Another complicated situation in spring is how one handles growth when different species are mixed in the same container. Again the grower faces two complicated situations (1) plants in the same container show widely different overall vigor and require different PGR consideration and (2) the growth regulation objectives vary for plants in the same container. For example in the first case we may have a dwarf banana or naturally dwarf angelonia in the center of a container that requires no PGR control, surrounded by an extremely vigorous petunia cultivar that requires aggressive PGR control. In the second case, we may want to increase branching on an ornamental millet plant in the center of the mixed container but also control vigor of sweet potato vines planted along the margins.
In both of these cases it is obvious that we can't apply single PGR treatment to the mixed container and achieve the desired results. The solution requires that we treat individual plants in the mixed container without adversely affecting the others in the same planting.
To control growth of the most vigorous varieties or species without affecting the less vigorous plants, the best option is to pre-treat the transplants as needed. A "plug dip" is a useful technique for this purpose. Here the plug flat is set in a tray of PGR solution and the rooting medium of the plugs is allowed to soak up the growth regulator. Allow the plug flat to sit in the solution for at least 5 minutes or until the plug root volume reaches the saturation point. Some growers will allow 30 minutes but the plug root volume should reach saturation will before that time. After soaking allow the plug sheet to sit on the bench overnight before transplanting into the final container. Only root-active PGRs such as A-Rest, Topflor, Bonzi and Sumagic (and their generic equivalents) can be used for this purpose. These chemicals can remain active in the root zone for months, and they tend not to migrate far from the treated soil. Therefore, control is conveyed to just the individually treated plugs and not the other plants in the mixed containers. For rates, use Bonzi at between 4-16 ppm, use Sumagic at 2-8 ppm, for Topflor use 3-12 ppm. If you have not tried this before and are growing in a northern climate, start with rates in the lower half of the recommended range.
With groups of plants that have different growth regulator requirements such as stimulating increased branching associated with Florel versus height control associated with growth retardants, the option again is to treat the plants separately. Here you can go one of two ways. Transplant the plants that require Florel into the final container & spray, and then fill in with plugs of the vigorous species that have already been treated with a root-active PGR as previously described. Alternatively, the container can be planted with the treated plugs, leaving a space for the Florel requiring plants to be dropped in at a later date. Grow the plants that require Florel in small containers (e.g. 4-6"pot) until the treatment schedule is complete and then drop them into the empty space. Growers will often plant an empty pot to facility the late transplant with minimal disruption to the other plants growing the mixed container.
See the 2015-2016 New England Floriculture Guide for latest information on plant growth regulators. Ordering info
Photo: Plant Growth Regulators - Combination Planters
Richard McAvoy, University of Connecticut
