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Day Neutral Spring-Flowering Perennials

In the third article of our series on perennial vernalization, Michigan State University researchers share the tricks of the trade.

by SONALI PADHYE, BETH FAUSEY, ERIK RUNKLE and ART CAMERON

IN the last two articles of this series, we introduced the concept of vernalization and discussed several practical aspects of delivering cold treatment to herbaceous perennial plants. We have also described the major flowering response categories of perennials based primarily on their response to vernalization and secondarily to photoperiod.

Based on their vernalization responses, herbaceous perennials can be categorized into the following three groups:

1) Plants that have an obligate or qualitative vernalization
     requirement: these plants do not flower without a cooling
     treatment.
2) Plants that have a facultative or quantitative vernalization
     requirement: these plants flower without cooling
     treatment, but cooling improves flowering characteristics
     such as percentage of plants flowering, reduced time to
     flower and increased flower number.
3) Plants that do not have a flowering response to a cooling
     treatment.

Plants that have an obligate vernalization requirement can be further categorized into the following two groups based on their photoperiodic response after vernalization:

1) Plants that are day neutral and therefore flower under any
photoperiod after vernalization.
2) Plants that require long days to flower after vernalization.

In this article, we describe specific responses of plants that require vernalization and are day neutral following the cooling treatment.

Researchers at Michigan State University have screened several hundred perennials for determining their vernalization response types and have found that many commercially important perennials require vernalization and are day-neutral (Figure 1A and B). Some examples include select cultivars of bugleweed (Ajuga reptans), sea thrift or sea pink (Armeria maritima), columbine (aquilegia hybrids), false spirea (astilbe hybrids), cheddar pinks (Dianthus gratianopolitanus), coral bells (heuchera hybrids), foamy bells (heucherella hybrids), candytuft (Iberis sempervirens), lungwort (pulmonaria hybrids) and saxifrage (saxifraga hybrids).

Figure 1A

Field-grown and cold-treated Astilbe thunbergii ‘Ostrich Plume’ flowers under short days and long days.

Figure 1B

Armeria latifolia is day neutral when vernalized at 41°F (5°C).

Time To Flower And Final Plant Size

When herbaceous perennials with an obligate vernalization response are forced, they are often propagated by cuttings or grown from seed, then grown for some period of time, cooled and subsequently forced and sold as actively growing perennials in flower. The final goal is to obtain a high quality, floriferous plant that is large enough to fill the pot so it is aesthetically appealing. Therefore, attaining the desired plant size and maximizing flower number are very important to ensure marketability. In general, the amount of vegetative growth and number of flowers produced depend on the time required to flower following vernalization. Specifically, we have defined two primary flowering time responses. The first response type consists of plants that bloom at the onset of spring, which we refer to as early-spring ephemerals. The second response category consists of late-spring perennials that bloom later in the spring.

Early-Spring Ephemerals

Vernalization-requiring, day-neutral perennials flower only following a cooling treatment. Hence, photoperiod does not regulate flowering. Due to this singular control of flowering, many species in this category flower early in the spring in their natural habitats or in the garden. In the greenhouse we have observed that following vernalization, many early-spring ephemerals flower within two to six weeks of forcing. For example, in our trials Ajuga reptans ‘Bronze Beauty’ flowered within two weeks (Figure 2) and Dianthus gratianopolitanus ‘Bath’s Pink’ flowered in four to five weeks of forcing at 68°F (20°C).

Figure 2

Complete flowering of Ajuga reptans ‘Bronze Beauty’ was achieved within just two weeks of forcing following vernalization treatment.

Bulking Requirement

Some perennials need to develop vegetative growth prior to cooling for them to be of sufficient size at flowering. For example, if rooted cuttings of Dianthus ‘Bath’s Pink’ are grown for four or five weeks before vernalization, they will not adequately fill a 5 1/2-inch container when forced. Also, the plants will only produce a handful flowers. Therefore, Dianthus ‘Bath’s Pink’ does not achieve its aesthetic potential when vernalized and forced without an extended period of bulking (Figure 3). Similarly, when Iberis sempervirens ‘Alexander’s White’ is vernalized without sufficient vegetative growth, it flowers in two to three weeks at 68°F (20°C), does not fill the pot and few flowers are produced. However, when sufficiently bulked prior to cooling, iberis ‘Alexander’s White’ produces copious flowers and produces a great potted flowering crop (Figure 4). Thus, most spring ephemerals should be bulked for at least six to 10 weeks prior to vernalization treatment (Table 1).

Figure 3

If Dianthus gratianopolitanus ‘Bath’s Pink’ is not bulked to final desired size prior to vernalization, plants do not fill the pot and produce few flowers.

Figure 4A

Figure 4B

When bulked sufficiently to fill the container prior to vernalization, Iberis sempervirens ‘Alexander’s White’ produces a mass of flowers within 3 weeks of forcing at 68°F (20°C).
4A. Side view of plants
4B. Top view of plants

Table 1
These day-neutral spring ephemerals require vernalization. We recommend that these plants be bulked to their desired finish size in their finish containers prior to vernalization. These species cooled as plugs will likely not adequately fill their finish container at flowering.
Species	Prop	Weeks of vernalization at 41°F	Weeks to flower at 68°F	Comments
Ajuga ‘Bronze Beauty,’ ‘Chocolate Chip,’ ‘Golden Beauty’	Veg.	≥6	2-6	Flowering of non-bulked ajuga was inconsistent. We recommend bulking prior to cooling to achieve 100% flowering. In our trials ‘Golden Beauty’ did not flower with ≤4 weeks of cooling. It has great foliage and plants could be marketed for their foliage alone.
Alchemilla mollis	Seed	≥9	6	Great perennial for color and foliage. Plants bulked for 8 weeks from a 128-cell plug and then vernalized for 9 weeks flowered completely.
Aquilegia straight species and older cultivars	Seed	varies 6-12	varies 5-8	These can make excellent garden plants but they have lengthy juvenile periods and extended cold requirements. Breeders have focused on reducing both by introducing A. flabellata and other species into hybrids
Aquilegia flabellata ‘Cameo,’ ‘Mini Star’	Seed	6-9	4	Cute dwarf columbines that grow to 6-10" tall. Need to be bulked for ~ 6 wks in their finish pot before cooling. ‘Mini Star’ needs less bulking.
Aquilegia × hybrida ‘Crimson Star,’ Musik and Songbird series	Seed	6-9	5	Striking flowers with nice fragrance and dense foliage. Due to juvenility factor, plants are best transplanted and grown for 3-6 weeks in the container.
Aquilegia ‘Origami Blue and White’	Seed	5	4	Very uniform cultivar with minimal juvenility and cold requirement. Plugs should be transplanted and grown for at least 3 weeks at 68°F (20°C) prior to a 5 week cold period for complete flowering. Other colors in the Origami series have been reported to require slightly longer periods of bulking and/or cooling.
Armeria maritima ‘Cottontail’ and related species	Seed or Veg.	6-10	3-5	Cold may not be absolutely required for all species but will decrease time to flower and increase flower number. Bulking before cold will also increase flower number.
Astilbe × hybrida numerous cvs.	Veg.	≥10	varies with cv. 5-9	Prior to cooling, plants may go dormant under SD, and thus plants should be bulked under LD when needed. However, they are day-neutral following cold. Often started from bare-root divisions.
Cimicifuga racemosa	Veg.	15*	6-7	Great garden plant with long bloom period. Bulking necessary to obtain attractive plants. Cold is absolutely required. Fifteen weeks was only cooling duration tested. When grown from bare-root divisions, plants were dormant and required cooling for emergence.
Dianthus gratianopolitanus ‘Bath’s Pink’	Veg.	6	5	Great garden plant in the North. Some new cultivars of this species only benefit from vernalization whereas several do not require vernalization.
Dianthus deltoides ‘Shrimp,’ ‘Zing Rose’ and many others	Seed or Veg.	≥5	5-8	Cold beneficial for ‘Zing Rose’ and required for ‘Shrimp’. When grown under low light, plants were floppy. Chemical growth regulation may be necessary under low light.
Dicentra eximia ‘Luxuriant’ and related	Veg.	15*	5-6	A great shade perennial often started from bare-root plants. Best to pot and establish prior to the cold period. Fifteen weeks was the only vernalization duration tested.
Euphorbia amygdaloides ‘Purpurea’	Seed or Veg.	5	2-3	Lime green flowers look attractive on purple-maroon foliage making it a great candidate for fall programs. Can be grown for its nice foliage alone. Depending on the size of starting material, bulking may not be essential. Plants with ~25 leaves yielded plants suitable for 5.5" containers when vernalized for 5 weeks.
Euphorbia polychroma (previously E. epithymoides)	Seed	15*	3	Provide great yellow splash in early spring. Needs to be grown in container for ≥6 weeks prior to cooling. Taller under long days.
Filipendula purpurea	Veg.	15*	3	Flowers for about 3 weeks. Airy flowers over compact plants. Great potential but we have only conducted limited tests. Poor flowering
Filipendula purpurea ‘Kakome’	Veg.	15*	3	Flowers for about 3 weeks. Airy flowers over compact plants. Great potential but we have only conducted limited tests. Poor flowering when cooled in plug tray.
Geranium dalmaticum	Veg.	6	7	Nice plant, especially in a container. Can be used in alpine gardens. Pink showy flowers on a small plant. Easy to flower but bulk before cold.
Geranium ‘Johnson’s Blue’	Veg.	15*	5-6	‘Johnson’s Blue’ typically flowers in June in Michigan and flowering continues for about 3 weeks. Longer blooming cultivars such as ‘Rozanne’ and ‘Jolly Bee’ do not require vernalization, but cooling decreased time to flower and increased flower number in ‘Rozanne’.
Geranium sanguineum ‘New Hampshire Purple’	Veg.	15*	5	Clear purple flowers on this compact geranium. Plants grown under high light produce more flowers. Many other cultivars to choose from.
Geum chiloense ‘Mrs. Bradshaw’	Veg.	15*	8	No plants flowered without vernalization. May need growth regulation. Seed propagated Geum coccineum ‘Borisii’ required vernalization for complete flowering.
Heuchera cvs.	Seed or Veg.	10	5-8	We have studied numerous Heuchera. Most (but not all) should be bulked before cold for best response. This is especially important for seed propagated and tissue culture plants.
Heucherella ‘Cranberry Ice,’ ‘Dayglow Pink,’ ‘Viking Ship’	Veg.	9-12	5-6	Heucherella are hybrids between Heuchera and Tiarella. Like both its parents, Heucherella cultivars prefer partial sun to full shade. These sterile hybrids are reported to be profuse and repeat bloomers.
Iberis sempervirens ‘Alexander’s White,’ ‘Snowflake’ and others	Seed or Veg.	≥8	2-3	Iberis blooms very quickly following vernalization. Cool temperatures can slow flowering. Bulking before cooling is a must.
Lychnis coronaria ‘Angel Blush’	Seed	15*	8	A great garden plant but difficult in container. Plants continue to unfold leaves in 41°F (5°C) cooler. Seedlings with 7-9 leaves did not achieve 100% flowering possibly due to juvenility issue. Bulking is recommended to overcome juvenility.
Phlox divaricata ‘Laphammi,’ ‘London Grove’	Seed or Veg.	15*	3	Very rapid flowering after the vernalization period has ended. Some cultivars including ‘May Breeze’ may need prolonged bulking to overcome juvenility prior to cooling.
Phlox subulata ‘Emerald Blue’ and numerous cultivars	Veg.	≥9	3-4	Obtaining pest-free starting material is essential for success. Flower buds form during the cold treatment. Without bulking, plants produce very few flowers.
Potentilla atrosanguinea ‘Mrs. Willmott’	Veg.	15*	6-7	This cultivar spreads in a container. Plants may benefit from staking or growth regulation. Many other Potentilla cultivars may have similar flowering requirements.
Pulmonaria saccharata ‘Peirres Perfect Pink’	Veg.	6	7	Grown mostly for foliage and not flowers. Flowers are short-lived. Buds were initiated in the cooler.
Saxifraga ‘Aureo-punctata,’ ‘London Pride,’ ‘Triumph’ and similar species and cultivars	Veg.	≥9	4-5	A great rock garden plant that can bloom uniformly. Already a successful pot plant in Europe. Keeping dry during greenhouse production is essential.
*only vernalization duration tested in the trial.

Forcing Aquilegia Cultivars

We have tested several aquilegia species and cultivars in our research program. They are all started from seed and most have a juvenile period, which needs to be overcome by bulking prior to vernalization. The duration of the juvenile period and vernalization requirement vary considerably among species and cultivars. In our studies, ‘Origami Blue and White’ flowered completely when plants from 128-cell plugs were bulked for three weeks at 68°F (20°C) and then cooled for five weeks at 41°F (5°C). With the same bulking and cooling treatments, ‘Winky Double Red and White’ flowered inconsistently while A. chrysantha did not bloom at all. Complete, rapid and uniform flowering of ‘Winky Double Red and White’ occurred after six weeks of bulking at 68°F (20°C) and 10 weeks of cooling at 41°F (5°C). Therefore, blanket statements for bulking and cooling are not possible with aquilegia cultivars. Many new aquilegia cultivars have been bred with fairly short juvenile periods (three weeks after transplant) and vernalization requirements (five weeks).

We have found that bulking in the finish container, rather than in a plug tray, improved flowering percentage and reduced time to flower of ‘Origami Blue and White,’ as well as other cultivars. Therefore, aquilegia should be potted first, then bulked, cooled and then forced. Many other early-spring ephemerals that require bulking prior to vernalization should also be potted and bulked in their final containers before cooling. This requires more space during bulking and cooling, but also produces a higher quality crop than when spring ephemerals are started from vernalized plugs.

Strategies To Bulk And Cool Outdoors

A common strategy to produce spring ephemerals is to pot plants in their final containers during summer or fall and bulk them before cooling. In the northern states, plants are usually grown in a greenhouse or hoophouse for several weeks and then temperature is slowly lowered during autumn. Plants are then vernalized in a cooler or a protective structure such as a minimally heated hoophouse and then forced. In southern states, this entire process of bulking, cooling and forcing can take place outdoors.

A key limitation with an outdoor cooling strategy is the possibility that a vernalization requirement may not be completely met in a mild winter, especially in the mid-South. For northern states such as Michigan, however, this is not a concern as the winters are long and cold enough to satisfy the cooling requirement of all herbaceous perennials.

Another cooling strategy is to grow perennials outdoors in the ground and harvest them as bare-root perennials, preferably after the first frost. When properly packed, the bare-root plants can be stored in a cooler (usually at 28°F) for several months. Bare-root field-grown plant material of desired size can be purchased as starting material. We have trialed several bare-root perennials with great success. For example, Dicentra eximia ‘Luxuriant’ exhibited a fantastic display of flowers when started from bareroot divisions (Figure 5A and 5B). Bareroot starting material may be particularly useful if resources to bulk and cool spring ephemerals are unavailable.

Figure 5A

Figure 5B

Field-grown bare-root starting material of Dicentra eximia ‘Luxuriant’ produced a great forced crop.
5A. Field-grown bare-root starting material
5B. Plants in flower when forced at 68°F (20°C).

Late-Spring Perennials

So far, we have discussed day-neutral species that are quick to flower following vernalization. There are a handful of species that require vernalization and are day-neutral but are relatively slow to flower (Table 2). In their natural habitats, such slow-flowering species tend to flower later in the spring. We refer to these plants as late-spring perennials. Campanula ‘Birch Hybrid,’ Delasperma cooperi and veronica ‘Red Fox’ are some examples of late-spring perennials.

Most of the late-spring perennials that we have tested produce considerable vegetative growth during forcing. Therefore, these can be vernalized as plugs and, when potted after vernalization, still produce a satisfactory crop. Although we have found only a few species and cultivars belonging to this category, they are important due to their lack of a bulking requirement. For example, when campanula ‘Birch Hybrid’ rooted tip cuttings were cooled for five to seven weeks at 41°F (5°C), they produced a beautiful display of thousands of flowers that lasted for three to four weeks at 68°F (20°C) (Figure 6A and 6B).

We tested the vernalization response of campanula ‘Birch Hybrid’ to a range of temperatures and durations. Campanula ‘Birch Hybrid’ flowered when plants were vernalized for five to seven weeks at 32°F to 46°F (0°C to 7.5°C). Interestingly, complete flowering occurred at temperatures as high as 55°F (12.5°C) when plants were vernalized for 12 weeks. Thus, campanula ‘Birch Hybrid’ can be effectively vernalized at a relatively broad range of temperatures (between 32°F to 55°F), depending on the duration of the treatment. In contrast, veronica ‘Red Fox’ flowered only when vernalized between 32°F to 46°C (0°C to 7.5°C) and longer durations at warmer vernalization temperatures resulted in incomplete flowering. Thus, although the effective vernalization temperature ranges vary between species, most can be vernalized successfully between 32°F to 46°F (0°C to 7.5°C).

Potential To Subsequently Rebloom In The Garden Most early-spring ephemerals and late-spring perennials produce one flush of flowers and they do not produce a second flush unless vernalized again (Figure 7). Therefore, many of these species should be precisely scheduled and sold before the flowering period is over. An important consideration is educating the buyers to expect a single flush of flowers in the spring, similar to spring flowering bulbs. A few exceptions exist, including some new hybrids of heucherella and campanula ‘Kent Belle,’ which may rebloom in the garden.

Table 2
These day-neutral plants are vegetatively propagated and do not require bulking prior to vernalization. Therefore, these plants can be cooled in a plug tray and then potted and forced.
Species	Prop	Vernalization duration at 41°F (wk)	Weeks to flower at 68°F	Comments
Campanula × ‘Birch Hybrid’	Veg.	≥5	6	Great pot plant. Thrives in cooler climates with high light. Supplemental light increases the number of flowers considerably.
Campanula ‘Kent Belle’	Veg.	15*	6-7	Fantastic campanula with large blue bell-shaped flowers on upright stems. Plants are larger reaching 24-28" tall when finished.
Delasperma cooperi	Veg.	15*	9	Trailing habit. Can become intertwined and this ruins subsequent flower display. Performed well in hanging baskets in our trial.
Veronica ‘Sunny Border Blue’	Veg.	6	9	PPA selection. Great plant though may be susceptible to powdery mildew. Important to have disease-free starting material.
Veronica spicata ‘Red Fox,’ ‘Icicle’	Veg.	6	7-9	These little veronicas have a shorter bloom period but retain their lower leaves better than ‘Sunny Border Blue’. Useful garden plants and are easily forced as smaller pot plants.
*only vernalization duration tested in the trial.

Figure 6A

Figure 6B

Following 5 to 7 weeks of vernalization treatment at 41°F (5°C), campanula ‘Birch Hybrid’ produces a fantastic display of thousands of flowers even when started from a small tip cutting.
6A. Tip cutting
6B. Forced plant

Figure 7

After the first flush of flowers, pulmonaria ‘Opal’ did not flower again in the gardens.

Summary

There are many great perennials that require vernalization for flowering and are day neutral. Most early-spring ephemerals flower quickly when forced at 68°F (20°C) and do not produce sufficient vegetative growth after cooling to fill the pots. To produce marketable plants from this group, bulking of plants in the final container is essential prior to vernalization. For seed-propagated aquilegia, Euphorbia polychroma, Alchemilla mollis and heuchera cultivars, bulking is essential to overcome juvenility. Several late-spring perennials also require vernalization and are day neutral but these flower more slowly during forcing at 68°F (20°C). These plants can produce sufficient vegetative growth after cooling, enabling growers to vernalize these plants in plug trays. Although we have only found a few plants that fit this category, they are important ones as they can be part of a "quick cropping" program.

About the authors:

Sonali Padhye is graduate research assistant, Erik Runkle is assistant professor and floriculture Extension specialist and Art Cameron is professor, Department of Horticulture, Michigan State University. Beth Fausey is floriculture program manager, Ohio State University Extension in Northwest Ohio. They would like to thank Cathy Whitman for her research contributions and private greenhouse companies that support floriculture research at Michigan State University.