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Life After Cold!
Herbaceous Perennials That Require Vernalization And Long-Day Photoperiods To Flower
by SONALI PADHYE, BETH FAUSEY, ERIK RUNKLE and ART CAMERON
FLOWER induction of most cold-hardy herbaceous perennials can be categorized based on their response to vernalization and photoperiod (See Table 1). In previous articles, we discussed general aspects of the vernalization process and the specifics for plants that have an obligate vernalization requirement and are day neutral for flowering. In this article, we discuss responses of perennials that require both vernalization and subsequent long days for flowering.
From an ecological point of view, the dual control of flowering by vernalization and long-day photoperiods ensures that plants will not flower during a warm spell late in the winter, and, in fact, will only flower in the summer after the day length exceeds a critical photoperiod. This strategy seems to be important to plants native to regions where the springs can be mild but are often interspersed by brief cold spells late in the season.
At Michigan State University, we have studied the flower induction requirements of more than 400 herbaceous perennial species and cultivars during the last 15 years. We have learned that many perennials require vernalization but, following cold, most are day neutral with respect to photoperiod. In fact, there are relatively few examples of plants that absolutely require both vernalization and long-day photoperiods (see Table 2). Some of the more popular plants in this group include Campanula punctata ‘Cherry Bells,’ Coreopsis grandiflora ‘Baby Sun,’ Coreopsis grandiflora ‘Sunray’ and Tanacetum niveum ‘Jackpot.’ In each of these selections, flowering is strictly regulated and occurs only after vernalization, and then only with exposure to long days during forcing.
A second category of perennials has an obligate vernalization requirement followed by a facultative response to long-day photoperiods (Table 2). Examples of plants in this category include Astilbe chinensis ‘Pumila,’ Campanula garganica, Digitalis thapsi ‘Spanish Peaks’ and Isotoma axillaris (previously known as Laurentia axillaris). For these plants, forcing under long days greatly improves flowering characteristics in one or more ways, including greater flowering percentage, reduced time to flower, increased bud number and improved branching. In fact, in many cases, long days are essentially required to obtain plants with acceptable horticultural quality. In both cases, the relatively complex flower induction requirements of the herbaceous perennials listed in Table 2 provide extra challenges for growers who wish to produce them in flower at specific times of the season.
In some instances, we have observed that when actively growing, vernalization-requiring perennials are held in the greenhouse for a prolonged period of time, sporadic flowering can occur. For example, during one trial with Campanula punctata ‘Cherry Bells,’ two plants out of 50 in the population flowered within six months when grown under a 16-hour photoperiod without cooling. This implies that botanically, campanula ‘Cherry Bells’ may not have an obligate vernalization requirement. However, growers are interested in complete, rapid, uniform and profuse flowering to obtain a saleable crop. Therefore, we have included campanula ‘Cherry Bells’ in the obligate vernalization category. It is important to note that categorization into vernalization and photoperiod responses is based on horticultural requirements for flowering.
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Table 1
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| Flower induction of a wide range of northern temperate herbaceous perennial plants can be categorized based on their response to vernalization and photoperiod. In our experience, short-day plants typically do not respond to vernalization. |
| Long-day photoperiod requirement |
Vernalization requirement |
| None |
Facultative |
Obligate |
| None |
These plants flower regardless of cooling or photoperiod |
Cold improves flowering but is not absolutely required; plants are day neutral |
Cold is absolutely required for flowering; plants are day-neutral after cold |
| Facultative |
These plants do not require cold but flowering is improved by long-day photoperiods |
Cold and long days both improve flowering percentage, number and/or quality, or potentially reduce time to flower |
Cold is absolutely required; long-days improve plant quality and/or reduce time to flower |
| Obligate |
These plants do not require cold but have an obligate long-day photoperiod requirement |
Cold is helpful, but long-day photoperiods are absolutely required for flowering |
Cold and long-day photoperiods are absolutely required for flowering |
| Table 2 |
| Species-specific information of herbaceous perennials that require cold and long-days during forcing for flowering. Propagation: seed (S), vegetative (V) or tissue culture (TC). |
| Species |
Prop |
Cold duration at 35 to 41°F (weeks) |
Long-day Photoperiods |
Weeks to flower at 68°F |
Comments |
| Astilbe chinensis ‘Pumila’ |
S/V |
9 |
Beneficial |
10 |
Great little astilbe but slow to flower. Plants were dormant under 9 h photoperiod and flowered after cooling. Flowering was hastened under 16 h photoperiod compared to 12 h photoperiod by up to a week. Bulk in fall prior to cold for the best flower show. |
| Brunnera macrophylla ‘Jack Frost’ |
V |
6 |
Required |
3-4 |
A great brunnera. Only a few plants flowered without cooling. Under SD, goes dormant and then requires LD for best growth. Although flowers are not necessary, they add to attractiveness. |
| Campanula garganica |
S/V |
15* |
Beneficial |
7 |
Charming campanula species. LD were very beneficial - decreased time to flower and increased flower number. |
Campanula punctata
‘Cherry Bells’ |
V/TC |
15* |
Required |
5-6 |
Very exotic campanulas with large drooping red flowers. Little to no flowering without vernalization and long days. |
Coreopsis grandiflora
‘Baby Sun’ |
S |
6 |
Required |
7 |
‘Baby Sun’ nice as a smaller Coreopsis with rebloom potential. |
| Coreopsis grandiflora ‘Sunray’ |
S |
6 |
Required |
7 |
‘Sunray’ can fill larger containers without a problem. Blooms after cold under long days. Short days can substitute for cold. |
| Digitalis thapsi ‘Spanish Peaks’ |
S |
15* |
Beneficial |
8-9 |
Plant Select choice from Colorado with compact growth habit and cute little flowers. Supplemental light nearly doubled flower number. Would not require PGRs. |
| Euphorbia ‘Jade Dragon’ |
V |
15* |
Beneficial |
3 |
0 and 15 weeks were the only cooling durations tested. We expect that 6 to 9 weeks would be adequate. During the force, flowers formed under both SD and LD but only opened under LD. Flowering in greenhouse was atypical compared to outdoors. Long bloom time in the garden. |
| Gaillardia ×grandiflora ‘Goblin’ |
S |
9-12 |
Beneficial |
7 |
Goblin is a facultative vernalization and facultative LD plant. Cold improved percent flowering, uniformity in flowering time, and hastened flowering. LD increased flower number and height and hastened flowering. Note that some of the new cultivars of Gaillardia including Fanfare and Oranges and Lemons do not require vernalization and in fact are day-neutral. |
| Hosta ‘Golden Tiara’ and ‘Golden Scepter’ |
V |
3 |
Beneficial |
12 |
Petite hosta cultivars. Good for borders or edging. Starting with multiple-eye divisions resulted in best results. Flowering is not necessary for sales but best growth after cold under LD. Leaf production can be slow under SD. |
| Isotoma (Laurentia) axillaris |
S/V |
6 |
Beneficial |
10 |
Fragrant lavender flowers in profusion! Marginally hardy in North - but fantastic nevertheless! LD highly beneficial. Can eventually bloom without cold - but vernalization treatment is highly beneficial (we would consider horticultural obligate) |
| Lavandula angustifolia ‘Hidcote’ and ‘Munstead’ |
S/V |
10-15 |
Beneficial |
7 |
Under long days 66% Hidcote plants flowered without cold and all plants flowered after cooling. No plants flowered under SD with or without cold. No Munstead plants flowered without cold under any photoperiod. After 12 wks of cooling, all plants flowered under LD and 60% flowering under SD. Obviously, this plant has a more complex response to vernalization and photoperiod. |
Oenothera fruticosa ‘Youngii-
Lapsley’ |
V |
≥3 |
Beneficial |
3 |
This species of Evening Primrose is very showy in bloom. Youngii-Lapsley did not bloom without cold. Plants were slower to bloom under short days and were shorter. In our trials, higher light greatly increased the number of flowers produced. |
| Penstemon barbatus ‘Elfin’ |
V |
6 |
Required |
5-6 |
Our limited studies indicate that this cultivar appears to require both cold and LD for flowering. |
| Rodgersia aesculifolia |
V |
15* |
Beneficial |
6-8 |
Has proven difficult in our production system. It needs plenty of size before initiating the cold treatment. These plants like to be kept on the moist side. |
| Tanacetum niveum ‘Jackpot’ |
S |
12 |
Required |
5 |
Silvery foliage and numerous daisy-like flowers. ≥12 weeks of cold resulted in complete flowering. Plants have a juvenility period that must be first be met by bulking prior to cooling. |
*0 and 15 weeks were the only duration of cooling tested in the study.
The results reported in this table are based on greenhouse trials conducted at Michigan State University. Altering production practices may affect outcomes of your trials. There is some evidence that high light levels (i.e. outdoors) may overcome some or all of the vernalization requirement in some plants.
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Herbaceous Perennials That Require Vernalization And Long-Day Photoperiods
Tanacetum niveum ‘Jackpot’
Tanacetum niveum ‘Jackpot’ has an obligate requirement for cold and long-day photoperiods. In our trials, complete flowering of tanacetum ‘Jackpot’ occurred only after cooling at 41°F for ≥12 weeks, and only when followed by forcing under long days (Figure 1). No plants flowered without cold regardless of the photoperiod. In fact, without a vernalization treatment, plants remained on the bench in a heated greenhouse for months and still remained vegetative. Following cold treatment, supplemental lighting increased the number of buds but did not affect time to flower in this cultivar. Tanacetum ‘Jackpot’ is seed-propagated and must be first grown to overcome the juvenile stage before exposure to cold.
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Figure 1
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Tanacetum niveum ‘Jackpot’ flowered after ≥12 weeks of cooling at 41°F, but only under long days.
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Coreopsis grandiflora ‘Sunray’
Coreopsis grandiflora ‘Sunray’ is another example of a perennial that flowers only after vernalization when grown under long days (Figure 2A). When seed propagated, plants should be bulked until 16 leaves are formed to overcome juvenility. We have found that ‘Sunray’ plants can be cooled in plug trays prior to forcing under long days. In separate studies, we learned that the critical photoperiod of coreopsis ‘Sunray’ was 12 hours and, therefore, plants must be forced under photoperiods of 12 hours or longer for flowering.
Interestingly, coreopsis ‘Sunray’ also flowered after exposure to short days followed by long days. Thus, in this cultivar, short days can substitute for vernalization (Figure 2B) and holding plants under nine-hour short days for six weeks is as effective as vernalization. This response type (short-long-day) is fairly unique in our experience.
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Figure 2A
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Coreopsis grandiflora ‘Sunray’* flowered after 16 days of vernalization at 41°F, followed by forcing under 16-h photoperiod.
*Please note that the effective duration of vernalization treatment is based on our trials on a clone of this cultivar.
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Figure 2B
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Coreopsis grandiflora ‘Sunray’* completely flowered in response to 42 days of 9-h short day treatment, followed by forcing under 16-h photoperiod.
*Please note that the effective duration of short day treatment is based on our trials on a clone of this cultivar.
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Herbaceous Perennials That Absolutely Require Vernalization And Have A Facultative Response To Long-Day Photoperiods
Campanula garganica
Campanula garganica did not flower without cooling in our studies and flowered under both long days and short days after 15 weeks at 41°F (Figure 3). However, more plants forced under long days flowered and long days reduced the time to flower by 25 days and increased the number of buds at first open flower. Thus, the response to long days is not completely obligate, but highly desirable for production of quality plants. Therefore, for commercial production, Campanula garganica should be vernalized and then forced under long days.
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Figure 3A
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Campanula garganica did not flower without cold under 9-h photoperiod or 16-h photoperiod provided by night interruption or high pressure sodium lamps.
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Figure 3B
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Though Campanula garganica flowered under short days and long days after vernalization at 41°F for 15 weeks, long days improved flowering characteristics.
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Digitalis thapsi ‘Spanish Peaks’
Digitalis thapsi ‘Spanish Peaks’ required cold to flower in our trials. No plants flowered without cold, even though they developed considerable size (Figure 4A). After 15 weeks at 41°F, about 50 percent of the plants flowered under short days while almost all of the plants flowered under long days (Figure 4B). Time to flower was not affected by the forcing photoperiod in this cultivar. The quality of ‘Spanish Peaks’ was greatly improved under long-day photoperiods by increasing light levels from 7 to 17 mol·m-2·d-1 (Figure 4B). We recommend vernalization treatment followed by forcing under bright, long days.
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Figure 4A
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Digitalis thapsi ‘Spanish Peaks’ did not flower under any photoperiod without vernalization.
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Figure 4B
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Following vernalization, more plants of Digitalis thapsi ‘Spanish Peaks’ flowered under LD (80-90%) than SD (30%), and increasing light levels further imcreased flowering percent.
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Isotoma axillaris
Isotoma (previously known as Laurentia) axillaris has a reputation of being a late bloomer when started from seed. In some garden books, it states that plants will not bloom until August or even September. However, we have learned that vernalization greatly accelerates flowering of isotoma (Figure 5). In our studies, all plants cooled for 10 weeks flowered when grown under long days. Flowering percentage of plants forced under short days increased from 30 percent to 80 percent as the duration of cooling increased from 10 and 15 weeks (Figure 5). Thus, exposure to longer durations of cold reduced the requirement of long days in Isotoma axillaris. Forcing under long days also reduced the time to flower by three to four weeks. For quick and uniform flowering, Isotoma axillaris should be vernalized at 41°F for six weeks and forced under long-day photoperiods. We have found that vernalization temperatures below 40°F are injurious to isotoma plugs and that temperatures as high as 55°F can be effective.
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Figure 5A
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Isotoma axillaris did not flower under any photoperiod without vernalization.
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Figure 5B
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Following vernalization, plants of Isotoma flowered poorly under short days, but readily under long-day photoperiods. Increasing light levels further improved flowering performance.
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Producing Plants With Dual Flower Induction Requirements
Perennials discussed in this article have both vernalization and long-day requirements for flowering. Some plants in this category should first be grown to an adequate size ("bulked") prior to cooling. The effect of photoperiod during bulking is species specific. For example, we successfully bulked Campanula punctata ‘Cherry Bells’ under short days prior to cooling. However, Astilbe chinensis ‘Pumila’ plants bulked under a nine-hour photoperiod became dormant, with cessation of growth and loss of leaves. This dormancy ended only after cooling.
We have vernalized many perennials under a nine-hour photoperiod at 41°F. We have often seen that many perennials that go dormant under short days also show browning or loss of leaves in the cooler. However, as in the case of Astilbe chinensis ‘Pumila,’ the roots and crown survive during the cooling and flourish later during the force. We have also compared the effect of cooling Coreopsis grandiflora ‘Sunray’ under short days and long days at 41°F. In this cultivar, the photoperiod during cooling did not affect the subsequent flowering response.
The best compromise temperature for vernalizing plugs and young herbaceous perennial plants is 41°F. Lower temperatures can injure some plants on occasion and higher temperatures may not always be as effective. Still, temperatures up to 55°F can be effective for vernalizing select perennials, including Isotoma axillaris.
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 former graduate student at Michigan State University and is currently floriculture program manager, Ohio State University Extension in Northwest Ohio. They would like to thank Cathy Whitman for her research contributions and private floriculture companies that support floriculture research at Michigan State University.
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