A million birds can't be wrong: Blackberries are juicy and delicious picked warm from the summer sun. If you crave fresh-picked, sun-warmed blackberries but balk at cultivating blackberry shrubs (Rubus fruticosus) because they gang up on you in the backyard, remember that growing aggressive plants in containers can help prevent them from taking over your garden.
You know blackberries aren't prima donnas if you've ever seen fields occupied by the spreading brambles. Just give them a location with at least six hours of sunshine a day and acidic, sandy, well-drained soil, and they'll flourish in U.S. Department of Agriculture plant hardiness zones 5 through 10.
It's not hard to find good reasons for planting blackberry shrubs in pots instead of your garden beds. If your backyard has alkaline soil or clay soil that doesn't drain quickly, cultivating berries in containers means you can avoid all the soil amendments required for growing brambles in the ground. Or if you live in a house with a small garden or none at all, containers may be your only option. Perhaps the most persuasive reason to think container gardening when it comes to blackberries, though, is their tendency to spread rapidly and clump densely, making an established "patch" of them difficult to eradicate.
Inviting a Shrub Into Your Container
Select a container for your blackberry shrub that holds 15 to 20 gallons. It should be at least 18 to 24 inches wide and 12 to 16 inches deep. If your container is deeper than 16 inches, use a trowel or shovel to add wood chips to the bottom to reduce the depth.
Fill your pot to within 6 inches of the top with 1 part peat moss to 1 part potting soil. Add several inches of organic, weed-free compost and blend well. Remove half of the soil with the trowel or shovel and set it aside for planting time.
Choose a blackberry cultivar or variety with an erect growth habit for your container, rather than one that trails. Erect blackberry species have thick, stiff canes that can usually support themselves without a trellis. It's also important to pick a cultivar that's guaranteed to be free of viruses, preferably a thornless variety, such as 'Black Satin' (Rubus subgenus Rubus Watson 'Black Satin', USDA zones 5 through 8) or 'Chester' (Rubus fruticosus 'Chester', USDA zones 5 through 9), These tend to be less vigorous and thus less likely to rapidly outgrow their containers.
Remove the shrub from its nursery container by squeezing on the sides gently. A container-grown plant can be planted at any time of the year, although early spring is best. Set the plant on top of the soil in its pot; then use the trowel or shovel to fill in around the plant with the soil you set aside. The plant should end up at the same depth at which it was planted in its nursery container. Add water slowly until it drips from the drain holes in the bottom of the container. Plant only one shrub per container.
Starting the second year, prune your container-grown blackberry shrubs early in the growing season in the same way you would if they were planted in the ground. First, sterilize the blades of the pruner by soaking them in a mix of 1 part water to 1 part rubbing alcohol; allow the blades to air-dry. Trim out any damaged canes or canes that rub against others. Then trim back second-year canes to between 40 and 42 inches and lateral branches to about 12 to 18 inches. Prune the shrubs again in winter to remove dead canes that produced fruit the prior summer. Dispose of all clippings.
Food and Water Count
Blackberries are thirsty shrubs to begin with, and container plants dry out faster than those in the ground. This means irrigation for potted blackberries is very important. Check to see if your shrub needs water by scratching the soil surface. If the top inch of soil is dry, add water slowly until the excess runs out of the holes in the bottom of the container. In hot, dry climates, you may have to check soil moisture and irrigate a few times a week or every day. Using a few inches of organic mulch to cover the soil helps hold in moisture, but keep it several inches away from the canes.
Blackberries may be injured by excess fertilizer. Fertilize your containers conservatively in late spring or early summer by mixing 1 tablespoon of 10-10-10 or similar water-soluble fertilizer or plant food with 1 gallon of water and pouring the mixture evenly over the soil surface in each container. Repeat four to six weeks later.
Cold Protection in Winter
If you live in an area with a cold climate, you'll have to help your containerized blackberries survive the winters. According to Extension Professor Dr. Leonard Perry, University of Vermont, you can place your potted shrubs in a garage or outbuilding during the coldest months. Alternatively, if your region is cool in winter but not cold, heap soil, compost, straw or other insulating material around the pot to help hold in heat. Or, if possible, sink the container into the ground.