Verbena bonariensis is the botanical name for what is commonly called purple top verbena or tall verbena. It is a perennial plant that grows taller than other verbena varieties, reaching heights of 3 to 6 feet. The flowers form in clusters at the tip of each vertical stem and persist through the fall. Propagating Verbena bonariensis can be accomplished by taking a herbaceous stem cutting and planting it to develop a new root system.
Wait until the early to midsummer when the Verbena bonariensis is in full bloom and has an abundance of stems. Locate a healthy plant to take the cutting from.
Take the cutting from the Verbena bonariensis early in the morning before the sun falls on the stem. Trace a healthy stem with no flowers back by at least 3 inches. Cut the stem off using clean hand pruners or a sharp knife.
Pull off all of the leaves from the lower 2 inches of the stem and throw them away.
Pour 1 to 2 tsp of rooting hormone powder out onto a plate. Dip the cut end of the verbena stem into the powder and then discard the remaining powder.
Fill a 4- to 5-inch diameter plant pot with compost until it is 1 inch from the top. Poke a 1-inch deep hole in the centre of the compost using your finger. Insert the powdered end of the verbena stem into the hole and press the compost up around it.
Water the soil until it is completely damp to the bottom. Place a clear plastic bag on top of the pot and slide a rubber band around the upper rim of the pot to hold the bag in place.
Place the pot in bright light but not direct sun. Check the pot after approximately six weeks for white roots coming out of the bottom.
You can take Verbena bonariensis cuttings all the way through September.