Powering up young plants

Here’s a guide for preplant treatments to finish annuals quickly.


Plant growth retardants are the go-to method for controlling plant size for many growers.
Photos: Martin Bergsma, Adobe Stock

Growing spring annuals is a series of sprints. Hundreds of different annuals are grown from plugs grown from seed or liners grown from cuttings. From the time young plants are transplanted into containers until they are picked up and placed on racks for shipping or onto benches for sale, space and time are tight. Efficient space use and conserving time and labor help make spring production more manageable. One of the most efficient ways to grow plants is to treat them at a young age, such as when they are still plugs or liners, prior to transplant. Much of the efficiency in treating young plants compared to finished plants is directly related to planting densities. For example, during the young plant phase, plants are grown commonly at densities ranging from 69 to 200 plants per square foot for a 100-cell tray of liners or 288-cell plug tray, respectively. Alternatively, during finishing densities of 9 (4-inch pots grown pot-to-pot) to 33 plants per square foot (1,204 trays) are common. Looking at the difference in plant densities, the efficiency of any treatment applied at the young plant age is bound to be better when compared to the same treatment applied during finishing. This article is going to discuss a few different ideas for things you can do to young plants prior to transplanting them for finishing to simplify finishing.

Controlling plant size during finishing is a near-universal challenge all growers face when producing annuals. The answer? Most of the time, it is going to be a plant growth retardant (PGR) application. Whether foliar sprays or substrate drenches, chemical growth control is used to suppress excessive growth and produce plants proportional to their finished container size. Since PGRs are so frequently applied during finishing, consider treating plants prior to transplanting. All growth-retarding chemicals can be applied as foliar sprays to young plants. However, remember that ethephon (Collate, Florel) applications can also delay flowering, which is likely unwanted for finishing plants in container sizes at or smaller than 4-inch containers. As with foliar sprays during finishing, apply PGR solutions at a volume of 2 quarts per 100 square feet, and maximize drying time on foliage by applying early or late in the day and turning off horizontal airflow fans. Another application method to consider for the young plant phase is plug or liner dips — where trays are placed into a PGR solution for 30 seconds to 2 minutes while solution is taken up by the growing substrate. Compared to foliar sprays, the application time and environment doesn’t really have any effect on the application, making it extremely flexible with respect to timing. However, manage substrate moisture so that plugs and liners are not bone-dry prior to treatment, otherwise excessive PGR solution will be absorbed and growth control will be excessive. Unlike foliar sprays, only ancymidol (Abide, A-Rest), flurprimidol (Topflor), paclobutrazol (Bonzi, Piccolo) and uniconazole (Concise, Sumagic) are taken up by roots and can be applied as dips.

Take steps to manage growth and achieve optimal results at all stages of the growing cycle.

While we want to inhibit excessive growth during finishing, we also want to promote branching. This can be done mechanically or chemically. Mechanically, plants are pinched or sheared to promote branching, and this is commonly done during finishing. However, this can also be done during the young plant stage for some species. Shearing a tray of 100 rooted cuttings is going to take much less time compared to the same plants transplanted into 4-inch containers. Always be sure to sanitize tools to minimize disease spread when working with susceptible crops. Chemical promotion of branching can be a good alternative to mechanical methods because it can save labor and minimize the risk of disease transmission. Both ethephon and benzyladenine (Configure) promote branching in annual bedding plants. However, as mentioned earlier, ethephon may not be a good choice for small container sizes due to the flowering delay effect.

However, if larger containers such as hanging baskets are being grown, the flowering delay may be desirable for the longer crop time. Regardless of container size, benzyladenine can be applied to reduce apical dominance and promote branch development. Not only does spraying benzyladenine save time compared to shearing or pinching, it also can reduce crop time. When plants are sheared or pinched, flower buds may be removed and, as a result, increase the time until flowering for sale. By using benzyladenine, branching can be enhanced without removing any young flower buds and avoiding any delay in flowering. Foliar benzyladenine sprays are a good choice for the young plant stage, and concentrations between 50 and 500 can be used, but in-house trialing will help you determine the best concentration(s) for your different crops. As with other PGR, benzyladenine solutions should be applied at 2 quarts per 100 square feet.

Annual bedding plants are best sold in flower. However, short finishing times or a lack of appropriate infrastructure can make flower induction during finishing more challenging. Consider initiating flowering during young plant production. Since plants are grown at much higher densities during propagation as opposed to finishing, the amount of infrastructure required to induce flowering of short-day and long-day plants is much less than compared to applying the same treatments during finishing. The majority of annual bedding plants have a long-day flowering response and using day-extension or night-interruption lighting is going to be the only way to induce flowering during naturally short days. A variety of lamps can be used, from low-intensity light-emitting diode “flowering lamps” to high-pressure sodium lamps, as long as at least 2 µmol·m–2·s–1 at plant height and sufficient far-red light are provided. For short-day plants, the use of black cloth to truncate naturally long days to create short days. Using black cloth with an aluminized outside can help reduce heat buildup under the cloth, which is helpful in avoiding heat delay as well as avoiding drought stress for young plants with a small substrate volume.

The strategies outlined in here for treating young plants are meant to help streamline the finishing and potentially short. It is important to emphasize that the techniques laid out in this article are to be used on young plants that you will be transplanting for finishing yourself — these should not be used on young plants that are grown and shipped off for others to finish. While many of the finished plant attributes we are trying to achieve by using these techniques are desired by nearly all growers (controlling height, enhancing branching, inducing flowering), you never know for sure how the young plants you produce for others are going to be grown. As a result, you do not want to make decisions for the people finishing the young plants you are growing.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu

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