The importance of sched-uling flowering is increasing as retail outlets require marketing plants with color and the need to minimize space, labor, and heating and electrical costs swells. The floriculture production programs at the University of Minnesota, St. Paul, Minn., and Michigan State University, East Lansing, Mich., have focused on identifying what factors affect flowering of ornamental crops and developing ways that growers can use that information to flower crops at desired times. Some of that work will be summarized in this article.
In addition, we are introducing three new concepts that we will elaborate on in future articles. These concepts and additional research information may greatly increase the ability of growers to schedule bedding plants effectively.
Seed-propagated plants transition from an “immature” to a “mature” state as they age. This same phenomenon occurs with animals, birds and many other life forms; most animals and birds are incapable of reproducing immediately after birth. They must first grow and develop to a specific age before they can produce offspring. Similarly, plants must reach a developmental stage before they can produce flowers. Therefore, growers cannot induce flowering until a crop has transitioned from an immature to a mature state.
Plant species differ in when they transition from immaturity (the juvenile stage) to maturity (the mature stage). For instance, many annual bedding plants transition from the juvenile to mature stage very early in development, usually during the first month after germination. Herbaceous perennials often take 3-6 months to transition to a mature state, whereas trees can require more than 10 years. In nearly all instances, plants that are propagated by stem cuttings are mature; thus, maturity is not an issue with these crops.
How can we tell when a plant is mature? The temperature at which we grow a crop controls how fast the crop develops. A crop grown for three weeks at 70° F will obviously be larger and have more leaves than a crop grown for three weeks at 60° F. Therefore, we have to measure a plant’s developmental age in a different way than time.
We believe the best way to measure plant maturity is to count the number of nodes that have unfolded on the main stem. (A node is the point of leaf attachment to the stem. Plants with an opposite leaf arrangement have two leaves per node, whereas plants with an alternative leaf arrangement on the stem have one leaf per node.)
For instance, petunia ‘White Storm’ and ‘Purple Wave’ transition from immature to mature plants when they have unfolded one and four nodes, respectively. In contrast, a perennial crop such as heuchera (coral bells) may need to unfold 14 nodes before seed-lings become mature plants.
Figure 1, left, shows the node numbers when different annuals and perennials transition from immature to mature plants. Remember, cuttings taken from stock plants are almost always mature and are capable of flowering and, thus, vegetative annuals and perennials are not included in the table.
Once a plant is mature, it can be induced to flower. The conditions that promote flowering vary with species. For instance, some species are induced to flower by the day length (or photoperiod) plants are grown under. Some species are induced to flower by the total amount of light plants receive per day. Some species, particularly perennials, are induced to flower by a period of cold temperature (optimally 42-48° F), which is known as a vernalization treatment. The sections below review how photo-period and total light per day affect flowering in bedding plants.
Photoperiod. Once plants are mature and have gained the comp-etence to flower, flowering in some species can be controlled by day length or photoperiod. Most plants can be categorized into five photoperiodic response groups based on how photoperiod influences their flowering:
Many bedding plants are photoperiodic, meaning time to flower is influenced by the photoperiod that plants are grown under. Figure 2, right, shows the photoperiodic response groups for 25 common bedding plants. Placing mature seedlings or cuttings under the photoperiod that induces flowering will result in the quickest flowering.
Total light per day. Some bedding plants flower quicker when grown under high light levels. Why is this? Our controlled studies have identified that some plants transition from the immature to mature stage earlier when seedlings are grown under a high daily light integral (DLI) (high total amount of light per day). When a plant flowers earlier under high light intensities (more total light per day), the plant has a facultative irradiance response. When flowering time is not accelerated by high light intensities, the plant has an irradiance indifferent response. Figure 2, right, shows the irradiance response groups of 25 bedding plants.
How do we measure the light a plant can use? A foot-candle meter is not the best way to measure lighting for plant growth because a foot-candle is based on the light a human eye can see and not what a plant can use. Plants respond to photosynthetic light, and, thus, the appropriate meter that should be used by growers when measuring lighting for plant growth is a quantum sensor or quantum meter.
A quantum sensor measures only the light a plant uses for photosynthesis. Even more helpful than a quantum sensor is a light integrator, which measures the number of moles of photosynthetic light per day as well as the intensity of that light at any point in time. Using irradiance response groups to help schedule flowering will require you to have a light integrator, since the irradiance response categories are based on the average DLI.
Facultative irradiance plants flower earlier as the moles of light Á per day increase. When does increasing light no longer result in earlier flowering? Our research, which is reinforced by growers’ experiences, suggests that growers will see a benefit with most bedding plants as total light per day increases from 2 to 12-14 moles. A few crops, such as nicotiana, benefit from an even higher DLI, but most benefit from up to 12-14 moles of light per day.
Scheduling flowering in bedding plants has been difficult in the past. Flowering times can vary from year to year and from greenhouse to greenhouse. We are introducing three new terms that, if used by the industry, will help growers schedule crops. In addition, we advocate that supply companies and researchers use these terms when developing crop schedules so there is consistency across the industry. The three new concepts we are proposing for bedding plant scheduling are: response groups, grow times and re-bloom time. Each of these is defined below.
Response groups. We are introducing the concept of “response groups” for bedding plant production. The industry is familiar with the concept of response groups for potted plants, such as poinsettia or chrysanthemum. The response group of a potted poinsettia or chrysanthemum is the time from the beginning of inductive conditions (short days with poinsettias and mums) until the time of open flowering when grown at 68/65° F day/night temp-eratures. The response group varies among species and cultivars.
We define response group of a bedding plant as the time from the start of inductive conditions on mature plants until flowering when grown at 68° F day and night. This is slightly different than response groups developed for potted plants in that both the day and night temperatures are 68° F. The rationale for this is there are a number of bedding plants (New Guinea impatiens, melampodium, cleome, vinca, etc.) that can dev-elop very slowly at temperatures less than 68° F. In addition, using a 68/65° F day/night temperature standard may not be the best standard temperature regime because day length varies during a year and in different parts of the country and, therefore, the flowering time will vary because the average daily temperature varies. Using a constant 68° F temperature standard eliminates this problem.
By using the response group concept, growers can identify exactly how long it will take from the start of an inductive treatment (such as long days for obligate long-day plants) until first flowering. This would greatly simplify scheduling! In time, we hope to develop tables where growers can have the response groups at different temperatures, but for now, we suggest an industry standard of reporting response times at the standard constant 68° F temperature.
Grow time. Some growers may not want to monitor when plants are transitioning from immature to mature plants or the information may simply not exist. How can we more easily schedule flowering in these crops? We need to resurrect the term “grow times,” but in a new way. Production times or grow times of some bedding plant crops have been reported in the past but are hard to compare because many are determined using different (and sometimes unmonitored) temperatures and photoperiods.
Therefore, we are resurrecting the term and defining it with standardized conditions. We are defining the grow time of a seed crop as the time it takes from germination until flowering under inductive conditions (juvenile period + response group) when grown at a constant 68° F. Such information would be very helpful for growers to schedule crops as well as enable comparisons of new varieties to existing bedding plant crops.
With time, such information should be provided to growers from university researchers and seed companies at a variety of temperatures. Currently, we are determining grow times for more than 50 bedding plants this winter and spring at the University of Minnesota in association with Wagner’s Greenhouses, Minneapolis and Hugo, Minn. Researchers at Michigan State University are determining grow times for approximately 20 seed-propagated annuals at varying temperatures (including constant 68° F). Eventually, there will likely be grow times for a variety of crops at different temperatures.
Re-bloom time. More and more growers are rooting nonrooted cuttings or are buying rooted cuttings. Scheduling these cuttings to have them in flower after pinching on a specific date can be difficult. John Erwin is presently working with Pleasant View Gardens, Loudon, N.H., to determine the time from pinching liners until re-blooming for some vegetative crops grown at temperatures ranging from 54 to 78° F. As a result of this work, we believe the industry needs a standardized term to report this information to help growers schedule flowering times after pinching across suppliers and universities.
We are proposing the term “re-bloom time” as the time it takes from pinching a cutting until the first flower opens on the shoots that arise after the pinch when grown at a constant 68° F. Re-bloom time certainly varies among species. For instance, the re-bloom time for angelonia ‘Angelface Blue’ grown at a constant 68° F is 56 days. In contrast, the re-bloom time for petunia ‘Supertunia Bordeaux’ grown at 68° F is 32 days. With this information, growers can effectively re-bloom different species for the same date by pinching those species at different times. We are currently working with a number of growers to develop re-bloom times for different vegetatively propagated crops.
Although we would like to have absolutely constant temperatures to effectively schedule a crop, it is impossible in commercial greenhouses. Therefore, we have been looking at ways to allow growers to realistically and more easily monitor crop development.
We are proposing a new way of monitoring crops that is similar to the concept of graphical tracking used for monitoring plant height. Instead of plant height, we are proposing establishing developmental tracks that will allow growers to determine if a crop is ahead or behind schedule at any point in development. A developmental track is based on two pieces of information: degree hours (or degree days) required to flower a crop under inductive conditions and the time in which you desire to do that.
For example, apple production time is often tied to the number of degree days that a plant accumulates in the spring. Degree days are calculated by accumulating the time that the temperature is above the base temperature of a plant (often 40° F). Apple trees bloom Á when they accumulate a specific number of degree days. We are using a similar concept to apply to bedding plant production.
If we know how many degree days, or degree hours, a plant needs to reach a flowering size, then a grower can determine on a daily or weekly basis what temperature he or she needs to grow the crop at to have it flower at a desired time. Conversely, a grower can tell how many days ahead or behind he or she is. We will be working with Paul Fisher at the University of Florida, Gainesville, Fla., to develop developmental tracks using computer software programs for different seed-propagated bedding plants and vegetative liners.
This concept will provide growers with a developmental track model where they will be able to determine if a crop is developmentally ahead or behind schedule at any point in development. This will allow growers to make temperature management decisions on a daily or weekly basis to ensure that their crops flower when they need them, they adjust shipping selling dates accordingly and/or they do not spend more money heating a crop than needed.
These new tools and concepts could greatly increase the ability of growers to effectively schedule bedding plants. The concept of response groups would allow growers to effectively schedule bedding plants as we do with chrysanthemum and poinsettias. The concept of grow times would increase the ease in scheduling crops from germination to finish for bedding plant growers. The concept of re-bloom times would allow growers to effectively schedule flowering of vegetative annual liners. Collectively, the application of these concepts could increase the ability to schedule crops and, if accepted as industry standards, allow direct comparisons between growers and provide a standard for information from suppliers and propagators.