Media, water quality impact propagation

Success in propagation from seed and vegetative cuttings can be greatly affected by the type of propagation media used and by water quality. The media used will impact root development and aeration for new seedlings or rooted cuttings. Water quality can impact misting efficiency and will impact media pH. The impact on media pH can affect further growth of the seedlings or cuttings.

Making a decision to purchase a commercial medium or to mix a medium on site requires understanding the characteristics of various media components and their impact on media physical properties. A successful grower will choose a medium that fits the type of propagation, along with the conditions under which the propagation occurs (temperature, light levels, watering practices, etc.).

Media components

The main components used in media are peat moss, coir, perlite and vermiculite. In some cases, mainly for vegetative propagation, bark can be used. Other components sometimes found in propagation mixes are bark ash, peanut hulls and rice hulls.

* Peat moss is an excellent component because of its good water- and nutrient-holding ability and low pH. It is essentially sterile, but not sterilized. Most of the peat moss used by North American growers is Canadian sphagnum peat.

* Coir is a byproduct of the coconut-processing industry. When the fiber is processed, the finer product left behind is known as coconut coir or coir pith. It is similar in properties to peat moss, but tends to have a higher bulk density and pH. One concern with coir is its sometimes high levels of sodium. Test the product before use and use a reliable supplier.

* Vermiculite and perlite are made from ores that are expanded at very high temperatures to form brownish vermiculite and white perlite. In general, vermiculite has some water- and nutrient-holding capacity, while perlite adds only aeration to a growing mix. Vermiculite tends to be higher in pH.

* Bark in commercial growing mixes is almost always southern pine bark, although other softwood barks may be used. It is a byproduct of the lumber and paper industries. Bark needs to be aged or composted before use. Composted bark adds weight and increases aeration. It also provides some nutrient-holding ability, which is very low in comparison to peat moss.

Composted bark continues to decompose in a growing mix and requires nitrogen as part of the process. Growers usually need to apply additional nitrogen or fertilizer when using bark-based mixes. Well-composted bark in a mix can be important when plants remain in the medium for a long time, or when they are grown on outdoors, since the bark does not compact in the pots.

* Other components, such as bark ash, peanut hulls and rice hulls are generally not used for propagation mixes. Peanut and rice hulls tend to be more locally available components and are sometimes used in mixes for vegetative propagation. Bark ash, which is basically a burnt bark byproduct of the lumber and paper industries, is a relatively fine product. It is used in some commercial mixes.

General properties of peat moss, bark and coir

Peat moss	Bark	Coir
Essentially sterile. May impart some disease resistance	Essentially sterile. May impart disease resistance.	Essentially sterile.
Source: Canada.	Source: United States and Canada.	Source: Sri Lanka, India and Mexico.
Requires pH modification.	Requires nitrogen addition and pH modification.	May require nitrogen addition.
Excellent component for propagation and production.	Best used for production.	Excellent component for propagation and production.
Grades and coarseness vary depending on processing.	Grades and coarseness vary depending on processing.	Most of coir presently available is similar. There may be some slight variations.
Most common problems are related to age or fineness.	Most common problems are related to improper composting.	Most common problems are high soluble salts and lack of understanding of properties.
Major concern: Environmental effects of harvesting. Should not be an issue.	Major concern: Improper composting, which can lead to nitrogen tie-up and poor plant growth.	Major concern: Sourcing, age of product used, high water-holding capacity.

General properties of perlite and vermiculite

Perlite	Vermiculite
No soluble salts.	Generally, low soluble salts.
Neutral pH.	Moderate to high pH.
Sterile.	Sterile.
High pore space.	High pore space.
High air space.	High air space.
Little to no water-holding capacity.	Moderate to high water-holding capacity.
Little to no cation-exchange capacity.	Moderate cation-exchange capacity.
Low bulk density.	Low bulk density.

General properties of bark ash, peanut hulls and rice hulls

Bark ash	Composted peanut hulls	Rice hulls
High pH	Moderate pH	Available composted and from parboiled processing. Moderate pH
High bulk density	Moderate bulk density	Low bulk density
Generally high in moisture	Similar properties to composted Bark	Low cation-exchange capacity
Low nitrogen draw	Low nitrogen draw	Low nitrogen draw

Propagation vs. production mixes

The properties of a propagation mix usually differ from one that is used for growing on. A propagation mix is exposed to high humidity. It also usually fills small containers or cells, as compared to larger containers for growing on. Because of these factors, a propagation mix needs to be fine in texture, but still allow for good aeration.

During propagation, little to no fertilization occurs until seeds have germinated or cuttings have rooted. The main chemical reaction that occurs in a propagation medium is the effect of the lime that is incorporated into the mix. The lime causes an increase in the medium pH. Propagation mixes are usually manufactured to have lower levels of lime and nutrients.

Propagation mixes used for seed germination need to be very fine in texture, as the germination usually occurs in small cells. These mixes generally contain fine peat moss and some aggregates such as perlite or vermiculite.

Sometimes the peat is replaced with coir. However, an additional aggregate is still needed. Many growers prefer for plug production is fine perlite. Fine perlite has no impact on the chemical properties of a mix and provides good aeration. A common ratio of peat to perlite for seed propagation mixes is about 4:1 to 5:1. Fine vermiculite can also be used, but it tends to have a higher pH and water-holding capacity. Coir added as a replacement for peat increases water-holding capacity, but keeps aeration about the same. Coir also tends to increase the medium pH, but not as much as vermiculite. It has generally been found that only a portion of the peat can be replaced by coir. Coir tends to be more granular and the root ball could fall apart when plugs are removed from the trays for transplanting.

Cutting propagation

Vegetative cutting propagation mixes can be coarser than those used for seed germination because cutting propagation is usually done in small containers or larger cells. However, many growers tend to use the same mix for both vegetative and seed propagation. Good water-holding capacity and adequate aeration are key.

In the case of cuttings, mixes containing bark are sometimes used, especially if the containers will be used to grow the plants on for some time after roots have formed. Regardless, the same chemical properties that are important for seed germination are also important for rooting of cuttings.

Commercial propagation mixes

Most propagation mixes offered by manufacturers tend to be peat/perlite or peat/vermiculite mixes. Some will contain coir. When selecting a propagation mix, growers should focus on quality and consistency.

The potential value of the plugs or cuttings is high in relation to the amount of mix used. Growers can request specs on the mixes and can usually find a mix to fit their specific needs.

Remember that the mix’s chemical properties are affected by water quality. That is why most plug and propagation mixes tend to be low in nutrients and lower in lime than standard growing on mixes. Avoid mixes with high soluble salts or pH.

Propagation mixes should also be tested for wet-out. A good wetting agent is essential for even and thorough distribution of water and nutrients. Some growers will add a wetting agent when they initially water in the seed or cuttings.

Water-quality impact

In the early stages of propagation, pH will generally begin to increase since the main reaction is that of solubilization of lime. A fertilizer charge has little impact since there is little to no nutrient uptake at this point. This is why, in most cases, the lime rate for growing mixes used in propagation tends to be lower than that in production mixes. Water high in alkalinity causes numerous pH problems during propagation unless it is treated before use.

Much research has been done on the effects of water quality on pH and on fertilizer needs for growing a crop. These same principles hold true (and are magnified) during both seed and vegetative propagation.

Seedlings and recently rooted cuttings are usually not provided a high rate of fertilizer. Therefore, the impact of fertilizer selection on pH is not that great. Fertilizer recommendations based on water quality do not hold as well in propagation, since growers generally use high-nitrate fertilizers at low rates.

Nitrate nitrogen causes a rise in the medium pH over time. As with the lime reaction, this can be magnified if water high in alkalinity is left untreated.

It is usually recommended that water for propagation be treated to bring alkalinity down to 40-60 (or lower) parts per million calcium carbonate. This range is low enough to slow the impact of lime and nitrate fertilizers on medium pH. Keeping the water at or below this range also tends to keep the irrigation mist heads clean. Test water once or twice per year to make sure alkalinity is staying in the proper range.

Mix your own

Growers making their own mixes should buy components from reliable suppliers. Peat moss and coir should be low in soluble salts.

Other components such as perlite and vermiculite are often overlooked before adding. This can be a problem, especially with vermiculite. Some vermiculite can have higher soluble salts and a higher pH. Growers should know vermiculite’s properties before incorporating it into a mix. Testing for soluble salts and pH is the first step. Growers should also test for total nutrients. The reason is some vermiculite can contain appreciable amounts of sodium. Most vermiculite tends to be low in sodium and high in potassium. Also test other mix components.

Once the main components have been selected, consider the chemical components such as lime, fertilizer and wetting agent. The nutrient charge should be relatively low so fertility can be controlled.

Most mix companies use a lower nutrient charge in their propagation media than in their standard growing on media. Growers may want to consider not adding a starter charge to their own propagation mix.

A wetting agent is a must, as proper water penetration and dispersion in the mix will help attain a more uniform crop. Follow label rates for wetting agents.

Most companies use a fine-grade dolomitic lime for their mixes. This lime tends to buffer pH well and provides calcium and magnesium for plant use. Many factors affect pH other than the lime. The most important of these is water quality, especially alkalinity. This can have a greater impact (especially high alkalinity) than the lime addition to the mix. Many propagation mixes have a lower lime charge than standard growing on mixes. This is especially true for seed propagation.

- Dan Jacques

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Dan Jacques is technical specialist, Sun Gro Horticulture, 2 High Meadow Road, Hadley, MA 01035; (413) 549-2793; fax (413) 259-4372; danj@sungro.com.