Oakland, Maryland

Hosta gardeners do not normally consider hosta plants as being among those ornamentals, such as chrysanthemums, poinsettias, spring bulbs and others, that are known to be dramatically responsive to the length of day. As a longtime observer of physiological responses in plants and now a learner about hostas, I am surprised at never having seen a reference on hostas being so described. It is obvious to me, however, that daylength is largely responsible for flowering time, vegetative growth cycle, induction of dormancy, axillary bud formation, late summer root development and other growth phenomena in hostas.

When we see in hosta registration descriptions that H. sieboldiana 'Elegans' blooms in mid-June, that H. plantaginea blooms in August and that H. 'Fall Bouquet' blooms in late September, we accept these descriptions as being accurate for the locations being indicated in the descriptions. We have no trouble acknowledging that the flowering period for each hosta is quite regular each year and is a function of the seasonal environment at the specified locations. We assume that minor variability may occur according to location. In a roundabout and perhaps subconscious way, we are thus recognizing that hosta are responsive to length of day or photoperiod. But we never think or talk about this important plant response determinant, much less understand it. A few points of clarification are perhaps in order to help us understand the role of daylength in hosta growth and development.

Depending on latitude location of our gardens, the length of day varies considerably during the growing season. Daylength, as we all know, is due to the angle of the earth's axis in relation to the sun. At the time of the vernal equinox (March 21) and the autumnal equinox (September 21) when the earth's axis is at a 90 degree angle to the sun, the daylength is 12 hours at all latitudes of the earth. The longest day occurs on June 21 but may vary in length from approximately 16 hours in northern latitudes such as in Canada, to around 14 hours as in much of Florida.

Photoperiodism of plants is simply defined as the growth response to varying lengths of daylight and darkness. These responses may include flowering dates, vegetative growth, formation of rosette foliage in plants, formation of tubers, bulbs, rhizomes and buds, formation of abscission layers in petioles and dormancy. Flowering is the best known response to photoperiodism in plants and is the most dramatic response in hostas.

Short-day plants were defined originally by plant scientists as those which flower when the daylength does not exceed a critical amount. The important factor, however, in initiation of flowering in shortday plants is the long period of uninterrupted darkness rather than the period of light. Short-day plants typically bloom in late summer and early fall.

Long-day plants were described initially by scientists as those which flower during the longest days of the year or when daylengths are equal or greater then some critical period of daylight. More recent interpretations of this phenomenon is that long-day plants are those that flower when they have received a critical period of darkness or less than that amount.

A third category, day-neutral plants, have some other type of stimulus rather than daylength or darkness which causes them to flower. In applying these flowering response categories to the Genus Hosta, it is quite likely that critical daylengths (or nightlengths) could vary from species to species or from cultivar to cultivar, thus having within the Genus, hostas that are short-day types, long-day types and day-neutral types.

An understanding of the complex mechanisms involved in the physiological response of the plants to daylength is important for a better understanding of the plants. Photoreceptors know as phytochromes (of which two types exist in cells of plant) are blue-green proteinaceous pigments which have the capacity to either inhibit flowering or help initiate flowering. One form designated as phytochrome red (PR) absorbs red light from the light spectrum. The other form, phytochrome far-red (PFR) absorbs far-red light which is of a longer wave length than red light. The unstable phytochrome far-red reverts back to phytochrome red during the hours of darkness. In the mixture of phytochrome red and phytochrome far-red within plant cells, the relative amounts of each substance, though minute, determine whether the plant will flower or not. In short-day plants a long period of darkness allows the phytochrome far-red to revert back to PR so that the plant has some minimum time during the 24 hour diurnal cycle with no phytochrome far-red present. This initiates flowering.

In long-day plants the phytochrome far-red induces flowering. Long days cause these plants to produce phytochrome far-red in predominate quantities. During the short night phytochrome far-red is changed to phytochrome red, but because the night is short, there is little time with no phytochrome far-red present, therefor the plant flowers rather than being inhibited from flowering.

In hostas it is obvious that there are long-day varieties which bloom early in season and short-day varieties that represent the late bloomers. In between are those hosta types which are either day-neutral types or types not able to respond physiologically to the phytochrome receptors in the plant's cells. Rather, they may respond to other internal or external stimuli.

Early bloomers (long-day types such as the sieboldianas and fortuneis) initiate their flowers as daylengths extend beyond a critical photoperiod that varies for each type. An average critical photoperiod for early types may be approximately 13-1/2 hours which occurs in late April or early May in the mid U.S. gardening latitudes.

Late bloomers (short-day types such as H. plantaginea, kikutiis, longpipes and their hybrids) probably initiate their flower primordia when daylengths are less than 13-1/2 hours or when hours of darkness are adequately long to allow phytochrome far-red reversion described previously to continue for an adequate period of night-time hours.

By now, readers have probably thought of exceptions to response or non-response of hostas to the photoperiod. I recall several exceptions also. For example, Kenn Skrupky of Rice Lake, Wisconsin, described to me how he grew H. plantaginea under fluorescent lights 24 hours a day during winter months. Sometime between mid to late April he removed the plants from the lights when they received perhaps 13-1/2 to 14 hours of normal daylength. After about three weeks he observed a flower scape emerging. So what is an August Lilly (presumably a short-day plant) doing when it blooms in June? Of course, Kenn was happy to have pollen of H. plantaginea to spread on early pod parents of his breeding program. This technique of modifying daylength to achieve out of season flowering should be a boon to scores of hosta hybridizers who normally have to be satisfied with hosta flowers in their specific photoperiod-induced season only.

Another exception to the rule of photoperiod response in hostas includes those few occasions when we encounter blooming out of season. At this moment I have about 20 small plants of H. 'Sum and Substance', which I propagated from leaf bud cuttings last fall. Plants are only about 10 inches tall now with about 10 leaves each. Three-year-old mother plants are perhaps two feet tall and bloom normally in July. My small plants were too young to flower in their normal flowering season but now in August have budded scapes about 15 inches high. They will bloom in September, not as short-day plants but probably as examples of day-neutral type plants blooming later than normal because they are juvenile.

We have all noticed that small plants, either recently out of tissue culture or single, small divisions usually do not flower in their normal flowering season the first year. Perhaps it is because they have not yet become large enough nor have become adequately established to demonstrate their response to either daylength or other external or internal stimuli.

And what about those early-started seedlings which have been growing under 24 hour lights since October? By March some begin to bloom, but not all of them. Those that bloom under lights are probably long-day types. The short-day types probably will bloom later when they are large enough to respond to their phytochrome receptors and when their nights become long enough.

What is the explanation when an apparent long-day plant develops a flowering scape in fall? I observed one small plant of H. montana 'Aureomarginata' with a bloom scape in late September. I suspect that this plant achieved a critical size or state of physiological maturity while the daylength was still adequately long (perhaps in late July) to allow the photoperiodic response to flower primordia initiation to proceed. This particular plant may not normally bloom before frost because growth is gradually slowing as a result of successively reduced daylengths and slowly dropping fall temperatures. If the plant where to be brought indoors in a greenhouse early enough where it may receive additional hours of light and higher temperatures, it probably would flower.

Is "re-blooming" a photoperiodic response? The term re-blooming, in my view, is a misnomer. We all have observed that any given division will bloom once early in the season. Its central growing point develops a flowering scape and it blooms in its normal season. Apical dominance is destroyed and other small divisions within the clump may then grow larger and, depending upon their ability to respond to external stimuli, may flower later than normal, out of season. In this case a division is not blooming again. Other divisions are simple blooming later.

I recall an anecdote on a possible photoperiodic response in hostas that is unique. Alan Tower told me about a nursery visitor, a Mexican person who lived in the mountains near Guadalajara, Mexico. Hostas five years old had never flowered there. Was this a photoperiodic response of perhaps long-day hosta varieties planted in a geographic latitude where most days are short? Unfortunately, detailed information was not obtained so we do not know for sure if failure to flower was a photoperiodic response.

As daylengths are reduced to about 12 hours in mid to late September, and as average temperatures gradually become lower, dormancy gradually becomes apparent in hostas. It is likely that the interaction of both shorter days and lower air and leaf tissue temperatures are the external stimuli responsible for initiating dormancy in hostas. Dormancy proceeds at a slow pace but once it has begun, it appears to be irreversible.

I hope these views (derived from observation and analysis only) will stimulate re-thinking various aspect of hosta physiology. There is nothing revolutionary about these concepts since they are merely a re-synthesis of plant knowledge applied to our beloved genus Hosta. Re-thinking photoperiodism in hostas certainly should suggest many new possibilities, new techniques and even new goals in hosta breeding.

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The American Hosta Society