Blackberries re-grow their wood each year. Last year’s canes produce fruit this year and then die as soon as the fruiting is complete. In the meanwhile, the new canes are growing vigorously and are well underway while the berries are ripening. As soon as the old canes die, they can be removed to make way for the new growth. This is the one major job of the season for blackberries.
Your goal is to create a chest high hedgerow (or box hedge) about three to four feet in width. Pruning the shoots as they grow is very important. The more you prune, the more berries you will have. Dr. Moore, one of the nation’s top experts at the University of Arkansas, said that if you did not prune a blackberry plant, you would have a vine about twenty feet long with a few berries on the tip end of the cane. The more you prune, the more the vines branch and the branching creates the berries.
Spraying is seldom needed for blackberries. When the fruit is ripe, thrips and stinkbugs can be a problem. Sevin will kill both of them and the fruit can be eaten soon after spraying (read the label). Sevin is not poisonous to humans or pets. Spots on the leaves indicate a fungal infection, particularly with thornless varieties, and will require a fungicide for control.
Mulching is very helpful. Besides conserving moisture in the soil it keeps the berries in the lower part of the bush clean. If you have bare ground under the bushes, rain will splash soil up onto the leaves and fruit, rendering some fruit unusable. Washing the fruit just before eating is much better than washing soon after harvest.
Blackberries require a lot of nitrogen. Most publications call for one pound of pure nitrogen per 100 feet of row before harvest and the same amount after harvest. Ammonium sulphate (21-0-0) is an inexpensive fertilizer and is good for the alkaline hill country soils. At 21% nitrogen it takes about 5# of it to make 1# of pure nitrogen. It also contains sulphur which helps acidify the soil and this releases iron to keep the plants green.
A common problem for blackberries in alkaline soils is that the plants cannot get the iron they need (iron chlorosis), which you will see as yellowing leaves with green veins. The quickest way to solve this problem is to use an iron product called “LibFer”, which will turn the plants green again in a few days. Acidifying the soil will do the trick, but is slower.
The key to success is applying lots of fertilizer and frequent watering. Heavy watering is counter productive. Prune the growing vines at every opportunity and make them grow just like a box hedge. Well-trained hedges can produce a gallon of berries per foot of row.