Herbaceous Perennials

Bill Devlin
Adams County Master Gardener

The topic of herbaceous perennials is so broad that I feel it necessary to address it as a multipart article.

First some definitions are appropriate. A perennial is a plant that lives longer than two years. That definition includes shrubs and trees. To exclude shrubs and trees we use the adjective herbaceous to narrow the definition to non-woody plants. A perennial is also a perennial in its native habitat, but may not be a perennial in a different climate zone. An example is a Norfolk Pine, that is an exotic pine native to Norfolk Island in the tropics off the coast of Australia, and can be grown in Pennsylvania if potted and taken in for the winter, but will not survive outside.

Out lining my four parts, Part I will discuss growth structures, bed preparation, and a few of the many varieties that bloom in March.

  • Part II will discuss dividing perennials for those who now have them as well as planting and transplanting, and a few varieties that bloom in April and May.
  • Part III will discuss after planting care, and a few varieties that bloom in June and July.
  • Part IV will discuss fall and winter care and a few varieties that bloom in August and September - October.

Growth Structures


Growth structures allow herbaceous perennials to live from one year to the next. These include bulbs, tubers, and rhizomes. Credit for these definitions is given to the on-line encyclopedia Wikopedia.

A bulb is an underground vertical shoot that has modified leaves (or thickened leaf bases) that are used as food storage organs by a dormant plant.

Tubers are various types of modified plant structures that are enlarged to store nutrients. They are used by plants to overwinter and re-grow the next year and as a means of asexual reproduction.

In botany, a rhizome is a horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes.

Bed Preparation

Successful perennial gardens start with thorough and thoughtful bed preparation. Some of the key points include: eliminating perennial weeds before turning the soil; insuring a well drained soil yet having it retain enough moisture for good plant growth; providing for sufficient organic matter in the soil; and adding fertilizer as needed.

Eliminating perennial weeds: The first step in soil preparation is to get rid of perennial weeds before you turn the first spade of soil. When establishing new beds in grassed areas or in areas where there is heavy weed growth, apply a non-selective, systemic herbicide such as Glyphosate to the area. Trade names for products containing glyphosate include Gallup, Landmaster, Pondmaster, Ranger, Roundup, Rodeo, and Touchdown. Apply this material to weeds that are actively growing, generally when temperatures are consistently above 50 degrees. Spring applications are good with fall being another time when weed control is good with this material.

Outline the shape of the bed with a garden hose and spray within the outline. It will take 7-14 days before you will see the weeds being killed. After the vegetation is brown, you can till the area. For weeds that are particularly aggressive, the first spray of Glyphosate may not control all of the plant. It is suggested that after tilling to leave the bed remain unplanted for a few weeks to see if any of the perennial weeds re-grow. If they do, a second application of Glyphosate will control the remaining weeds. It is a good idea to not be too much in a rush to plant without getting all the weeds under control otherwise you end up fighting those weeds while trying to grow perennials.

Providing drainage: Well-drained soil is essential in order to grow perennials successfully but is most critical when it comes to over-wintering perennials. More perennials are killed by soils that stay wet over the winter than by the actual cold temperatures. To ensure a well-drained site, avoid planting in low-lying areas. During bed preparation, add organic matter at a rate of about 25-30 percent by volume of soil. This translates to adding about 3-4 inches of organic matter on top of the bed and working it into about 10-12 inches of soil.

In areas that tend to have less than good drainage, raising the bed either with timbers, rocks, landscape bricks or similar materials will greatly improve drainage and your chances of growing and maintaining a perennial bed. Drainage can be checked by simply digging a hole 8-12 inches deep and filling it with water. Let it drain and fill it again. If this water drains in less than 1 hour, drainage should be satisfactory.

Adding organic matter: Organic matter is the key to improving less than great soils. There is no easy short cut and no magic soil preparation material that can take its place. Organic matter helps to improve the physical and biological properties of soils when added in sufficient amounts and to sufficient depths.

The bottom line is, don't short cut this part of bed preparation. Organic matter improves the structure and aeration of clay soil and improves moisture and nutrient retention in sandy soil. There are a variety of organic matter materials that can be used depending on availability, preference and cost. Materials to consider would include compost, peat moss, composted barks, leaf compost, mushroom compost, and composted manure. For large areas with a bulk requirement, while not an endorsement consider Mason-Dixon Farms Dairy, an Adams County wholesale dairy as a source for bulk professionally composted manure. Other dairies and horse farms may be contacted, but avoid fresh, non-composted manure. Local stores carry retail bagged quantities also.

Fertilizer rates: Generally, the fertilizer requirements for new beds consist of adding about two pounds of 5-10-5 fertilizer per 100 square feet of garden bed area. You will till this in at the time of bed preparation. If you have a new yard, consider obtaining a soil test. Kits are available at a nominal charge at the Ag Center.

  • March Blooming Perennials
  • Winter Aconite-Eranthis hyemalis
  • Myrtle Spurge-Euphorbia myrsinites
  • Snowdrops-Galanthus nivalis
  • Christmas Rose-Helleborus niger
  • Lenten Rose-Helleborus orientalis
  • Bloodroot-Sanguinaria canadensis
  • Spring Beauty-Claytonia virginica

Planting & Transplanting

Perennials can be purchased in a number of ways. The most common way is plants in quart, one or two gallon containers. These plants are already growing and afford the gardener the flexibility to select and plant through the growing season. Another way is bare-root or packaged plants. These are obtained through mail order or at garden centers and are sold as dormant material. These are available for spring planting only. If these materials are received at a time that they can not be planted immediately, keep the plants cool and keep the roots moist. They can be held for several weeks this way, thus assuring their survival prior to planting.

When to plant: Most perennials are best planted in the spring. However, with the availability of material in containers, the planting season often extends well into the summer and early fall with autumn planting continuing until the first of October. The earlier perennials are planted the better the root system will be when the plant enters the winter. Late fall plantings can sometimes result in frost heaving and loss of perennials.

Planting depth: Containerized perennials should be planted at the same depth they were grown in the container. Planting too high results in plants drying out and too low invites crown rots. Some perennials such as bleeding heart, iris and peony need shallow planting in order to flower properly. Containerized plants should be watered before planting and bare root perennials should be soaked in water for one hour prior to planting in order to re-hydrate the plants.

Transplanting: Most perennials are transplanted in the spring as growth starts or in the late summer or early fall. It is usually best to wait until the plants have flowered and then cut back by half just prior to moving. If plants are moved out of season, they may need to be shaded for several days to allow them to recover.

Dividing Perennials

A common maintenance chore in a perennial garden is that of dividing. There is no set rule as to when to divide perennials. Some may need division every 3-5 years, some 8-10 years and some would rather you not bother them at all.

Perennials will send signals to let you know that they would like to be divided. The signals to watch out for include: flowering is reduced with the flowers getting smaller; the growth in the center of the plant dies out leaving a hole with all the growth around the edges; plant loses vigor; plant starts to flop or open up needing staking; or it just may have outgrown its bounds. These are the signs to look for and not a date on the calendar.

If division is indicated, spring is the preferred time to divide. Some fleshy rooted perennials such as poppy, peony, and iris are best divided in the late summer to very early fall.

Division is usually started when growth resumes in the spring. The process starts by digging around the plant and then lifting the entire clump out of the ground. Then, using a spade or sharp knife, start to cut the clump up so that each clump is the size of a quart or gallon sized perennial.

Discard the old, dead center and trim off any damaged roots. The divisions should be kept moist and shaded while you prepare the new planting site. After replanting, water well and protect the divisions from drying out.

Division is no more complicated than this. Some perennials may be more difficult to divide than others because of their very tenacious root system. Division has as its primary goal, the rejuvenation of the perennial planting so it can continue to perform the way it was intended. Many home gardeners have found that the process of division is more traumatic to them, the gardener, than it is to the perennial.

April Blooming Perennials

  • Bugleweed-Ajuga reptans
  • Basket of Gold-Aurinia saxatilis
  • Pasque Flower-Pulsatilla vulgaris
  • Rock Cress-Arabis caucasica
  • Purple Rockcress-Aubretia deltoidea
  • Glory-of-the-Snow-Chionodoxa luciliae
  • Old-fashioned Bleeding Heart-Dicentra spectabilis
  • Fringed bleeding Heart-Dicentra eximia
  • Candytuft-Iberis sempervirens
  • Netted Iris-Iris reticulata
  • Grape Hyacinth-Muscari armeniacum, M. botryoides
  • Creeping Phlox-Phlox subulata Squill-Scilla siberica
  • Early Tulips, Narcissus, and Hyacinth Virginia Bluebells-Mertensia virginica
  • Jack-in-the-Pulpit-Arisaema triphyllum
  • Marsh Marigold-Caltha palustris
  • Trout Lily-Erythronium americanum
  • Prairie Smoke-Geum triflorum

May Blooming Perennials

  • Lady's Mantle-Alchemilla mollis
  • Common Columbine-Aquilegia canadensis C
  • olumbine-Aquilegia x hybrida
  • Sea Pink-Armeria maritima
  • Blue False Indigo-Baptisia australis
  • Mountain Bluet-Centaurea montana
  • Snow-in-Summer-Cerastium tomentosum
  • Delphinium-Delphinium x elatum
  • Cottage Pink-Dianthus plumarius
  • Gas Plant-Dictamnus albus
  • Leopard's Bane-Doronicum orientale
  • Peony-Paeonia hybrids
  • Oriental Poppy-Papaver orientale
  • Wild Geranium-Geranium maculatum
  • Wild Ginger-Asarum canadense
  • Golden Alexander-Zizia aurea
  • Wild Sweet William-Phlox divaricata

After Planting Care

Mulching: Mulch provides a number of benefits. They help to make the garden appear neater, conserve soil moisture, retard weed growth and moderate soil temperatures. There are a variety of materials that can be used as mulch. Examples would be bark, dry grass clippings, and hulls of various sorts. Mulch should not be applied right up to the crown of the plant to avoid problems with crown rots. Leave some air space between the mulch and the crown.

New perennial beds are mulched right after planting with about 2 inches of mulch. Additional mulch is applied annually as needed so that the overall depth doesn't exceed 2 inches. Apply additional mulch in the spring as soils start to warm. Most perennials will not need additional mulch in the winter if soils have been properly prepared and the drainage is good. The exception would be for perennials that have been transplanted or planted late in autumn. Here, a 3-4 inch layer of loose mulch like straw, or evergreen boughs applied after the soil is frozen, helps to avoid frost heaving.

Watering: Water is a vital part in getting newly planted perennial gardens established. Soak the plants initially after planting and then check regularly to prevent drying out. Mulching helps to cut down on watering frequency. The general rule of thumb of one inch of water per week for established plantings holds true. Less frequent but deep watering encourage perennials to root more deeply and thus become better able to handle drought conditions.

The most common and time efficient way to water perennial gardens is to use soaker hoses. Many perennial gardeners will snake a soaker hose through the garden and leave it there all summer. When water is needed they will connect it to a faucet and turn it on. To make the hose invisible, bury it just under the mulch.

Fertilization: Most perennials do not require large amounts of fertilizer if the soils have been prepared properly. Many over-fertilized perennials will produce excessive, soft growth and produce very few flowers. Many times perennials will tend to "lodge" or open up when over-fertilized.

As a general rule, unless a soil test indicates otherwise, perennials can benefit from one pound of nitrogen per 1000 sq. ft. Granular fertilizers with a formulation of 12-12-12, 10-10-10, 5-10-5 or similar are sufficient.

Weed Control: Weeds that do appear in perennial gardens are often best controlled by shallow cultivation. If the weeds are perennial in nature, quick action is needed so that the infestation does not get out of hand. Cultivation again is the key, or you can make an very selective and directed application of glyophosphate to the weed. Use a foam paint brush to make such applications without the fear of damaging surrounding perennials.

June Blooming Perennials

  • Astilbe-Astilbe spp.
  • Silver Mound Artemisia-Artemisia schmidtiana 'Silver Mound'
  • Silver King Artemisia-Artemisia ludoviciana 'Silver King'
  • Carpathian Harebell-Campanula carpatica
  • Peach-Leaf Bellflower-Campanula persicifolia
  • Blanket Flower-Gaillardia x grandiflora
  • Coral Bells-Heuchera spp.
  • Rock Soapwort-Saponaria ocymoides
  • Pincushion Flower-Scabiosa caucasica
  • Stokes Aster-Stokesia laevis
  • Spiderwort-Tradescantia x andersoniana
  • Veronica-Veronica spicata, V. longifolia
  • Pale Purple Coneflower-Echinacea pallida
  • American Bellflower-Campanula americana

July Blooming Perennials

  • Fern-leaf Yarrow-Achillea filipendulina
  • Common Yarrow-Achillea millefolium
  • Blackberry Lily-Belamcanda chinensis
  • Bugbane-Cimicifuga simplex
  • Tickseed-Coreopsis grandiflora
  • Threadleaf Coreopsis-Coreopsis verticillata
  • Purple Coneflower-Echinacea purpurea
  • Globe Thistle-Echinops ritro
  • Sea Holly-Eryngium amethystinum
  • Babys Breath-Gypsophila paniculata
  • Helen's Flower, Sneezeweed-Helenium autumnale
  • Sunflower Heliopsis-Heliopsis helianthoides
  • Hibiscus-Hibiscus moscheutos
  • Hosta, Plantain Lily-Hosta spp.
  • Blazing Star-Liatris spicata
  • Sea Lavender-Limonium latifolium
  • Cardinal Flower-Lobelia cardinalis
  • Bee Balm-Monarda didyma
  • Russian Sage-Perovskia atriplicifolia
  • Garden Phlox-Phlox paniculata
  • False Dragonhead-Physostegia virginiana
  • Butterfly Weed-Asclepias tuberosa
  • Rattlesnake Master-Eryngium yuccifolium
  • Great Blue Lobelia-Lobelia siphilitica
  • Culver-Root-Veronicastrum virginicum
  • Pink Turtlehead-Chelone glabra
  • Balloonflower-Platycodon grandiflorus
  • Perennial Blue Salvia-Salvia x superba

Fall & Winter Care

Many perennials are better left standing over the winter than cutting them down. There are several reasons for this. In addition to many of the perennials having attractive foliage and/or seed heads, they offer food resources for birds. Many birds find the seeds of perennials particularly tasty.

The stems of perennials also offer a place for some birds to hide during the winter. With some marginally hardy perennials, leaving the stems up for the winter aids in overwintering. The foliage helps to insulate the crowns. Mums seem to benefit a great deal from this practice. Another reason to leave stems stand is that if the perennial is a late riser in the spring, the stems will help to mark the spot and prevent any accidental digging in the area that might harm the underground portions of the plant.

Cutting back perennials in the fall may be something you would want to do especially if you were bothered by foliage diseases. Removing the old foliage would be a positive in this case as it helps to reduce the amount of innoculum present to re-infest next year's foliage. Removing foliage can also be one of pure aesthetics. Some gardeners like to see standing perennials in the winter and others don't. When perennials are cut down, do so after they have gone dormant. This is usually after the plants have experienced several hard frosts. Cut the plants down to within 2-3 inches of the crown. Cutting too close can result in winter injury on some perennials due to the fact that the buds for next year's growth are right at the surface or higher and not below the soil line.

August Blooming Perennials

  • Monkshood-Aconitum napellus
  • New York Aster-Aster novii-belgii
  • New England Aster-Aster novae-angliae
  • Leadwort-Ceratostigma plumbaginoides
  • Garden Mum-Dendranthema x grandiflora
  • Red Hot Poker-Kniphofia hybrids
  • Joe-Pye Weed-Eupatorium maculatum
  • Closed Gentian-Gentiana andressii
  • Black-Eyed Susan-Rudbeckia fulgida var. sullivantii 'Goldsturm'
  • Showy Sedum-Hylotelephium x 'Autumn Joy'
  • September-October Blooming Perennials
  • Monkshood-Aconitum
  • Aster-Aster
  • Coreopsis-Coreopsis
  • Bleeding Heart-Dicentra eximia
  • Purple Coneflower-Echinacea
  • Globe Thistle-Echinops
  • Blanket Flower-Gaillardia
  • Sneezeweed-Helenium
  • Phlox-Phlox
  • Balloon Flower-Platycodon
  • Black-Eyed Susan-Rudbeckia
  • Sedum-Hylotelephium

This series has presented native and non-native perennials organized by month of bloom. I will also present one local greenhouse's inventory of perennial plants stocked ordered alphabetically. Pictures of these plants are available at the Lady Bird Johnson Wildflower Center website, listed at the end of this article. Also, by just entering the name of one of these plants in your computer search box, you will also find many pictures of various varieties of these herbaceous perennials.

  • Achillea
  • Ajuga
  • Aquilegia
  • Ameria
  • Astilbe
  • Bellis
  • Buddleia
  • Caryopteris
  • Campanuia
  • Chrysanthemum
  • Coreopsis
  • Delosperma
  • Delphinium
  • Dianthus
  • Dicentra
  • Digitalis
  • Echinacea
  • Fern
  • Gailardia
  • Gauithera
  • Geranium
  • Grasses
    • Acorus
    • Carex
    • Miscanthus
    • Panicum
    • Pennisetum
    • Phalaris
    • Schizachryium
    • Spodiopgon
  • Heliopsis
  • Hemerocallis (Daylily)
  • Heuchera
  • Heucherilla
  • Hosta (20 varieties)
  • Iberis
  • Lamium
  • Lavendula
  • Liatris
  • Lirope
  • Lupinus
  • Monarda
  • Mondo Grass
  • Myosotis
  • Papaver
  • Penstemon
  • Pervoskia
  • Phlox (tall upright)
  • Phlox (creeping)
  • Platycodon
  • Polemonium
  • Rudbeckia
  • Salvia
  • Seabiosa
  • Sedum
  • Verbena
  • Veronica

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