Either way, boron is recommended. There are two ways to apply boron to dahlias. One is to mix boron with the preplant fertilizer at a rate to supply 2 lbs per acre. Use caution, however. It is easy to apply too much boron.
Excessive amounts cause the foliage to burn.
Too much nitrogen may increase foliage growth at the expense of flowers. Crop fertilization based on North Carolina soil tests. North Carolina Department of Agriculture and Consumer Services, Agronomic Division. Circular No. 1.