Why did my blackberry plant fail to set fruit?

Question

I planted a dwarf everbearing blackberry ("Baby Cakes" cultivar) in a container this past spring. After growing vigorously through the summer to fill a 30-35 gallon trough, it flowered voluminously on the primocanes in the fall, but none of the flowers developed into fruit.

Why might this have happened, and is there anything I can do to encourage fruiting in the future? I have read that blackberries can suffer from incurable "sterility," in which plants reliably flower but never fruit. Is that a likely explanation in this instance?

Additional (possibly irrelevant) background: I am in zone 7a. The plant is on a roof deck in full sun. There is abundant pollinator activity where it is planted, although the blackberry plant does not seem to be the bees' favorite. Right after that plant was planted in the spring, it produced a single bloom on a floricane, which did develop into a berry.

UPDATE: With the benefit of another growing season of experience, it is clear that the plant is not sterile. In the spring/summer after I wrote my question, the bush set abundant fruit.

However, it once again did not produce much fruit in the fall. The blooms from the late summer/early fall generally failed to develop into fruit. In the mid-fall, a decent number of blooms did start to develop into fruit, but those fruit were a week or two short of maturity when the cold weather hit. This seems possibly consistent with the theory given below that the plant is not truly everbearing in my climate, at least when it is young, although its modest success in setting fruit this past fall could be a good harbinger for future years.

Answer 1

According to Monrovia, 'Baby Cakes™ is an exciting, new, thornless dwarf blackberry that presents a fireworks-like display of large, juicy berries in the summer! Its compact habit makes it perfect for patio pots. In most regions, this blackberry will produce twice in one season. Produces on second-year canes.'

It says in most regions it produce twice in one season, but not in all regions. It also says it produces on second-year canes. Your primocanes were created this summer, so should not expect them to bare fruit until they are in their second.

Working as a nurseryman for many years, I have often found that when new varieties come out, they sell them as the most amazing berries to ever come on the market, but when it comes to the home garden, we discover otherwise. You have to remember the growers have ideal growing conditions, where they are able to get these plants to produce in ways that you and I will never be able to reproduce.

Monrovia has three locations in the United States. The two biggest are in Oregon in a region known for producing amazing edible and ornamental plants for the nursery trade. The largest growing operation is in California, where they have some of the most prime weather for growing amazing plants. Bountiful sunlight, warmth and very long seasons.

I think it is one of these two things or a little of both;

Maybe it needed time to settle in, before it starts to show you ever-bearing characteristics.

OR

The grower is able to produce what only can be produced under the most ideal conditions. Ideal conditions start with knowing the exact chemical make-up of their soil, knowing when and how exactly to fertilise this particular plant. Not just blackberries, but this variety of blackberry. You will not have some expensive equipment to read data and laboratories to analyse your growing conditions.

In the end you will have fruit that you grew yourself. You can enjoy it knowing you did that.

Answer 2

My inclination would be to blame the pollinators. I have a very similar problem with pear flowers. The trees get many visits from bumble bees but they don't seem to do a thorough job of the pollination or they are not in large enough numbers. Honey bees are much more conscientious, but still the pear flowers are not one of their favourites and they will prefer to travel to get better nectar and pollen.

There are some plants, such as Bidens, which are magnets for bees. See if you can get some Bidens hanging baskets or pots and arrange things so that the bees have to get past the blackberries to get at the Bidens.

At least as an experiment to confirm my suggestion, you might consider hand pollination. With a fine artists paint brush or feather, dab over your flowers keeping in mind that they are self-fruitful so you don't have to look for male flowers. Your job is simply to make sure that loose pollen actively gets moved about on the flower. When walking by the pot in flower give the pot a vigorous wiggle or shake from time to time, preferably in sync with Brubeck's "Take Five".