Originally from South America, Verbena bonariensis is a perennial in the southern United States and has naturalized there, but it isn't considered invasive. The plant, also known as tall verbena, has grayish-green basal foliage and lavender flowers atop three-foot stems that lend a transparency to the garden. An annual in areas north of USDA Plant Hardiness Zone 7, this verbena makes up for its susceptibility to the cold by seeding itself throughout the garden for years. The plant prefers full sun and is drought-tolerant. Gardeners pressed for time can let nature do the propagating, but for those who want a little control, Verbena bonariensis will respond to traditional propagation methods.
Leave the flower stalks intact on your Verbena bonariensis plants and let the seeds ripen. Harvest the seeds in early November. Cut off the flower stalks, take them indoors and shake each flower head over the open envelope. The seeds will be small and brown. Do not bother trying to separate the seeds from the other dried material from the flower.
Set the envelope in a cool, dry place for winter.
Transfer the seeds into a zip-top in late February and chill them in the refrigerator for about a month.
Take the bags from the refrigerator. Fill the pots with the seed-starting mix and press down gently. The soil level should be about an inch from the pot rim. Place the seeds in an amount the size of a dime on the surface of the seed-starting mix. Do not cover them. Water the pots from the bottom by placing them in the kitchen sink in about an inch of water. Let the water drain.
Place the pots in a warm, dark place such as a heated basement. According to gardening expert Rob of the Rob's Plants website, the seeds should germinate in about three weeks. Keep the seed-starting mix moist but not soggy.
Move the pots after the seeds germinate into a sunny window or under fluorescent lights for 10 hours a day. Keep the soil moist.
Keep one or two of the healthiest seedlings in each pot after the plants are three inches tall. Remove the rest by gently pulling them out.
Move the pots outside during the day in late April or early May to get the plants accustomed to the outdoor light levels and temperatures. Set the pots in a shaded, protected location and bring them in at night. Check daily to see if the soil is dry an inch below the surface. If it is, water until the soil is moist. After about a week, leave the plants outside at night if the temperatures will be above 40 degrees Fahrenheit. Transplant into the garden after all danger of frost is past.
Take stem tip cuttings from the plants anytime during the growing season, according to Fine Gardening online. Before taking the cuttings, fill the pots with the potting mix until the mix is an inch from the rim of the pot.
Remove the bottom set of leaves with the scissors. Cut the stem just below where the bottom leaves were cut off. Dip the plant in the rooting hormone, making sure to cover the cuts you've made on the stem with the hormone. Although rooting hormone products aren't toxic, Steve Silk, writing for Fine Gardening online, recommends using plastic gloves while performing this step.
Make a hole with the tip of your index finger about an inch deep. Set the cutting in the hole, pushing down lightly. Push the soil around the stem to give stability to the cutting. Set the pots with your cuttings into a sink filled with water to about an inch deep. When the soil is moist, let the water drain.
Place the cuttings outside in a shady location. Keep the soil moist but not soggy. After about two weeks, the cuttings will begin to root into the soil. When you feel some resistance when gently pulling on the stem, the plants have rooted. Steve Silk recommends getting the plants used to more sun and wind for three days before planting in the garden. Prune the tips of the plants to encourage a full-looking plant.
Divide Verbena bonariensis in early spring in the areas where it is a perennial. Water the plant the day before so that the soil is moist.
Lift the entire clump, being careful to get under the rootball with the shovel without damaging too many roots.
Lay the clump on its side and take a knife and cut the clump into quarters, allowing each section to have enough roots and top growth to survive on its own. Replant in a sunny area with well-drained soil.