MSU Extension Floriculture & Greenhouse Crop Production

Improving Perennial Flowering

Cold and light are two factors that can greatly influence the flowering of some perennials. by BETH FAUSEY, SONALI PADHYE, ERIK RUNKLE and ART CAMERON.

In the previousfour articles of thisseries, we discussed the general aspects of vernalization and the specific responses of cold-requiring herbaceous perennials with day-neutral and long-day photoperiod requirements. In this article, we focus on the facultative nature of vernalization.

Perennials with a facultative vernalization response often benefit from a cold treatment at 35ºF to 45°F but do not absolutely require exposure to cold for flowering. These perennials may flower faster, have sturdierstems, increased vigor and have a more abundant flower display following a cold treatment compared to nonvernalized plants.

In some cases, high DLIs, long day lengths(at least 16 hours) or both can partially or fully replace the need for vernalization. Thisis particularly important during winter production and forcing of perennials when natural day lengths are short and the DLI islow. For example, flowering of non-vernalized perennialssuch as Lychnis floscuculi‘Jenny’is generally poor, delayed and non-uniform unless plants are grown under long day lengths and with high irradiance (Figure 1). When DLI islow outdoors, many plantsrequire supplemental lighting for at least 16 hoursto achieve the necessary DLI for adequate flowering and plant quality. Although flowering occurs under low-light conditions, it may not be comparable to the flowering response achieved undersimilar photoperiod and DLI conditionsfollowing vernalization (Figure 1).

Figure 1. Flowering of non-vernalized Lychnis flos-cuculi ‘Jenny’ exposed to a 9-hour short day with low light (top left), a 16-hour long day with low light (top center), and a 16-hour long day with high light (top right; 30 percent higher daily light integral). In contrast, vernalized Lychnis flos-cuculi ‘Jenny’plants flower profusely under both short and long days following a 15-week vernalization treatment at 41°F.

Figure 1. Flowering of non-vernalized Lychnis flos-cuculi ‘Jenny’ exposed to a 9-hour short day with low light (top left), a 16-hour long day with low light (top center), and a 16-hour long day with high light (top right; 30 percent higher daily light integral). In contrast, vernalized Lychnis flos-cuculi ‘Jenny’plants flower profusely under both short and long days following a 15-week vernalization treatment at 41°F. days. Nearly all plantsflowered under both short and long daysfollowing a 15-week vernalization treatment at 41°F. In addition, vernalized plants had more vigorous growth and improved flowering uniformity. Flowering time also was reduced by two to four weeks.

Corydalis‘Blackberry Wine’responded similarly with only 10 percent to 20 percent of non-vernalized plants flowering under short days or long days with low light, while all plants flowered following a 15-week vernalization treatment at 41°F. Vernalized plants flowered three to four weeks earlier under long days and 10 weeks earlier under short days compared to non-cooled plants.

Digitalis. In the past, we found that foxglove species and cultivars generally require or benefit from exposure to five or more weeks of vernalization at 41ºF before forcing. Although some foxgloves, such as‘Carillon’ and‘Foxy,’may flower without cold, flowering of vernalized plants is generally more rapid and uniform (Figure 2). We also found that flowering percentage of digitalis ‘Foxy’was highly dependent on the greenhouse light environment and increased when plants were grown under an elevated average DLI. With adequate light, all plants flowered under long days. Increasing vernalization treatment improved flowering under short days, but flowering never reached 100 percent.

Leucanthemum. We have evaluated a number ofshasta daisy cultivars and all benefit from a vernalization treatment (Table 2). In particular, leucanthemum‘Snowcap’ is an obligate long-day plant without cold and a facultative longday plant following cold treatment. About two-thirds of plantsflower without vernalization, but only if grown under a photoperiod of at least 16 hours or provided with a four-hour night interruption (Figure 3a). Exposing plantsto a cold treatment of at least five weeks at 41°F greatly improvesflowering uniformity. Flowering also occurs under shorter day lengths after vernalization but is delayed compared to when grown under long days (Figure 3b). Vernalized plants also form three to four times as many flowers under long days than short days.

at least five weeks at 41°F greatly improvesflowering uniformity. Flowering also occurs under shorter day lengths after vernalization but is delayed compared to when grown under long days (Figure 3b). Vernalized plants also form three to four times as many flowers under long days than short days.

In contrast to‘Snowcap,’ leucanthemum‘Becky’ is an obligate long-day plant regardless of vernalization and does not flower under short day lengths. Plants flowered completely without a cold treatment only when grown under long days and with ample irradiance. Only 60 percent of non-cooled ‘Becky’flowered when grown under a 16-hour day and a low DLI. In contrast, all plants flowered when grown under long days with ~30 percent more light (Figure 4). All plants flowered under long days following vernalization.

Maximizing Production With Facultative Vernalization Requirements

Although the majority of the perennials listed in Table 1 do not absolutely require cold for flowering, flowering percentage, timing, uniformity and plant quality are improved when plants are vernalized. Therefore, when possible, we recommend that growers provide a vernalization treatment (at least six weeks at 41°F) for these perennials. In some perennials, a high DLI can compensate for no cold treatment and vernalization can override the requirement for a particular photoperiod to induce flowering. If forcing non-vernalized perennials to flower and natural light levels are low, maximize the greenhouse light environment by providing long-day lighting and increasing the DLI by using supplemental photosynthetic lighting.