Jennifer Schultz Nelson
Extension Educator, Horticulture
I've been wondering for the last month or so why my blackberries have not set fruit. People have said to me in the past that since I have my Ph.D. in Horticulture I must not kill plants or make mistakes in my garden. On the contrary I tell them-- I just kill plants and make mistakes in a far more complicated manner than most. My blackberries are no exception.
Blackberries are members of the genus Rubus, one of the most diverse genera in the flowering plant world, containing hundreds of species. Rubus is the genus also known as the "brambles", which includes raspberries and blackberries.
I grew up eating raspberries in our home garden, but never blackberries. I am from northern Illinois, and most blackberries aren't reliably hardy there. A good cultivar for colder climates is 'Illini Hardy'. University of Illinois developed this very cold hardy cultivar, which produces great tasting fruit if you can manage to get past the millions of thorns on its canes without losing a pint of blood.
I much prefer the thornless blackberry cultivars, the first of which, 'Everthornless', was developed at University of Illinois by my favorite professor, Dr. Bob Skirvin.
I have one thornless blackberry cultivar in my garden, called 'Triple Crown'. This is the fourth growing season for my small patch of three plants. Last year they produced over a gallon of blackberries, each as big as your thumb. It was more than enough to make some great tasting jam. Excited by this relatively large harvest (well, the largest I've had to date), last fall I decided to pay these plants some extra attention and make sure they were even better this year. In my excitement I ignored some important details on how blackberries grow differently than my raspberries.
Blackberries and raspberries are perennial crops, but the shoots, or canes that grow are individually biennial. That means that each individual cane will only live for two years.
The first year of growth, the canes are called "primocanes". In blackberries and some raspberries, the primocanes do not produce flowers and fruit. In the second year of growth, the canes are called "floricanes" and they produce flowers and fruits. If you just let blackberries and raspberries grow without pruning or cutting back, there would be a mixture of primocanes and floricanes present at any given time.
There are raspberry cultivars available that are called primocane fruiting or autumn fruiting types. These will flower and set fruit on the primocanes, which means you can harvest fruit from them the first year they are planted. They will set fruit on the same canes in the second year (now called the floricanes), but in general this second crop is of lesser quality. These are the kind of raspberries I have in my garden.
Recommended pruning for blackberries and raspberries in spring is to remove the dead canes, leaving both primocanes and floricanes intact. Remember that floricane fruiting raspberry cultivars and all blackberries must be pruned this way if you want to harvest any fruit—they only produce fruit on the floricanes, so you can't remove those and expect to harvest any fruit!
Autumn or primocane fruiting cultivars may be cut back to the ground after last harvest in the fall. This channels all the plant's energy into producing primocanes the following spring, which for these types yield the best quality fruit. It also reduces the chances of diseases overwintering in plant debris.
This is where my big mistake comes in. Last fall, for the first time, I mowed my raspberries down to the ground. They are primocane, or autumn fruiting types. So all that came up this spring was primocanes, and I have a bumper crop this year. That's the good news.
The bad news is that I mowed my blackberries down the same way. I forgot that although I had a lot of floricanes which flowered and produced fruit last year and should have been removed, there were also plenty of primocanes, which would have produced fruit this year, had I not mowed them down last fall.
I should have only removed the canes that had flowered and produced fruit last year, then in early spring trimmed away any parts of the primocanes that had been killed by winter cold. The ends of blackberry canes are prone to winter kill, so pruning should wait until the spring.
Instead I am looking at a tangled mass of primocanes this year, wondering where the fruit is. So this fall I will just leave them alone and trim the canes early next spring to a manageable height, probably about four feet tall. Then I should have an enormous crop, since the plants had an unexpected vacation from producing fruit this year!