What to do with runt Blackberry plant?

I planted a row of 11 Ouachita blackberry plants as bare root cuttings in mid Feb. (along with a lesser number of other varieties) Of the 11, initially 10 plants came up within 2-3 weeks and are now a bit less than knee high on average, the 11th one started to show signs of life a couple of weeks later (about a month ago), but has grown very little since, it is still only 2-3 inches tall and has a handfull of small fingernail size leaves…

My question is should I leave it or should I pull it to prevent whatever might be stunting its growth from spreading?

You did not mention the source of the cuttings. Did you propagate them? Did they come from a nursery as virus indexed plants, or tissue culture plants? Its hard hard to say which is the best choice since blackberries can suffer from many virus problems, some which are contagious and destructive. Its not unusual for new plants to be virus infected, but tissue culture plants suffer from less problems. I know of one blackberry research trial that failed because so many of the new plants suffered major virus problems, although the plants came from multiple certified nursery.

To be safe, I would move the runted plant to another location away from other brambles and see how it performs. If it fails to thrive, send it to the trashcan, not the compost pile. Be on the lookout for orange blisters on the leaf surface. If you see them you have orange rust - get rid of the effected plants fast! It causes stunted plants and is fatal. Ouchita is not highly succeptable to this disease like Navaho is

How did they get rooted? My guess is that one had many fewer roots. If all the cuttings came from the same plant it is doubtful its a virus. But if the leaves grow funny-shaped pull it immediately.

I planted some blackberries a few years ago, some of the small tissue culture plants were almost dead and they grew maybe 4" in the first year. They then got about 2’ tall in the 2nd year. I am hoping for big growth this year,.

The blackberries came from one of the authorized propagatoion nurseries and while the root development varried, I don’t recall anything particularly off about this specific one. At this point I will probably go ahead and pull it to be safer as soon as things dry out some from this weeks rain.

I may give it a few more days as it has grown visibly in since I checked on it 48 hours ago.