There is a pretty easy and effective way of reproducing blackberries from root cuttings.
When its the best time to propagate blackberries from root cuttings?
Best time to reproduce your blackberry plants its in winter or early spring, when the carbohydrates are still in the roots of blackberries. So lets say between December and March, depends on each variety, on how much hibernation or chilling hours each variety needs. In general, you should propagate them before the bud break occurs.
Not all the varieties will propagate the same with this method, but in a nutshell, the principle it is the same.
How to propagate your blackberry plants from root cuttings?
1. Prepare the soil, which should be pretty much the same as for planting blackberries, but in finer granulation if possible, so the big particles won't disturb you at transplanting them. How to prepare the soil, we discussed in blackberry soil preparation for containers.
2. Collect healthy looking roots from your blackberry plants, which should be at least 2mm (1/8inch) thick, and put them in a bucket with water, so they don't dry out.
3. Cut the roots to a desired length, which should be at least 5cm (2inch) in length, and place those root cuttings into a container with soil.
4. Cover your root cuttings with a bit of soil, just as much so the roots get covered. 1cm (0.4in) of soil is more than enough.
5. Water the soil, so it will be moist, but not to much, make sure your container has the drainage holes in the bottom.
6. Cover the container with plastic foil for moisture retention, and place it somewhere on a bright place with a room temperature. The higher the temperature, the faster they will begin to sprout.
7. Depends on a blackberry variety and the propagation temperature, but somewhere after 2 to 3 weeks, young blackberry plants should start to sprout out of the soil.
8. Wait until they grow up to 3 small leaves, then gently dig them out of the soil. Now if the root is too long, you can cut the sprouted root cutting along with the root to 2.5cm (1inch) in length, and dig the non-sprouted root back into the container.
9. Gently transplant the blackberry root cuttings into a new pot or container, where they will start to develop their own roots. Place the containers into a propagator or under a foil where you still want to retain the same humidity as they had in first container.
10. After 2-3 weeks, you can start lowering the humidity by opening the ventilation holes in propagator or making the holes into the foil, this is called hardening off process. Just sure you don't take their humidity away too quick.
11. Take care of your propagated blackberries from root cuttings!