One of the easiest ways to propagate red valerian is by dividing it. Perennials with tuberous roots like valerian can simply be split in two and then planted as two separate plants. The best time to divide red valerian is in spring just before it produces its new growth for the season. But while red valerian tolerates division, it still delivers a shock to the plant. Make sure your plant is well-watered and well-fed before dividing it. Sick or weakened valerians are unlikely to survive division.
Dig the red valerian up or remove it from its pot. Take care not to damage its roots. Use a shovel to dig a circle around in-ground valerian that is roughly 1.5 times the circumference of its leaves. Loosen potted valerian by running a knife around the inside edge of the pot. Then pull the plant out carefully (do not force it) by its base.
Split the valerian's root ball in two. Use a sharp, disinfected knife to cut the plant right down the middle from the top down.
Plant each half of the valerian plant. One can return to its original pot or container. Plant the second in a new container or spot in your garden. Plant the valerian at the same height in the soil as it was previously growing.
Water both valerian plants until the soil runs out of its pot. Garden-grown valerian should be watered until the soil is moist to the depth of its roots. Continue to keep the valerian's soil moist at all times until the plant establishes itself and produces new growth. Then resume your regular watering schedule.