Top 10 novice grower questions

Features - Growing Media

Here are some common questions that new growers have about growing media.


Starting with and maintaining a good growing medium is critical to producing a high quality crop. New and inexperienced growers often have numerous questions about what growing media they should be using and how to get maximum performance. Even experienced growers can encounter problems with media, whether it’s a substrate they’ve used for years or something new that they are experimenting with.

Long term storage outdoors of packaged growing media should be avoided. Sunlight can degrade and eventually disintegrate the plastic packaging.1. Does a growing mix have a shelf life?
It is always preferable to use a growing mix that is as fresh as possible, because certain chemical and physical properties can change during storage. The degree of change depends on both storage time and conditions.
Whether bagged, baled or bulk, after storage, a mix can dry out becoming dusty and difficult to wet. A mix stored outdoors in humid, rainy conditions can absorb moisture and become wet and moldy. During handling and potting, a wet mix is easily compacted, resulting in decreased aeration porosity. While unsightly, the mold is not pathogenic and quickly goes away on its own.
Most mixes contain a starter fertilizer. As mixes age during storage, naturally occurring micro-organisms can consume some of this fertilizer, reducing the electrical conductivity and nitrogen content. If an older mix is used side by side with a fresh, new mix, some initial growth differences might be seen, especially if the first fertilizer application is delayed.
Sunlight can degrade and eventually disintegrate the plastic bags or bales used as packaging, allowing for contamination by weed seeds or disease organisms. Although there are a lot of “what ifs,” growing mixes are best used within six months of manufacture.

2. The water pH is very high. Will this affect the growing mix and the crops grown in it?
Water pH is a measure of solution acidity or basisity. It is an easy assumption that the pH of the irrigation water can affect the growing mix pH. However, water pH alone has little impact on the mix. Instead, another property of water, it’s alkalinity level, affects the mix pH. Water with a high pH can, but does not always, have high alkalinity and should prompt a grower to do complete water analysis to determine alkalinity level.
Irrigation water high in alkalinity can induce rising growing mix pH, resulting in trace element deficiencies in pH sensitive crops like petunia and calibrachoa. Considered as a single factor, high water pH is a bigger factor when preparing pesticide solutions rather than its effect on the growing mix.
Low pH water can also cause problems. The reduced bicarbonate concentration in low pH water can cause declining growing mix pH, increasing the potential for trace element toxicity problems in sensitive plants like geranium and marigold.

3. Is there an advantage to using coir in the mix?
Coir, derived from coconut husks, is somewhat peat-like in physical nature but there are differences. Because coir is denser than peat during watering-in, growing mixes with a high percentage of coir are less prone to settling down from the top of the pot than peat mixes.
Coir is naturally higher in pH than peat. Compared to peat, coir contains higher levels of potassium and sodium, resulting in higher soluble salt levels. There is also some evidence that mixes containing coir are less subject to fungus gnat infestations.

4. How long can I store a mix that contains a controlled-release fertilizer?
A good practice to follow with a growing mix containing controlled-release fertilizer is to use it as soon as possible. With common greenhouse-use formulations, each controlled-release fertilizer has a longevity factor, or release rate, listed on the label (e.g. three to four months, 180 days, etc.). The release rate is dependent on the medium temperature in a moist growing mix. The higher the temperature, the quicker the release rate. Because growing mixes contain some moisture, the release process for a controlled-release fertilizer begins soon after the mix is made, resulting in increasing soluble salt levels.
During storage, fertilizer release rate is affected by temperature, time, a growing mix’s degree of moisture and the fertilizer’s longevity factor. While any mix containing a controlled-release fertilizer can be safely stored for a week or two before use, soluble salt levels should be checked after longer storage periods prior to using the mix.
If necessary, the salt level can be reduced by leaching after pots are filled. The best management practice would be to order only that amount of mix that can be used before leaching becomes necessary. Packaged mixes should be stored cool and dry to minimize high temperatures and moisture absorption.

5. Does bark tie up nitrogen?
All organic mix components undergo natural decomposition. Bacteria and fungi are responsible for this process. The micro-organisms consume nitrogen along with the organic material, and the nitrogen becomes “tied-up” within the cellular structure. In the growing mix, these micro-organisms compete with the plants for nitrogen. Growing mixes made with readily decomposable organic material can support a large population of micro-organisms.
Fresh bark taken directly from trees is subject to rapid decomposition. To compensate for this, growing media manufacturers age or compost the bark. The resulting composted bark is resistant to further rapid decomposition.
Properly processed pine bark is important in the production of clean growing mixes.
Incorporation of aged or composted bark in a mix supports a much less active population of micro-organisms than fresh bark, consuming or “tying up” less nitrogen.  Properly aged or composted bark ties up only slightly more nitrogen than peat moss.

6. Are bark mixes “disease suppressive”?
Aged or composted bark contains many species of naturally occurring micro-organisms. Some of these micro-organisms are relatives of the bacteria and fungi used in commercially available biocontrol products. When used in a growing mix, properly processed bark suppresses some disease pathogens for a short time. These suppressive properties should not be solely relied upon for disease control.
Mix components that are processed properly, including bark, becomes populated with micro-organisms that suppress certain fungal diseases. It is important to note that the word “suppressive” is not the same as “prevention”. Disease problems can occur in suppressive mixes under conditions of poor sanitation or heavy disease pressure. Suppressive mixes should be used as a disease-control tool along with customary fungicide and sanitation programs.

7. Why is the growing mix pH so low when tested fresh from the bag?
Growing mix ingredients pine bark and peat are naturally low in pH, in the 3.5 to 4.5 range. To adjust the pH up to the desired 5.5 to 6.5 range for crop production, limestone must be added. Once containers are filled with the mix and it’s moistened, the limestone starts to react and raise the mix pH. However limestone does not react instantly; it takes a period of three to five days for the reaction and the subsequent pH increase to occur.
A freshly manufactured mix straight from the bag can be low in pH, and only after the mix is watered does the limestone become activated. As the mix ages during storage, a slow pH increase can occur.

8. There is green scum on the surface of the mix. When the mix dries, the scum turns brown and then water doesn’t penetrate the surface. What causes this?
The growth of algae results in green scum on the growing mix surface. When dry, the scum sets up into an impermeable barrier that makes watering difficult. Algae, which are plants, thrive in the greenhouse environment. Any consistently moist area in a greenhouse is subject to algae growth.
Excessive algae growth occurs when mix conditions are too wet.  Over-watering, poor air circulation and/or compacted growing media result in conditions that inhibit drying and encourage algae growth.  Proper water and environmental management are needed to minimize algae problems.

9. Isn’t there a shortage of peat land in Canada? Isn’t peat harvesting
depleting the wetlands?

The North American peat industry takes environmental concerns very seriously. For example, when a peat bog is at the end of its productive life, it is mandatory that the area be restored to a functioning wetland. These restored areas can become ecologically balanced systems within five to 20 years.
There is a misconception that peat is in short supply and that the harvest rate is not sustainable. Peat is a renewable resource that is accumulating 70 times faster than it is being harvested. North America has more than 270 million acres of peat lands with only about 40,000 acres (0.016 percent) used for peat production. There are also millions of acres in national parks and other preserves that can never be touched.

10. Where can I find more information on the environmental concerns with peat moss?
The Canadian Sphagnum Peat Moss Association web site has many resources on peat harvesting and the environment. 
Bob Steinkamp and Michael Tilley are technical services managers, Jamie Gibson is director of research and development, and Hugh Poole is director of technical services, Conrad Fafard Inc., Fafard Technical Services, (864) 224-7989, Ext. 2382;;