A raspberry is not quite a tree, but I hope this is the best place for this.
I family emergency has meant that our 12 x 24m and 100 plant raspberry patch has been left untended for a few years. Half is overrun by weeds, and there are still small plants that easily transplant, but where the woodchip went down thick enough, we have a forest of raspberry canes. It is now early summer in NZ, and so the second year canes are giving us our first abundant crop, and I will cut them in a month when they stop producing, but this year's canes are now a thick forest almost as high as me. I want to thin them so they are pickable and don't mildew, and also to establish another patch in our "pick your own" paddock.
They are a first-year autumn and second-year summer variety.
If I cut the canes down about a foot high to move them, what will happen? I am not so interested in fruit this year, but production in following years.
Most temperate cane fruit, in my experience, can tolerate a hard prune. My first winter having too many tiny puppies in what I had thought was a fine-sized back yard resulted in an unintentional hard-prune over the winter. They all came back with a vengeance. These were a variety that yielded twice a season, and we got fruit late that summer.
In retrospect, I think I would have probably thought about pruning off the late-season blossoms to encourage more vegetative plant growth, but I wouldn't worry about them dying.
Although this might not hold true for all types, or if they aren't used to growing back all their above-ground vegetation because of a hard winter. Do you get winter?
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein
If you need to thin the numbers of canes, you need to prune at the soil surface or even below the soil surface.
If you want a "brier patch" then cut them about 20 cm above the soil level.
Raspberry and Blackberry react the same when it comes to pruning and the best time to do the pruning is near the start of winter (late fall)
Since NZ is heading into summer, pruning now will make a cane produce side sprouts (double or quadruple the cane number).
So depending upon what you are wanting the plants to do production wise, that is what will determine your best pruning part of the year.
As to cutting down to a foot high - what Bryant said.
As to moving them -- if this is the start of summer for you it would be a good idea to wait until late fall so that the plants are shutting down and the temperatures are not an issue anymore.
Raspberries have a shallow root system and are usually not very happy about summer heat unless shaded. When transplanted, the roots will be extra sensitive because of having been damaged. Also, consider that thinning them out also means allowing more sun to reach the ground and potentially dry out the soil. If there is still heavy mulch present then that's OK.
But, you know, in general, this is how raspberries work:
- first year: uh, did we plant enough?
- second year: okay, maybe
- third year and onwards: hey, does anybody need young raspberry plants? pleeeease?
Annie, we've got a couple of sizable raspberry patches here and the newer one is mostly made up of runners and sprouts from the first that I dug up when they made their way into walkways and garden beds. I just dig around with a narrow trenching spade, then lift them with a decent clump of soil around the roots. If it's spring or summer I pot them up and put them in a shady spot until they start growing. In autumn or winter I just plant them out where I want them to go.
We mulched heavily with wood chips winter before last and we're really noticing the difference this year. All the new growth canes are much thicker and the berries are bigger than they used to be. The old patch was getting pretty tired before we piled on the mulch and now it's looking great. Just picked a handful in there before it got dark this evening.
We have two 50 foot rows of raspberries here. I prune them back regularly in order to share excess canes with friends or to simply thin them out. I wait till the dormant season and cut them back to within three or four inches of the ground then dig. I've given hundreds of clumps away over the years with no issues.