Root Substrate Quality for Poinsettias

There is no one growing medium that is best for producing poinsettias. Plants have evolved with the ability to grow in almost anything, in a multitude of environments. It is precisely this ability that allows growers to manipulate plant growth for specific markets and conditions. Poinsettias can be successfully produced in a wide variety of growing media or "substrates." The selection of a substrate is dependent on the availability of components and equipment, the size of the growing operation, the expertise of the operator, and irrigation management. The keys to proper management of substrates are: (1) the attitude of the grower and (2) the people involved in selection, use, and handling of the materials.

A substrate is supposed to serve four functions: (1) to provide water; (2) to supply nutrients; (3) to permit gas exchange to and from the roots; and (4) to provide support for the plants. Unfortunately, this has been misconstrued to mean that they are properties that are immediately present after blending of the components. The only function that is "guaranteed" after blending is plant support. The other three are still grower-controlled. Otherwise, why would the same mix perform differently for different growers?

A more successful approach to substrates is to think about creating and managing the sub-surface environment for the poinsettia. The sub-surface environment is created in the following steps: (1) selection of components; (2) blending these components and additives; (3) filling pots, and (4) initial watering of pots after transplanting. A useful way of thinking about this process is to envision steps of a staircase. The creation of this subsurface environment is accomplished by "climbing" each step in order until you reach the top. The sub-surface environment is not "set" until the poinsettias are placed in the greenhouse and watered-in. However, once the environment is set, it is not constant, but changes as roots grow and explore the substrate. Even during the day, the environment can change hourly, as water is removed by the plant and replaced by the grower.

Desirable Properties

Poinsettia substrates should have a light bulk density to ease handling and shipping. For this reason, organic matter is usually a large percentage of the final mix. The organic matter must persist throughout the crop, not cause significant volume changes, and must be chemically stable to prevent nitrogen depletion in the crop. Initial nutrient content (soluble salts) should be low, so as not to damage sensitive young plants and seedlings. Cation exchange capacity should be high for ample nutrient reserves (6-15 meq/100 cc). The pH range should be in the range of 5.8 to 6.2 for soilless mixes and 6.0 to 6.5 for substrates that contain mineral soil at volumes =20%. Air and water volume contents of substrates are not properties of the substrates alone, but are affected by the container size, handling and flat filling, and watering practices.

Substrates are made up of two or more components. Common components are mineral soil, sand, sphagnum peat moss, vermiculite, pine bark, perlite, polystyrene beads, and rockwool. Soil-based substrates contain some portion of field mineral soil, such as a sandy-loam. Poinsettias of equal quality can be produced from soil-based and soilless mixes. However, the irrigation and fertility regimes are markedly different. The peat-lite mixes are generally more uniform, light weight, drain more quickly, provide better aeration and hold more water and nutrients than soil-based mixes. Although soil-based mixes are still used, the majority of growers use peat-lite mixes they either mix themselves or obtain commercially.

A major difference between soil-based and soilless mixes is total pore space. Most mineral soils have approximately 50% solids and 50% pore space by volume. Most organically-based substrates will have between 75 and 85% pore space. This increased pore space improves the water and air holding capacities of substrates in containers. It also makes these soilless mixes more susceptible to mishandling during blending and pot filling.

Another major difference between soil-based and soilless mixes is the unavailable water content. When water is applied to a substrate, a thin film of water binds so tightly to the substrate particles, that the root cannot pull it from the soil and is not available to the plant. Mineral soils and soil-based mixes generally have only 5 to 10% of the volume of the substrate contain water unavailable to the plant. However, soilless mixes have 20 to 25% of their volume filled with water the plant cannot "see." Because total porosity and unavailable water in soilless substrates are so different from soil-based mixes, growers must not treat them similarly, but need a fundamentally different attitude when using soilless substrates.

Container size and the sub-surface environment

Air and water relations of the sub-surface environment are affected by the size of the container used. There are three "fates" of water when applied to the surface of a substrate. Some water moves through the substrate and out of the bottom (gravitational water). Some binds tightly to the substrate particles through hydrogen bonding and become unavailable to the plant (hygroscopic). Most of the water becomes "suspended" in the pores of substrate through a combination of hydrogen bonding, capillary action, and gravity. The forces of hydrogen bonding and capillary action are determined by the substrate ingredients and bulk density. The force of gravity is effected by the height of the container. The taller the container, the more gravitational "pull" on the mix. The more gravitational pull, the more the mix will drain, the less water the mix can hold, and the more air space will increase.

The Percentage of Container Volume Table shows the air and water content of four substrates in 6-inch and 4-inch pots. In 6-inch pots, the soil-based mix had only 7% air space while the soilless mixes were 19 to 23%. As container size decreased, all mixes dropped in air space and increased in water content. As air space decreases, the chance of plant damage due to over watering increases. Generally, the higher the air space in these ranges, the better root growth, and the better start for poinsettia cuttings.

Percentages of container volume occupied by water and air at container capacity for four substrates.

Substrate Mix

Water/Air (%)

6 in. std. pot

4 in. std. pot

1 peat moss: 1 vermiculite

Water (%)

67.9

75.2

Air (%)

19.0

11.7

1 peat moss: 1 rockwool

Water (%)

68.4

76.0

Air (%)

23.4

15.7

3 pine bark: 1 sand: 1 peat moss

Water (%)

51.5

57.6

Air (%)

18.9

12.9

1 soil: 1 peat moss: 1 sand

Water (%)

47.2

51.2

Air (%)

7.4

3.4

Substrate-container handling-watering relationships

Container filling also influences the air and water relations of a substrate. The Effects of Filling and Packing Table shows the effects of compaction on a peat:vermiculite mix. For a 6-inch standard pot, air space dropped from 23% to 9% with increasing packing density, while unavailable water content rose from 21% to 30%. Substrate compaction reduces air space, increases the chance of over watering, and reduces the ability of the mix to provide water and nutrients to the plant.

Whether filling by hand or by machine, containers should be filled to excess, then brushed or scraped level to the top of the container. Care should be taken not to stack containers directly over one another, as this also increases compaction. Excessive shrinkage in the container after initial watering is caused by low moisture content in the substrate, and not from light container packing.

If excessive shrinkage is a problem, the proper correction is to increase the water content in the substrate prior to filling the container - not packing more substrate into the container. Maximum effect will occur when the moistened substrate is allowed to equilibrate overnight, then agitated just prior to container filling.

Effects of filling and packing containers on a peat:vermiculite substrate’s water and air space.

Handling Procedure

Water/Air (%)

6" std. pot

4" std. pot

Filled and brushed

Available water

43%

51%

Unavailable water

21%

21%

Air space

23%

15%

Filled, tapped on bench twice

Available water

44%

52%

Unavailable water

26%

26%

Air space

15%

9%

Filled, pressed and refilled

Available water

45%

49%

Unavailable water

30%

30%

Air space

9%

4%