Height control may be the single most common reason why Texas growers are unsuccessful with poinsettias. Although this is often blamed on poor performance by a chemical, height control is effected by many things.

Light is the most critical factor influencing the finished height of poinsettias. Growers are largely at the mercy of Mother Nature when it comes to environmental conditions, but there are some things which can be done to moderate their effects. Poinsettias require “full sun” for the majority of the growing season. This means that any shade cloth, whitewash, or other materials that might have been used to keep young cutting turgid should be removed after the plants are well established. Since many growers use shading to make up for an inadequate cooling system, poinsettias are often produced under too low of light conditions. The ability to control or moderate light intensity is a key to maintaining a good finished height.

Spacing is another important factor which influences plant height. Most 6-inch pinched poinsettias are grown on 12″-18″ centers. However, many growers try to squeeze every square inch out of their facility and “jam” their plants as close as 9″. It only stands to reason, the closer plants are spaced, the taller they will be. Good height control begins with adequate spacing. Some growers achieve wider finish spacing by using “aisle boards”. These are generally 1″ X 6″ slats, cut to fit between benches in the aisles. Of course this cuts down on how closely you can move through the plants, but it does offer a means of increasing the space between plants during the critical finishing period.

If a grower has provided good light and adequate spacing, height may be further controlled by use of a chemical growth regulator. At present several plant growth regulators (PGRs) are labeled for use on poinsettias.

The method of applying a growth regulator not only influences the effectiveness of height control but also its cost. Several studies have shown that drench applications are not effective in growing media containing bark. This is largely attributed to the chelating properties of the bark particles.

Also, a cost break down shows that drenches of either Cycocel or A-Rest are not economically feasible. Based on this information it is clear why most Texas growers currently use foliar PGR applications for height control of potted poinsettias.

Although Cycocel is labeled for poinsettias, caution must be used in its application. When Cycocel is applied to dry plants under high temperatures leaf yellowing may occur. Most Texas growers avoid this situation by thoroughly irrigating their plants before applying Cycocel. Most applications are then made at the end of the day, in the evening or when temperatures are below 75 degrees F.

Generally speaking, poinsettias will grow out of a “Cycocel chlorosis” but studies have shown that crop time may be increased by as much as three weeks. Therefore it pays to be careful in making you application.

Timing is also important in maximizing the effectiveness of growth regulators. In general, two applications of Cycocel are made to a 6″ pinched crop. The first (1:40) is applied approximately three weeks after panning, a second application (1:80) about five weeks later. Although more diluted applications of Cycocel may be made later in the growing season, studies have shown that bract size and number may be reduced.

Tank mixes of B-Nine and Cycocel are also widely used for controlling the height of potted poinsettias. Although B-Nine is not particularly effective on poinsettias alone, when combined with Cycocel this combination works extremely well. The B-Nine is thought to act as a carrier for the Cycocel which increases its effectiveness and decreases the potential for phytotoxicity.

Bonzi and Sumagic are also effective chemicals for poinsettia height control. However, growers have had mixed results with both of these chemicals. These Super PGRs can be over used, causing plants to be stunted and delayed. Most recently it has been shown that the volume of spray material applied per 100 square feet is as critical as the concentration of chemical used. Without question, application technique is the most important factor in the use of Bonzi or Sumagic.

A-Rest is another chemical used to control the height of potted poinsettias. This material is used primarily as a late season drench. A-Rest can be applied after plants begin to show color to help reduce stem elongation. Although most PGRs applied at this stage of growth cause reductions in bract size and number, A-Rest does not generally have these effects.