Are you looking for something colorful to stand out on your benches as the spring wanes and the summer starts? Something that not only survives, but thrives as the days (and your greenhouse) get warmer and brighter? Look no further than the herbaceous perennial Asclepias tuberosa, or butterfly weed. In 2017, butterfly weed was named the Perennial Plant Association’s 2017 Perennial Plant of the Year, and for good reason. Butterfly weed is an excellent low-maintenance herbaceous perennial that thrives in the summer sun and heat. But the real attraction is the plant’s flowering. Butterfly weed has an extended flowering period, usually with at least two waves of flowering in the landscape. Aside from its landscape performance and ornamental appeal, butterfly weed is best-known not only as a food source for monarch butterflies, but also as their preferred plant for them to lay their eggs on. With the increasing interest in monarch butterfly populations, butterfly weed has been selling out the past few years from the demand.
There are a few different cultivars with ornamental appeal available. The cultivar ‘Hello Yellow’ features bright yellow flowers, while the cultivar ‘Gay Butterflies’ contains a mix of colors, including shades of red, orange and yellow. Depending on your customer, there may be an interest in purchasing native genetics. If you are interested in producing a native genotype, check with your local seed supplier to see what they have, as well as their source if you want to be sure you are using sources you feel comfortable calling “native.”
You can finish butterfly weed in a variety of container sizes, from small quarts to gallons. Regardless of what size container you finish your crop in, seedling plugs are the best starting material for this crop. You can use a large plug, from 50- to 128-cell trays for reduced crop time. However, butterfly weed can also be easily finished from smaller plugs such as a 288-cell tray, though multiple plugs should be used when finishing larger containers from small plugs. Butterfly weed does not require a vernalization or cold treatment to initiate flowering and will flower its first year from seed, making it a relatively straightforward perennial to schedule and finish.
Growing butterfly weed early in the season can be challenging if you don’t have lights available in your facility. Under the short days of earlier spring, butterfly weed growth is minimal, as plants cease developing and can form tubers. However, either day-extension or night-interruption lighting is useful in creating long days to promote growth. Later-season crops, whether grown in a greenhouse or outdoors, won’t require lighting, neither to increase the quantity of light to promote growth nor extend the day length. The longer days of April and onward are sufficient for avoiding dormancy, and by this time the ambient light intensity in your greenhouse is likely reaching or surpassing the 10 to 12 mol·m–2·d–1 minimum daily light integral for good crop growth.
In addition, this is a warm-season perennial. In the landscape this is one of the later perennials to emerge, as it grows in response to warmer soil and air temperatures. This is another reason why early-season crops of butterfly weed need warmer temperatures than other perennials for forcing. However, this is also what makes this crop a great season-extender; this crop is well-suited to the warm temperatures outdoors and in the greenhouse that come after the exodus of spring annuals from the benches.
Unlike the related swamp milkweed (Asclepias incarnata), butterfly weed does not respond well to overwatering or excessive substrate moisture. Careful moisture management is important for butterfly weed. Early crops grown under lower light and cooler temperatures in the greenhouse may need a looser mix to promote substrate drainage and drying-down, whereas later crops can utilize a tighter mix since warmer temperatures and higher light intensities will promote transpiration and drying down. Like irrigation, butterfly weed is not a heavy-feeding greenhouse crop. Using 100 to 150 ppm N from a complete, balanced, water-soluble fertilizer or 3 to 5 lbs/yd3 of controlled-release fertilizer (three- to four-month release period) produces healthy, marketable container-grown plants. Remember to reduce the amount of controlled-release fertilizer used for growing containerized butterfly weed in the summer heat, as the increased temperature will cause nutrients to release more quickly.
Butterfly weed is a crop with bold and beautiful flowers, and the potential for visual and ecological impact, in addition to being well-suited for the warm and bright days from the tail end of spring into summer. Consider featuring this plant as part of any summer crop program.