Draw out the heart shape for your bamboo garden onto the ground with chalk spray. This will guide your digging.
Dig out a trench that is 1 foot deep and 2 feet wide.
Set up the cardboard so that it is 2 inches away from the walls of your trench. Use boxes for support between the two cardboard pieces. The pieces must be stiff enough to support the weight of the concrete as it sets.
Mix the concrete according to the manufacturer’s directions and pour it into the space between the walls of your trench and the cardboard. The easiest way to do this is with a shovel. You’ll want the concrete completely flush with the top of the ground.
Let the concrete dry completely. This takes between 24 to 48 hours. Follow the recommendations given by the manufacturer for best results.
Backfill the trench with dirt, leaving the cardboard boxes and pieces in place. The cardboard will decompose naturally, so there is no point in upsetting the wall.
Tamp down the dirt with your feet and keep filling until the trough is completely filled.
Plant your bamboo rhizomes 1 foot apart around the entire heart shaped bed. The concrete will keep the rhizomes from spreading outside of the bed. However, you’ll still want to keep an eye on the bamboo as it can occasionally jump the wall and spread into the yard.
Pick two young bamboo shoots out of your developing bed. The shoots should be next to each other at the base.
Bend the shoots into a heart shape.
Bind the shoots at the base and the top fold where they touch with plant binding tape. The heart shape will grow as the bamboo shoots grow and remain in shape.