Yard and garden: fall-flowering perennials

AMES, Iowa — Large gardens are colorful throughout the growing season. Many perennials can be grown in Iowa to provide blooms and interest year after year in late summer and fall. In this article, horticulturists at Iowa State University Extension and Outreach offer tips on selecting and planting tall, late-blooming perennials.

What are some good perennials that bloom in late summer and fall?

Perennials for full sun locations that bloom well in Iowa late in the growing season include pincushion flower (Scabiosa species), calaminth (Calamintha nepeta), balloon flower (Platycodon species), Russian sage (Perovskia atriplicifolia), anemone (Anemone × hybrida), New York aster (Symphyotrichum novi-belgii), showy stonecrop (Hylotelephium species) and goldenrod (Solidatego hybrids ).

Several perennials native to Iowa that bloom late in the season include black-eyed Susan (Rudbeckia hirta), rattlesnake (Eryngium yuccifolium), false sunflower (Heliopsis helianthoides), verbena (Verbena stricta), l grey-headed coneflower (Ratibida species), sneeze (Helenium autumnale) and boltonia (Boltonia asteroides).

For areas with moist soils, native Iowa perennials to try include Joe-Pye grass (Eutrochium purpureum), ironweed (Vernonia fasciculata), mountain mint (Pycnanthemum virginianum), obedient plant ( Physostegia virginiana) and Culver’s root (Veronicastrum virginicum). Other perennials such as hardy hibiscus (Hibiscus species) and pink tortoiseshell (Chelone lyonii) also bloom late in the growing season.

Finally, several ornamental grasses develop their most attractive characteristics in late summer and fall. These perennials have seed heads that mature and leaves that develop fall color in the fall. These grasses include Japanese silver grass (cultivars of Miscanthus sinensis) and feather grass (Saccharum ravennae, formerly Erianthus ravennae), as well as natives such as Indian grass (Sorghastrum nutans), sea oats from the northern (Chasmanthium latifolium), switchgrass (Panicum virgatum), and little bluestem (Schizachyrium scoparium).

Which shade perennials bloom in late summer and fall?

Perennials such as corydalis (Pseudofumaria lutea), coral bells (Heuchera hybrids), golden streak (Ligularia dentata), bugbane (Actaea simplex and A. racemosa), seafoam (Tiarella species), white wood aster (Eurybia divaricatus) and toad lily (Tricyrtis formosana and T. hirta) flower well in shady sites late in the gardening season.

Can I plant perennials in the fall?

Late summer or early fall is an excellent time to plant many perennials. This is also a good time to move or divide perennials, such as peony, daylily, garden phlox and oriental poppy. After planting, frequently check the soil moisture in the original root ball and in the surrounding soil. Water when the top 1 to 2 inches of soil feels dry to the touch. Continue to water as needed until the soil freezes.

To help them overwinter well, perennials planted in late summer or early fall should be mulched with 4 to 6 inches of straw, pine needles or other material at the end of fall, usually mid-November to late November in much of Iowa. Don’t apply mulch too early as it can slow the dormancy process, leaving plants more vulnerable to cold damage as they may not be fully dormant. Mulching helps prevent repeated freezing and thawing of the soil that can lift or heave plants off the ground, drying out exposed plant crowns and roots, which can cause serious damage or death. Leave the mulch in place until early spring. Remove the mulch as soon as it thaws between mid-March and late March. Winter mulching is generally not necessary once the plants are fully established one to two years after planting.

Will chrysanthemums planted in the fall survive the winter?

Unfortunately, chrysanthemums, even those labeled as hardy, generally do not overwinter when planted in the fall, even when given winter protection. Chrysanthemums have shallow, fibrous roots. Repeated freezing and thawing of the ground in winter (especially when there is no snow cover) can lift chrysanthemums from the ground, exposing roots and severely damaging or destroying plants.

Flowering mums purchased in late summer or early fall should be considered temporary additions to the landscape, much like many other annuals such as pansies, snapdragons, or ornamental kale. Spring is the best time to plant perennial chrysanthemums in Iowa. Mums planted in the spring have the entire growing season to establish themselves and generally survive the winter much better than those planted in the fall.

Shareable photo: 1. Chelone lyonii. 2. Pink hibiscus.

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