I have several high-yielding ‘Black Satin’ thornless blackberry bushes. The blackberries are large and plump, however, they are so tart I can only use them primarily for jams. The bushes are about four years old with good sun exposure. Is there something missing that might sweeten them, or is ‘Black Satin’ supposed to be tart? What about fertilizers – what should I be using? Currently the bushes get frequent waterings, but I don’t fertilize or spray.

The ‘Black Satin’ cultivar of blackberry is a semi-erect, thornless blackberry. The thornless varieties are generally less sweet than the erect (thorny) berry. Research data does not show any greater yield from erect blackberries than from thornless blackberries. Erect types generally do not require a trellis, so other than the thorns cultivation is the same. ‘Black Satin’ is considered to be a tart berry. It is recommended for jams, jellies, and pies. So your experience is not atypical. ‘Black Satin’ has only a fair winter hardiness, so with the fairly mild winters that we have experienced over the last couple of years, you most likely have seen little winter die back.

No, there is not anything that can be done to make ‘Black Satin’ grow any sweeter. Blackberries benefit from a great deal of organic matter in the soil. As the amount of organic matter increases, the need for fertilization drastically decreases. Three to four top dressings of fully cured compost would be recommended, as well as an early spring and mid-summer very light feeding of a balanced fertilizer (10-10-10). Be careful not to apply too much nitrogen. The recommended application rate for an established planting of blackberries is only 30 pounds of actual nitrogen per acre per year. Too much nitrogen will yield a lot of leaves but poor fruit production.

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