Fig. 1. These two different kalanchoe cultivars were grown under a range of day lengths. A flower represents plants that were flowering, while a leaf represents plants that remained vegetative. For both cultivars, the critical day length is 12 hours.
Diagram courtesy of Christopher J. Currey

Spring is well underway. Many of you are already selling annuals and perennials and, if you aren’t yet, you will be soon. One of the most important factors in marketing annuals and perennials is to have crops in flower when they are being marketed. This is a great way for plants to “advertise” their ornamental qualities to prospective consumers and increase sales. In order to have our crops flowering in time for retailing, we need to control flowering.

Flowering control includes both inducing (promoting) and inhibiting flowering. Whether we are trying to induce or inhibit flowering is largely related to container size. For plants grown in small containers such as flats, we are usually trying to accomplish rapid flower induction. The larger the finishing container is, we are usually trying to inhibit flowering for a period of time in order to get adequate vegetative plant growth, allowing the plant to bulk up and fill in the container before we induce flowering. The most common mechanism controlling flowering of spring crops is the day length or photoperiod.

There are three photoperiodic flowering response groups: 1) day-neutral plants, which are unaffected by day length; 2) long-day plants, which flower in response to long days; and 3) short-day plants, which flower in response to short days. For day-neutral plants, the day length neither induces nor inhibits flowering. Long days inhibit flowering of short day plants and promote flowering of long day plants, whereas short days inhibit flowering of long day plants and promote flowering of short day plants.

So, what is a “short” day? Well, the easiest answer is: It depends. What it depends on is the critical photoperiod for plants. For short day plants, the critical photoperiod is that photoperiod at or below which flowering is induced. This can vary among plants. In Fig. 1, we see that the critical photoperiod is 12 hours. At day lengths of 12 hours and less, flowering occurs, whereas plants remain vegetative when the days are longer than 12 hours. For long-day plants, the critical photoperiod is the photoperiod at or above which flowering is induced. Like short day plants, this varies widely with species. For a “safe bet” without knowing the critical photoperiods for every plant, 9 to 11 hours can act as a short day, while days 15 or 16 hours is usually a “long” day.

For very early spring crops or for growers with early market dates, you may have natural short days in your greenhouse. However, now that we are past March 21, day lengths across the country are greater than 12 hours, meaning we likely have days that are “longer” than “shorter” for many plant species. So how can you provide short days for those short-day plants that you are trying to induce flowering or for those long day plants that you want to keep vegetative? Black cloth, blackout cloth, or light deprivation curtains — these are all synonyms — are the only way to create short days when the natural days are long (Fig. 2). By pulling black cloth to cover your crops, these curtains will block the light entirely from your crop (unlike shade cloth, which just reduces light intensity). They are pulled over the crop at the end of the day and removed at the beginning of the day. If these cloths are being pulled manually, the hours tend to coincide with when the workforce arrives and when they leave for the day; that is why 8 or 9 hour day lengths are common. When selecting black cloth, get aluminized cloth that has a reflective material on outer surface of the cloth. This will reduce the amount of heat building up under the cloth when it is pulled at the end of the day.

Fig. 2. Black cloth, used to truncate day length and create short days, is seen here below shade cloth used to reduce light intensity.
Photo: Christopher J. Currey

Many greenhouses do not have shade cloth, which can pose a challenge for providing short days. Instead of investing in a system for your entire greenhouse, consider investing in a smaller area in your propagation space. For crops like marigolds and cosmos, providing short days during the plug stage may be sufficient for inducing flowering even after they are transplanted and placed into long days. Sometimes only two weeks of short days is all that is required to induce flowering. This can be a simple way to induce flowering without making extensive investments.

Managing flowering for your spring annuals can be a challenge. Short days can provide the long day plants the ability to bulk up, or promote flowering of short day plants. Consider the photoperiodic flowering requirements of your crops and their container size to determine if you should be providing short days for your plants. While black cloth is the only way to create short days later in the spring, you may be able to provide short days early in production during the young plant stage and only make an investment that would be a fraction of outfitting your entire greenhouse.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.