Herbaceous Perennial Production

J. Raymond Kessler, Jr.

 

            Growing consumer demand for herbaceous garden perennials has encouraged more greenhouse growers to consider these crops as part of their production program. However, for growers accustomed to producing annuals, perennials present a whole new set of production challenges. Unlike annuals, herbaceous perennials growing outdoors die to the ground and go dormant in the fall (except a few evergreen species) and then over-winter as fleshy roots or rhizomes. The shoot portion of the plant then grows again each spring producing vegetative structures followed by flowers and seed. Because of this lifestyle, many perennial species have developed strategies for determining what time of year to grow, when to flower, when to go dormant and when seed should germinate. Unlike annuals that flower over a long period, most perennials also have a limited season of bloom ranging from two to six weeks. Perennials may be classified as spring, summer or fall bloomers.

            A further complication for growers is that there are thousands of species and cultivars of perennials native to widely different environments from all over the world. As a consequence, cultural requirements and time of flower varies tremendously. This diversity makes perennials appealing to customers, but makes managing production difficult. Growers desiring to force perennials in containers for the spring market must therefore understand and manipulate factors controlling growth and flowering to meet market timing.

 

Life Cycle

            Like most higher plants, herbaceous perennials germinate from seed, experience a juvenile period, become reproductive, produce seed and eventually die (Figure 1). However, several points in the live cycle are of interest to growers because they may be obstacles to production or they can be manipulated to control crop scheduling. For example, seed of many species exhibit some type of seed dormancy that may require scarification to overcome hard seediness or stratification to satisfy a low-temperature requirement before rapid, uniform germination can occur. Young plants of many perennial species must also reach a certain minimum stage or size (based on number of leaves) before they attain the ability to initiate flower buds (Table 1). This period of vegetative growth is referred to as juvenility. Once juvenility is completed, many perennials do not automatically flower but require a period of cold (vernalization) and/or a specific day length to initiate flower buds. Many named perennial cultivars that are vegetatively propagated also exhibit a kind of juvenility because rooted cuttings must also reach a minimum stage or size before they attain the ability to flowers.

            In the fall, most perennials enter a vegetative dormancy to protect the plant from harsh winter conditions. In many species, this is triggered by the onset of short day lengths. A cold period is often then required to satisfy dormancy. Most species require a minimum of six to eight weeks at 40°F in a cooler or eight to 12 weeks in a minimum heated greenhouse (>35°F, <45°F), enough heat to keep temperatures just above freezing. Perennials may also be overwintered in containers outdoors as long as they are protected from freezing by covering with some type of thermoblanket such as microfoam. Watering during cold treatment, especially in a cooler, is fairly critical, do not allow plants to dry out or to remain too wet for long periods. Cold treatment is beneficial to most species, even those that do not, strictly speaking, require cold to flower. Cold treatment often reduces time to emergence, increased plant uniformity, reduces time to flower and allow plants time to develop a larger crown to fill large containers.

            In some species, typically late spring and summer blooming species, the cold period must be followed by long day lengths for flower buds to initiate. These plants can be flowered earlier by providing nigh-break lighting using incandescent lights designed to provide 10 to 20 footcandles. Apply the lighting as a four hour night break from 10:00 PM to 2:00 AM. Perennials that do not require long days are not adversely affected by night-break lights, although stem length may be longer. Night-break lighting can be discontinued once natural day lengths exceed 13 hours. The exact requirements for flowering all herbaceous perennials are not known but many are listed in Table 2.

 

Propagation

            The main methods for propagation of perennials are by seeds, divisions or stem cuttings. Seed is widely used for true species and a few cultivars that come true from seed. However, most of the improved cultivars must be propagated by asexual methods. Many perennial seeds can be germinated easily, but careful attention must be given to the importance of obtaining fresh seed, the conditions necessary to satisfy dormancy (if any) and providing the correct germination environment. Seeds of many perennial species germinate readily if the seed is collected fresh and sown immediately. If seed storage is necessary, store seed at 40 to 45°F and 40 to 50% relative humidity in sealed containers.

            Seeds of many perennial species do not require pre-germination treatment, but some germinate more rapidly and uniformly when provided stratification or scarification. Stratification is accomplished by sowing seed in moist sand or seedling mix and placing them in a refrigerator at 35° to 40°F for four to six weeks. Scarification is a process of cutting or abrasion of the seed coat or soaking seed in hot water (170° to 210°F) to allow rapid absorption of water during germination. Seed are frequently sown in plug trays similar to annuals. Germination of perennial seed occurs most rapidly and uniformly under warm (60° to 75°F), humid conditions. High humidity can be provided using intermittent fine mist or covering with a pane of glass or plastic bag. Generally, perennial seed germinate best when sown on a well-drained medium that is low in soluble salts with a 5.5-6.0 pH. Seed of perennial species vary in the optimum temperature range for germination. Three germination temperatures are often used: cool (60-65°F), moderate (70-75°F) or warm (80-85°F). The humidity and temperature should be reduced and light intensity increased once the first true leaf is visible. Perennials that can be started from seed are in Table 3.

            Division of field-grown stock plants into individual offsets is a technically simple way of propagating perennials, but is often inefficient in terms of labor cost and maintaining large stock blocks. The number of offsets that can be obtained varies widely with the plant species, some yield thousands while others only a half dozen. Offsets are generally established in trays or large plugs before transplanting to the final container.

            Stem cuttings are a rapid and increasingly popular method of propagating perennials. Cuttings can be harvested from field-grown stock plants in the spring, or from greenhouse-grown stock and stuck in flats or plug trays under intermittent mist. A rooting hormone is often unnecessary and plants are ready to transplant in three to six weeks.

 

Facilities

            Growers who want to start perennials from seed and/or vegetative cutting will need propagation facilities appropriate for these methods. This will also include a refrigerator if the seed of those species requiring stratification will be handled. A minimum heat greenhouse or large walk-in cooler will be required if the grower want to provide cold treatment. The greenhouse can be a simple Quonset-style house to protect dormant plants from freezing. Forcing greenhouse should have the ability to provide water, fertilization, insect and disease prevention and temperature and photoperiod control. Many growers establish separate houses or sections for two temperature zones, a cool night temperature area and a warm night temperature area.

 

Forcing Temperature

            Early and mid-spring blooming perennials are more compact if they are grown at low night temperatures (45° to 55°F), but more time will be needed for forcing to a marketable plant. Warmer night temperatures (60 to 68°F) can be used for most perennials which reduces the time needed for forcing a marketable plant, but may cause stretching, especially under low light intensity.

 

Container Medium

            No single type of container medium has proven to be the best for growing perennials. However, the medium should be exceptionally well-drained (minimum 15% porosity), hold sufficient water (35%-50% water holding capacity) and fertilizer, not bo too light in weight (40-65 lbs./ft3 bulk density) and be free of pathogens and weed seed. The potting medium should not compact or shrink excessively over time.

            The choice of media and media components frequently depends on the production system. Peat-lite media composed of sphagnum peat moss, vermiculite and perlite is often used when perennials are forced in a greenhouse. When perennials are finished in a nursery setting, various combinations of pine bark, peat and sand are often used. A nursery-type mix might consist of 60% composted bark, 20% peat and 20% coarse sand. Regardless, the pH of the medium should be adjusted to 6.0-6.5 using dolomitic limestone according to a soil test. Micro-nutrients and slow-release may also be mixed with the media according to manufacturers directions.

 

Planting

            Avoid planting perennial transplants too deep to reduce crown rot. Plant the root ball level with medium surface. Use 33% - 50% shade and periodic misting for about 2 weeks to establish during hot sunny conditions. Avoid over watering to reduce root and crown rot, but allow the potting medium to dry somewhat between irrigations. This encourages root growth, but don't allow young plants to wilt.

            Perennial transplants must allow sufficient growing time to establish roots throughout pot) in late summer to early fall before overwintering. Allow 4 weeks for quarts or 6 weeks for gallons prior to the onset of cold weather and covering for the winter.

 

Over-wintering

            Remember that the roots of perennials are more vulnerable to cold damage than shoots during freezing temperatures because roots don't harden off or become dormant like shoots and the thin pot doesn't insulate the roots very much. Choose a well-drained overwintering site to avoid winter loss from excessive moisture. Utilize gravel and/or drainage tile if needed. Plants are usually spaced close together for space efficiency and improved insulation. Cutback dead / dying foliage to reduce foliar diseases (evergeen types should not be cutback). In a poly-covered greenhouse, cover the house as late as possible, when night temperatures are consistently in the mid 30s. Use white poly instead of clear to minimize heat buildup. Plants can also be covered with fleece or thermal blankets out in the nursery area. Water them thoroughly and apply a rodent bait and fungicidal spray for Botrytis before covering. In either case, monitor plants and be prepared to irrigate as needed. Also monitor evergreen types for foliar diseases and treat accordingly. Uncover perennials as early as possible in spring. Gradually uncover the plants by opening doors and ends first, then as temperatures warm, slit poly along the ridge and finally remove the cover. However, it is best to retain the capacity to recover if a late cold snap occurs.

 

Irrigation

            In a traditional nursery production system, overhead irrigation is cheap and simple, but it can increased foliar diseases especially if plants are kept wet overnight. The coverage patterns of impact sprinklers may be affected by wind and as the plant canopy closes, its harder to provide sufficient water to the container medium. Use drip or ebb-and-flow systems if possible. Leaching with each irrigation is best to avoid soluble salt accumulation. Test the quality of the irrigation water (test for nutrients, EC, pH, alkalinity) and monitor EC and pH of the container medium during production. Saturated paste EC for the medium should be 1.0-2.0 mmhos/cm.

 

Fertilization

            To date, little work has been done on fertilization requirements for perennials. Generally, fertilizer rates should be adjusted for the growing temperature. When growing plants at cooler temperatures (45°-60°F), 50 to 100 ppm nitrogen two to three times a week should be sufficient for most species. At warmer temperatures (>60°F), increase the rate to 100 to 200 ppm nitrogen at every watering. Reduce fertilization in late summer, early fall to avoid soft growth in overwintering and do not fertilize during vernalization (<45°F).

            A calcium nitrate and potassium nitrate mix or 15-0-15 gives better results than high ammonium fertilizers such as 20-20-20, especially at cooler temperatures. Use a complete fertilizer such as 20-10-20 or 15-16-17 at warmer temperatures. If micro-nutrients were not incorporated in the mix, use a fertilizer formulation that includes micro-nutrients. Once flowering begins, reduce the rate to about 100 ppm nitrogen. Slow-release fertilizer is often used on container-grown perennials in a nursery setting either as a top-dressing or incorporating in the mix. As a starting point, follow manufacturers directions for rates.

 

Plant Growth Retardants

            Most early spring blooming perennials require little height control unless temperatures are too high and/or light intensity is too low. However, many late spring and summer blooming perennial species bolt during flowering and grow too tall for market acceptance in containers and pose problems in shipping. Consider growing cultivars that are more compact than the species (Rudbeckia fulgida ‘Goldsturm’) and do not allow plants to crowd during production.

            B-Nine is the most commonly used plant growth retardant on perennials and is applied as a spray at 3000 to 5000 ppm. Cycocel at 750 to 1,500 ppm can be used on balloon flower, Dianthus and English daisy. A-Rest, Bonzi and Sumagic are useful on some crops. Check the label for a list of perennials registered for a product.

 

Production Systems

            Unlike many greenhouse crops, there is a wide range of production systems that can be used to produce perennials. The choice of which system to use depends on the market. Define who is the customer, when will plants be marketed and how much time is available to grow the plants? The answers to these questions will help define the market container size, the range of plants to grow and the type of operation required for production. To generalize, perennial production can follow one of three regimes:

Small-size plugs - Plug flats in the 288 to 375 size range are used primarily for seed propagated perennials. Seeds are sown in July to August for transplanting into 6-inch pots or quarts in September to October. Seed of slow-growing species or those planted in gallon pots may be sown in June. In either case, plants must complete the juvenile stage before vernalization in November, December and January. Once vernalization is complete, plants are placed under forcing conditions until flower.

Medium-size plugs - Plug flats in the 50 to 125 size range may be used for seeds, cuttings or offsets for planting in 4-inch, quarts or gallon pots in the fall. Many growers who do not wish to propagate perennials purchase perennial plugs from specialists propagators. Purchased plugs may be received in the fall for potting in the final container and vernalization by the grower. Or plugs ( 50-72 count trays) may be received from January to March as already vernalized plugs (vernalized by propagator) that are potted in the final container and placed under forcing conditions.

Large-size plants - Perennials are received from specialists propagators as bare-root plants or large field-grown plugs. These may be planted in the fall for vernalization or in early spring already vernalized and ready to plant and force. Large size plants are mostly planted into one- to three-gallon pots.

            One critical decision is whether to grow plugs or purchase them from a specialists propagator. Growing perennial plugs in one season and then vernalizing them for spring sales requires more skill and careful planning than purchasing already vernalized plugs. Propagation and potting to final containers occurs in late summer and fall, which may not be a busy time for some growers. Propagation has inherent risks and requires special facilities. Plants must then grow sufficiently to overcome juvenility and have sufficient root growth for vernalization. However, the cost per plug can by less with growing your own plugs.

            Perennials may also be treated as spring bedding plants. The starting point may be vegetative propagation, sown seed or small to medium-size plugs that are then planted into 4-inch pots or large-cell market flats in early spring. Those species that grow rapidly and do not require special conditions will grow and flower for the spring bedding plant market. Those that grow slowly or have special requirements for flowering are sold green. Because customers are often reluctant to purchase plants that are not in flower, “green packs” should be have color labels or point-of-purchase material.

 

Insect Problems

            With the wide range of plant species frequently included in perennial production, it is important to practice early detection and learn to identify insect pests and their distinguishing symptoms. Chewing insects such as caterpillars, beetles, grasshoppers and slugs and snails can cause extensive damage to the foliage of perennials in a short period of time, especially in outdoor production. Boring insects cause damage when larvae bore into the plant and feed on internal tissues. European corn borer and stalk borer attack the stems while leafminers cause white serpentine trails in the leaves. Insects that feed on the sap of plants include aphids, mites, thrips and whitefly. These insects leave symptoms such as small yellow or brown specks, chlorotic spots or streaking and often twisted, stunted or curled growth. Good sanitation practices in the production area and inspection and quarantine of incoming plants will help reduce the introduction of insect pests. Before applying pesticide controls, identify the pest and apply the appropriate pesticide at the most vulnerable stage in the life cycle.

 

Disease Problems

            Like insects, early detection and learning to identify disease pests and their distinguishing symptoms is a key to disease control. Botrytis blight, root and stem rots, powdery mildew, leaf spots and viral diseases have been problems on perennials. Often learning the species and cultivars that are susceptible to a particular disease problem is part of the early detection process. Learning the environmental conditions and cultural practices that favor disease development can also help avoid serious problems. Sanitation, inspecting incoming plant material, isolation or destruction of infected plants are all part of a prevention program. It is important to get a definitive identification of the pathogen before applying a fungicidal control program.

 

Marketing

            Perennial consumers may be placed into three categories: 1) Spring consumers - those who traditionally purchased bedding plants in the spring and wish to expand into perennials to try something new. These consumers frequently required information which starts at the very beginning about perennial selection, culture in the garden and what can be expected from the plants. 2) Plant collectors or specialists - these are often gardeners with more experience that are looking for something unusual or different from what they have. These consumers require a wide selection of plants or they may specialize in a particular genus such as hosta, daylily or Iris. 3) Landscaping consumers - commercial or amateur landscapers who may purchase a large number of one kind of plant, often in larger containers.

            Each perennial grower must define the kind of market and the type of consumer their business will serve. However, providing information to help customers succeed with perennials and then providing an adequate selection of plants and containers sizes at a reasonable price are keys to success in any market. Perennials offer the grower a way to expand sales beyond the traditional spring market. Because of the wide selection of species and cultivars that bloom in different seasons, groups of flowering perennials can be featured in the sales area each week from early spring through fall, compelling the customer to make repeated visits to see what is in bloom next.

 


perennialproduction.gif

Figure 1


Table 1. Juvenility requirement for response to flower initiation treatments of herbaceous perennials.

Species / cultivar

Common name

Number of leaves

Achillea filipendulina ‘Cloth of Gold’

Fern-leaf Yarrow

13 or more

Aquilegia (most species)

Columbine

12 to 15 or more

Aster alpinus

Alpine Aster

at least 15

Astilbe arendsii

Astilbe

at least 6

Chrysanthemum coccineum

Painted daisy

at least 15

Coreopsis grandiflora ‘Sunray’

Tickseed

16 or more

Delphinium × elataum

Delphinium

4 to 5

Echinacea purpurea

Purple Coneflower

at least 4

Euphorbia epithymoides

Cushion Spurge

more than 8

Goniolimon tatarica

German Statice

more than 14

Heuchera sanguinea

Coral Bells

at least 16

Lavandula angustifolia

Lavender

40 to 50

Lobelia × speciosa ‘Compliment Scarlet’

Lobelia

6 to 7

Papaver orientale ‘Brilliant’

Oriental Poppy

more than 14

Physostegia virginiana

Obedient Plant

at least 10

Rudbeckia fulgida ‘Goldsturm’

Gloriosa Daisy

at least 10

Veronica spicata ‘Blue’

Speedwell

6 to 8

* D.A. Bailey and H. Scoggins. 1996. Perennials: Basics of profitable production (part II). North Carolina Flower Growers’ Bulletin. 41(6): 1-11.


Table 2. Forcing requirements of several herbaceous perennials.

Plant

Vernalization

Daylength during forcing1

Weeks forcing at 60°F NT

Achillea filipendula ‘Cloth of Gold’

Required

LD

4-6

Achillea millefolium ‘Red Beauty’

Required

LD

4-6

Aquilegia × hybrida

Required

none

6-9

Anemone sylvestris

Beneficial

(LD)

-

Armeria

Required

none

8

Asclepias tuberosa

Required

LD

-

Aster alpinus ‘Goliath’

Required

none

-

Aster novae-angliae

none

SD

-

Astilbe arendsii

Required

(LD)

4

Campanula carpatica ‘Blue Chips’

none

LD

10-11

Campanula glomerata ‘Joan Elliott’

Required

none

4

Convallaria sp.

Required

none

3

Coreopsis grandiflora ‘Sunray’

Required

LD

10-11

Coreopsis grandiflora ‘Early Sunrise’

none

LD

6-8

Coreopsis verticillata ‘Moonbeam’

none

LD

8-10

Delphinium elatum ‘Blue Mirror’

Beneficial

none

-

Dianthus plumaris ‘Cottage Pink’

Required

none

-

Dianthus deltoides ‘Zing Rose’

Beneficial

none

-

Dicentra spectabilis

Required

none

4-5

Echinacea purpurea

Beneficial

LD

-

Echinops bannaticus ‘Taplow Blue’

Required

LD

11-13

Euphorbia polychroma

Required

none

4

Gaillardia grandiflora ‘Goblin’

Beneficial

LD

-

Gaura lindheimeri ‘Siskyou Pink’

none

none

-

Geranium × hybrida ‘Johnson’s Blue’

Required

none

-

Gypsophila paniculata ‘Double Snowflake’

Required

LD

-

Heuchera sanguinea ‘Blessingham Hybrids’

Required

none

8-10

Hibiscus × hybrida ‘Disco Belle’

none

LD

-

Iberis sempervirens ‘Snowflake’

Required

none

-

Iris sp.

Required

none

4

Lavendula angustifolia ‘Hidcote’

Required

LD

-

Leucanthemum × superbum

Beneficial

LD

4-7

Liatris spicata “Kobold’

Required

LD

4

Linium perenne ‘Sapphire’

Required

none

-

Lobelia × speciosa ‘Compliment Scarlet’

Required

(LD)

-

Monarda didyma ‘Croftway Pink’

none

LD

-

Oenothera macrocaropa

Beneficial

LD

-

Phlox paniculata ‘Eva Cullum’

Required

LD

-

Phlox subulata

Beneficial

(LD)

-

Physostegia virginiana ‘Alba’

Required

LD

10-13

Platycodon grandiflorus ‘Sentimental Blue’

Beneficial

(LD)

11-13

Rudbeckia fulgida ‘Goldstrum’

Beneficial

LD

12-14

Salvia superba ‘Blue Queen’

Beneficial

(LD)

-

Sanguinaria canadensis

Required

none

4-7

Saponaria

Required

none

4

Scabiosa caucasica ‘Buttery Blue’

Beneficial

(LD)

-

Sedum spectabilis ‘Autumn Joy’

Beneficial

SD

14-18

Tradescantia × andersoniana ‘Innocence’

Required

none

4

Verbena × ‘Homestead Purple’

none

none

-

Veronica longifolia ‘Sunny Border Blue’

Required

none

9-12

Veronica spicata

Required

none

-

Viola

Required

none

3-4

1 Parentheses indicate condition beneficial but not required.


Table 3. Herbaceous perennials propagated from seed.

 

Genus

Temperature

Seed cover

WTF1

Genus

Temperature

Seed cover

WTF1

Achillea

70-75

None

8

Lavandula

60-65

Light

11

Alcea

70-75

Heavy

4

Leotopodium

70-75

Heavy

10

Alchemilla

70-75

None

10

Leucanthemum

70-75

None

8

Anaphalis

70-75

None

9

Liatris

70-75

None

10

Anchusa

70-75

Heavy

7

Limonium

70-75

None

10

Anemone

60-65

Heavy

12

Linaria

70-75

None

9

Anthemis

70-75

None

9

Linum

70-75

None

8

Aquilegia

70-75

Heavy

9

Lobelia

70-75

Heavy

10

Arabis

70-75

Heavy

8

Lunaria

70-75

Heavy

9

Arenaria

60-65

Heavy

8

Lupinus

70-75

Heavy

5

Armeria

60-65

Light

10

Lychnis

70-75

None

8

Asclepias

70-75

None

9

Monarda

70-75

None

8

Aster

70-75

None

8

Miscanthus

70-75

None

8

Astilbe

70-75

Heavy

10

Myosotis

70-75

None

7

Aubrieta

70-75

Heavy

8

Nepeta

70-75

None

7

Aurinia

70-75

None

8

Oenothera

70-75

Light

9

Baptisia

60-65

Light

9

Papaver alpinum

60-65

Light

10

Bellis

70-75

None

7

Papaver orientale

70-75

Light

10

Bergenia

70-75

None

10

Penstemon

70-75

None

8

Campanula

70-75

None

10

Physalis

70-75

None

7

Catananche

70-75

None

9

Physostegia

70-75

None

9

Centaurea

70-75

Heavy

8

Platycodon

70-75

None

8

Centranthus

60-65

None

8

Polemonium

70-75

None

9

Cerastium

70-75

None

8

Potentilla

70-75

None

8

Cheiranthus

70-75

None

7

Primula

60-65

Light

10

Coreopsis

70-75

None

9

Pulsatilla

60-65

Light

12

Coronilla

60-65

Light

7

Rodgersia

70-75

None

10

Cortaderia

70-75

None

8

Rudbeckia fulgida

80-85

None

9

Cymbalaria

70-75

None

8

Rudbeckia hirta

70-75

None

8

Delphinium

60-65

Heavy

8

Rudbeckia triloba

70-75

None

9

Dianthus

70-75

None

8

Sagina

70-75

None

7

Digitalis

70-75

None

8

Salvia

70-75

None

9

Doronicum

80-85

Heavy

8

Santolina

70-75

None

10

Echinacea

70-75

Heavy

8

Saponaria

60-65

Heavy

8

Echinops

70-75

Heavy

8

Scabiosa

70-75

Heavy

6

Euphorbia

70-75

Heavy

8

Sedum

70-75

None

9

Festuca

70-75

None

8

Sempervivum

70-75

Heavy

13

Gaillardia

70-75

Heavy

8

Sidalcea

60-65

Light

8

Geum

70-75

None

10

Silene

70-75

None

8

Gypsophila

70-75

None

8

Stachys

70-75

None

6

Helenium

70-75

None

11

Stokesia

70-75

Light

8

Helianthemum

70-75

None

9

Tanacetum

70-75

None

9

Heliopsis

70-75

None

9

Teucrium

70-75

Light

11

Heuchera

70-75

Heavy

11

Thalictrum

70-75

Light

10

Hibiscus

70-75

Light

5

Thymus

70-75

None

9

Iberis

60-65

Light

8

Trachelium

70-75

None

9

Kniphofia

70-75

Heavy

10

Veronica

70-75

None

7

Lathyrus

70-75

Light

8

Viola

60-65

Heavy

7

1 Weeks to finish in 128 celled plug tray.

P. Karlovich. 1995. Perennial plugs from seed. PPGA News.