Cook up a Storm with Amsonia Storm Cloud

Jeremy Windemuller
Perennial growers are typically familiar with amsonia since it easily fills in the garden with its easy-to-grow, shrub-like habit.

Storm Cloud is a fantastic new cultivar most noted for its incredibly dark emerging foliage in early spring. It naturally blooms in late spring, perfect for retail sales—although you could market it with its emerging foliage. It does grow a little differently than Amsonia hubrichtii, a different species, so here are some tips and tricks we’ve found for success in our trials.

Potting & timing
The best way to grow the highest quality finished amsonia is by starting with 72-ct plugs, planting one plug per premium 1-gal. pot from late May to early July (in Zone 6) and growing them outdoors for sale the following spring. Bury the crown slightly below the soil line to encourage eye development below the soil line.

Amsonia may also be started from bare root, planting one root per trade 1-gal. pot in early spring. Bury the crown slightly below the soil line just as if growing from plugs. Grow these plants indoors at a cool growing temperature of 55 to 60F (12 to 15C) and hold them at 40 to 50F (4 to 10C) when finished. Allow for six to eight weeks finishing time in trade 1-gal. pots for bare root.

Whether starting with plugs or bare root material, use a well-drained soil mix composed of bark and peat with a pH of 5.8 to 6.5.

Amsonia requires vernalization to bloom to achieve its mature upright habit and to produce multiple stems per pot. (Bare root plants purchased in spring will already have been vernalized.) 72-ct plugs started the summer prior will need to go through a 10- to 12-week vernalization (at 40 to 50F/4 to 10C) in order to have blooming plants the following spring.

Cultural recommendations
Fertility: After new leaves emerge, use a water-soluble fertilizer at 50 to 100 ppm Nitrogen constant liquid feed at every watering or incorporate slow-release fertilizer at a medium rate. The recommended EC rate is 2.0 to 3.0 using the pour-through method.

Amsonia prefers moderate moisture and requires a well-drained potting media. Allow plants to dry between waterings.

Amsonia is daylength neutral and doesn’t require artificial lighting to flower. However, high light levels are recommended during the growing season—this is a full-sun plant. Grow this crop under clear plastic indoors or in direct sunlight outdoors.

Growing temperature: Grow amsonia indoors from bare root at 55 to 60F (12 to 15C) or grow them outdoors from 72-ct plugs under natural temperatures outside in summer. Do not force plants to bloom in spring under warm temperatures or it will cause them to stretch excessively and quickly become overgrown. Forcing under cool 50 to 60F (10 to 15C) temperatures results in more compact and desirable finished plants.

Overwintering: The best way to overwinter amsonia is in a minimally heated greenhouse kept slightly above freezing. However, they may also be overwintered outdoors under a heavy frost blanket or in an unheated greenhouse. Bait liberally for rodents over the winter and do not let plants sit wet, or crown and root rots may settle in.

Pests & diseases
This native perennial is quite pest and disease resistant when grown properly (leaf pot and rust). Generally, pest free, but occasionally scout for aphids.

Finishing Tips
Spacing: Amsonia Storm Cloud has a vase-shaped habit, so spacing isn’t typically required before plants are sold at retail in spring. If holding after flowering, give the plants moderate spacing. Good spacing also allows for better airflow, which prevents foliar diseases.

Pinching: DO NOT PINCH an emerging amsonia once they’ve been vernalized! Doing so will likely eliminate flower production that year. Also, do not deadhead spent blooms, as that will remove what will become decorative seed pods in the fall. However, when growing finished plants from 72-ct plugs the summer prior, it’s appropriate to shear the plants back by half in midsummer to create a fuller habit and clean up the habit. Don’t cut hard into the stem. Since those plants won’t be sold until the following spring, you won’t have to worry about eliminating those flowers.

PGRs: After the plants are 4- to 5-in. tall, apply a uniconazole drench at 1 ppm or as a spray at 5 ppm. An additional spray may be necessary seven to 10 days later. GT

Jeremy Windemuller is a grower and trial manager for Walters Gardens, Inc. in Zeeland, Michigan.