wait for the seed heads to develop. The leaves have great flavor, too. Usually when the seed heads mature the plant slows down its growing. Also, don't forget to feed the dill they love a good manure or fish oil mix.