Blackberry bush doesn't produce fruit

s.osbaldiston

I have a 'waldo' thornless blackberry plant growing as a self supporting bush. I have had it for about four years and it generally produces one small group of flowers which in turn gives about 2 berries. It is about 18" tall and about 18" wide and is full of foliage. What is going wrong?

pansyface

It sounds extremely small. Has it not grown in all that time or have you kept it small by trimming it?

It should be much larger. If it hasn’t grown, I would suggest that you find it a situation that suits it better. It might repay you by flowering.

If you have been trimming it to keep it small, I think you’ve been cutting off growth that would have flowered the following year.

Lyn

With a blackberry, any branch that has fruit on needs to be cut right down to the ground, it only produces fruit once. 

Obelixx

Blackberries need space and also support for their canes.  Each year they produce new canes from the base which should be loosely tied up in a vertical bunch so they don't break.  Meanwhile, the ones they produced new the previous year need to be fanned out either diagonally or horizontally and tied to wire or cane or trellis supports.  These will flower and fruit.  

Once fruiting is over, those canes need to be pruned out at the base and then the new canes tied in to the supports to make a new fruiting fan.  A good mulch of organic matter such as well-rotted manure or garden compost and a good feed will keep them growing well.

This is what Ken Muir, a famous soft fruit nurseryman, has to say on his website.  Note the size at the end:

A relatively new early thornless blackberry, ripening mid to late July. Good resistance to cane and leaf spot and purple blotch. Thornless canes are semi-erect and moderately vigorous but are rather brittle and can be damaged in exposed situations - they should be tied in early. The fruit is of excellent quality, large and very firm with an intensely glossy black colour. Makes superb jam. The flavour is excellent. Requires 1.8m (6ft) of wall space.