Wednesday, February 6, 2008

Share my joy-Plant growth medium

The best way to learn something is to teach it. Therefore, I don't see any reason why I should be the only one learning about floriculture for my next and (hopefully) last written PhD qualifying exam. So, I will share here what I've learned today that I hope will be useful for tomorrow's exam. If you see any egregious errors please feel free to point them out to me, even if it is after my exam. Since this is likely to be a lot of material I'll divide it up into several posts. And because I'm learning at the same time I'm teaching, these posts will be a work in progress until tomorrow, and subject to change.

Disclaimer: This should not be considered an instruction manual for the culture of ornamental plants. The information presented here is most certainly incomplete, if for no other reason than that it presumes a level of basic knowledge. This is simply to complement what I think I already know with the details that I believe are important to know for my exam, that I was not (fully) aware of when I started studying this morning. Of some of these facts I was aware, and this serves to refresh my memory. Also, because I live in the South, things like cold-tolerance are of little relevance.

The five main categories I think will be important are:

1. Growth medium
-different soilless medium components
-waterholding capacity
-EC (electrical conductivity)
2. Nutritional deficiencies
3. Plant growth regulators
4. Post-harvest longevity of flowers
5. Legal protection of cultivar propagation

So, here we go.

Common soilless medium components
*Sphagnum peat moss-Consists of incompletely decayed plant material (at least 90% organic matter on a dry weight basis), usually formed in cool climates under low oxygen conditions. It typically has a very low pH, high CEC, and high water-holding capacity. It can, however, compact very easily, creating problems with the structure of the medium, and it is rather pricey.
*Rice hulls-Light weight, low CEC, cheaper than peat, high C:N ratio, decomposes easily (changing the structure in the process), microorganisms that break down the carbon will likely use up some of the N provided by fertilization, pH higher than that of peat moss.
*COIR-medium-high CEC, pH higher than that of peat moss, CEC lower than that of peat moss. Less expensive than peat, and no compaction issues.
*Yard clippings-Variable CEC (but lower than that of peat), high C:N and therefore medium structure changes because of breakdown, inconsistent availability (supply too irregular for large-scale growers), pH higher than that of peat.

Waterholding capacity
This is the volume of water in the growth medium after irrigation and drainage.

Optimum pH values for soilless medium are 5.5-6.0 according to the University of Massachusetts floriculture webpage and 6.2-6.5 for media with 20% or more soil. Individual requirements of crops vary, with Dianthus and Marigold on the high pH end, and Petunia and Salvia on the low pH end. The main problem with higher or lower pH is nutrient availability. For example, high pH results in iron deficiency, because iron becomes chemically unavailable to the plant, while low pH causes iron excess.

Electrical conductivity (EC) is an expression that measures the amount of salts in the medium. Low levels are not good, because it means that nutrients are not available to the plant. High levels are not good, because the plant would have to transpire too much water in order to take up nutrients, causing the plant to wilt (fertilizer burn). The following table is taken from Hanlon et al. (2002)

Species Type





Very High

------------------- dS/m -------------------

Woody Ornamentals


0.7 to 1.0

1.0 to 1.5

1.5 to 3.0


Bedding and Pot Plants


0.8 to 2.0

2.0 to 3.5

3.5 to 5.0


If the EC is too high, the medium can be leached out with high quality (low salt) irrigation water (EC 0.1-2.0).

That's it for growth medium.

Hanlon, E.A., McNeal, B.L., and Kidder, G. (2002) Soil and container media electrical conductivity interpretations. Circular 1092. Soil and water science department, Florida cooperative extension service, Institute of Food and Agricultural Sciences, University of Florida.

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