Some of the most popular and exciting bedding plants are propagated by stem-tip cuttings. Just like seed-propagated bedding plants, controlling growth of vegetatively propagated plants grown in containers can be a challenge. Starting out with compact rooted cuttings is the best way to begin every crop. However, it is not common for a tray of cuttings to finish taller than you or your customers would like. This article is going to focus on what you can do to avoid excessive liner growth and unwanted stretch.
One of the first things that can prevent excessive stretch is planning and scheduling crops. Getting cuttings stuck, rooted, and shipped and/or planted on time will help minimize unwanted growth. If cuttings are sitting in their trays on the bench waiting for shipping or transplanting, the extra time allows for more growth than is desired or necessary. Extenuating circumstances, such as inclement weather, can prevent shipping. However, proper planning and scheduling can help avoid situations like labor shortages.
When plants are initiated to flower, stems start to elongate as flowers are being formed. This can cause not only unwanted flowering, but excessive stretch. To minimize extra stem elongation from flowering, cuttings should be kept vegetative when possible. Cultivars with a photoperiodic flowering response would require non-inductive photoperiods; short-day plants should be kept under long days and long-day plants should be kept under short days. One of the main objectives of flower breeders is to diminish or eliminate photoperiod requirements for many annuals. This has resulted in many crops that are day neutral, or insensitive to photoperiod. Ethephon is a plant growth regulator (PGR) than can cause flower abortion and promote vegetative growth. However, ethephon can also diminish root growth and development. Make lower-volume applications (i.e. < 2 quarts per 100 ft2) and do not spray to “run-off” when treating cuttings to avoid excess solution being absorbed by substrate and negatively affecting root development.
Another method of keeping cuttings compact prior to transplant is to shear or pinch cuttings (Fig. 1). There are several benefits to this approach. First, shearing or pinching can remove excessive stem growth and keep cuttings from getting too large. Secondly, many vegetative annuals benefit from a pinch or two, which promote branching (Fig. 1) and result in a higher-quality finished plant compared to unpinched plants. Shearing and pinching can be done manually by hand or with a machine; the former will require more labor while the latter will require specialized equipment. Crops with numerous stems, such as verbena and bacopa, are well-suited to this method of height control. Cuttings with a strong central leader, such as New Guinea impatiens or zonal geranium cuttings, are not.
One convenient method of controlling stem elongation of floriculture crops is applying PGRs that inhibit gibberellic acid synthesis, or the GA inhibitors. While there is some evidence that dipping cuttings in GA-inhibiting compounds prior to sticking can control growth, this method is not well understood or widely used. Similarly, a liner dip is a common application method for applying PGRs to control growth, but this is more for controlling the growth occurring after transplanting. Foliar sprays are the best application for controlling cutting growth in the liner tray. For active ingredients that can be taken up by roots, including ancymidol, flurprimidol, paclobutrazol and uniconazole, be careful about applying too much solution and allowing the substrate to absorb the excess. Depending on the leaf size and density of cuttings in the tray, this may also warrant applying lower-volume sprays, as mentioned above.
Because the plant growth is usually controlled during finishing, starting out with stretched liners can only make this a bigger challenge. Proper scheduling, flowering control, pinching, and growth regulators can all be used to produce shorter and more compact cuttings, which will help you grow the high-quality finished plants you and your customers want.