The name "lilac verbena" may refer to the plant Verbena lilacina found in Western states or the ornamental annual or perennial Verbena hortensis and hybrids that have lilac-colored cultivars. In either case, verbenas prefer silty, well-drained soils. Growing these flowers in clay soil may prove a challenge unless the site or the soil can be amended.
Clay soils are characterized by super-fine particles that hold water and nutrients extremely well. If you can grab a handful of damp soil and clutch it in your fist, and it stays together in large clumps, the soil has high clay content. To test the permeability of your soil, dig a hole 1 foot in diameter and 1 foot deep, and then fill it with water. If it takes less than two hours to drain completely, you shouldn't have too many problems. If it takes more than four hours, the roots of your verbena may drown or be vulnerable to disease.
Verbenas can absorb nutrients best at a pH range of 6.0 to 7.5, and many clay soils can have a much higher alkalinity. Alkaline soils inhibit nutrient uptake even when sufficient nutrients are in the soil. Acidic soils can "burn" lilac verbena because they absorb more nutrients than they need. When clay soil holds too much water, it can invite anaerobic bacteria to infest the roots of verbenas. Anaerobic bacteria thrive in the oxygen-deprived conditions, such as waterlogged soil. Beneficial aerobic bacteria, on the other hand, die off.
Grow verbena on slopes of your yard, in raised planters or on a mounds so that the drainage of your clay soil isn't as much of a concern. If there is no such spot in your yard or garden and you can't amend the soil, grow lilac verbena in pots. Since all varieties of lilac verbena attract butterflies, plant them amongst other butterfly-friendly plants in a wind-sheltered part of your yard or garden
Test the permeability of your soil. Amend with compost and sharp sand to improve the drainage, and then test the pH with a home test kit or by sending a sample to your local extension service. If the pH of your well water or municipal water supply is extremely acidic or alkaline, keep an eye on the pH of the soil through time and amend as necessary. Compost added before planting and used as mulch should be more than sufficient to keep lilac verbena growing and blooming through the growing season.
Plant lilac verbena amongst butterfly-attracting plants that tolerate clay soils. Golden yarrow and yellow cosmos tolerate clay soil, attract butterflies and produce blooms that contrast lilac hues dramatically. Echinacea, butterfly weed and dill are all host plants for butterfly larvae and can be planted near lilac verbena in amended clay soils.