Weed infestations can develop rapidly because common greenhouse weeds grow quickly and produce hundreds of seeds that spread many feet from the parent plant.
Commercial growing media are designed to hold air and water and to provide mechanical support for plant roots. Growers expect their media to be uniform and consistent from bag to bag, provide a proper chemical environment for the plants with adjusted pH rates and be weed- and disease-free.
A growing medium supports the plant, but it does not grow the plant. From watering and fertilizing to controlling insects and diseases to managing environmental conditions, the grower makes the final decisions to produce a plant.
What happens outside the growing media manufacturers’ control can be quite complex. Storage conditions, algae development and fertilization decisions are just some of the issues growers face.
1. Proper storage
Store packaged growing media in a cool, dry, low-light environment. Several elements of commercial growing media can change during storage. Long storage time can cause media to dry out. When media lack sufficient moisture, they may be more difficult to wet. To help retain moisture, packaged media should be stored on pallets or other structures and not directly on the ground.
Storage can cause chemical changes, such as an increase in pH and/or a decrease in soluble salts and nitrogen levels. While media containing a controlled-release fertilizer can typically be safely stored for one to two weeks prior to use, you should check the soluble salt levels after longer storage periods. Test media stored for six months or longer to determine whether any chemical changes have occurred and to compensate for any changes as necessary.
Use growing media within six months of manufacture. Media are packaged in plastic bags or bulk totes to reduce moisture loss, yet media can still dry out during long periods of storage. More specifically, the wetting agent incorporated into the media may degrade over time thus causing problems.
Depending on the circumstances and the product, dry media can be difficult to wet, particularly with baled media, as opposed to bagged, because bales are often manufactured at lower moisture levels and stored for longer periods of time. Pre-filled pots and flats can also dry out rapidly when using media stored for longer periods of time. Consider applying a liquid wetting agent through an injector to older mixes, but be sure to follow label rates strictly to ensure proper coverage.
If storage conditions have been ideal and a mix has been manufactured, shipped and warehouse-stored within six months, but you still find the medium to be dry, then you should check with the supplier to address your concerns.
A quick test for evaluating media wettability:
- Fill a polystyrene cup with 2 fluid ounces of water.
- Scoop a level teaspoon of the growing mix from the suspect batch and add it to the water.
- Measure the length of time the mix takes to saturate.
- Typically mixes that do not saturate within five seconds have lost some wetting properties.
Reduce the incidence of wind-dispersed weed seed in packaged media and containers. Weed infestations can develop rapidly because common greenhouse weeds like oxalis and bittercress grow quickly and can produce hundreds of seeds that spread many feet from the parent plant.
The source of weed seed can be hard to determine. Seed can blow through the greenhouse vents or come in contact with plant liners. Irrigating with pond water can result in weed problems because of weed seeds blowing into the pond. A few weeds allowed to go to seed can result in a major infestation.
Growers should identify weeds in containers and outlying areas of the production plot (such as in adjacent crops, under benches, outside greenhouses near openings and louvers and local fields) to document weed species on site. Never apply pre-emergent herbicides in closed environments like greenhouses and cold frames.
Only a few post-emergent herbicides are recommended for use in greenhouses. Ideally, post-emergent herbicide sprays should only be applied when greenhouses are empty. Avoid herbicide drift on plants inside closed environments as well as applying herbicide sprays around greenhouse air-intake sites.
4. Molds and algae
Produce plants under the proper environmental conditions. Processed bark and other organic materials contain a high population of naturally occurring, harmless micro-organisms. Under certain environmental conditions (moisture, temperature, light, etc.), these organisms may be detectable, even in freshly opened media bags.
Media stored in direct sunlight can develop a slime mold in the area between the mix and the bag where condensate forms. Occasionally, slime molds may also form on the medium surface soon after potting. Though unsightly, the mold is not harmful to plants and should go away on its own in less than two weeks. In addition to slime mold, mycelial growth from saprophytic fungi may hold together bark mix components forming clumps which can impede the filling of containers.
The growth of algae can result in the presence of green scum on the growing medium surface. When dry, the scum can form an impermeable barrier that makes watering difficult. Excessive algae growth can also occur when the medium is too wet. Over watering/fertilization, poor air circulation and/or a compacted medium can result in conditions that inhibit drying and encourage algae growth. Proper water, fertility and environmental management are required to minimize algae problems.
Provide good air movement in environments where opened, packaged media present a foul smell. The mixing of organic materials with water and fertilizer can generate significant microbial activity. If too much water is present and a mix is compressed more than usual due to the palletizing operation, this microbial activity can turn anaerobic (without oxygen). These conditions can generate organic acids similar to cider, sour milk, etc., resulting in an unpleasant odor, especially in a closed environment.
Aeration during the potting operation should be sufficient to eliminate the organic acids that cause the odors, but this usually takes a few hours. The odors are typically short-lived after aeration and should cause no concerns related to plant health.
6. Using the same mix for all crops
Irrigation management is critical. Use fine to coarse mix components that contribute to air space. It can be challenging to develop one universal mix suitable for all crops.
As a rule of thumb, use a finer mix for young plant production and a coarser mix for finished plants. When propagating in shallow trays, use a coarser-textured mix to improve drainage. In larger containers, a mix’s water-holding properties can be improved by adding coir, peat or bark fines.
Propagating and growing in the same mix usually involves the management of water and the use of peat-based media or media containing fine bark particles. Popular peat-based mixes, ranging from 70-85 percent peat with the remaining percentage in perlite, have worked well for growers wishing to keep things simple with just one mix. Growers who use bark for rooting cuttings and producing finished plants typically have no more than 30 percent fine bark in their mixes.
To select a mix based on watering preferences, remember these general principles. Just because two mixes are made with the same percentages of components (e.g. bark and peat) does not necessarily mean they will have identical watering properties. The component sources used may not be the same for both mixes.
7. Organic media
If you are looking to use an organic growing medium, it needs to contain materials approved by the National Organic Program. Many conventional growing media are nearly “all natural”, but the small amount of synthetic materials they contain, such as chemical fertilizers and wetting agents, prohibit them from being classified as organic by the Organic Materials Review Institute. Commercially available organic media products require just a few changes to meet certification standards established by the National Organic Program to make them appropriate for use by organic producers.
The peat, bark, perlite, vermiculite, dolomitic limestone and gypsum in most conventional media are acceptable by organic standards. Incorporation of synthetic wetting agents and a pre-plant nutrient charge (a water-soluble complete fertilizer or a slow-release fertilizer such as urea formaldehyde) disqualify media from organic certification.
Several organic fertilizers are appropriate for use as pre-plant nutrient amendments, with the goal to provide three to six weeks of nutrients as a starter charge. Pelletized fertilizers such as chicken or turkey manure or chicken litter work well for this purpose.
8. Physical property conundrums
Know your mix’s physical property values. How do you determine the best growing mix for any given crop? Most crops can grow in almost any mix if managed properly. However, you increase your chances of success with careful mix selection and good on-site production practices. While many factors must be considered, the degree of water retention is the most important element to consider when selecting a mix.
Water retention is governed by a mix’s porosity balance. There are three kinds of pore spaces (macro, micro and air filled).
A mix with many air-filled pores and few water-filled pores requires frequent watering and is considered a “dry” mix. A mix with few air-filled pores and many water-filled pores is a “wet” mix requiring less irrigation. This mix is typically poorly aerated and watering has to be performed carefully.
The porosity balance of a mix is controlled by coarseness or fineness of its components. A mix containing numerous coarse particles and many fine particles has many air-filled pores and fewer water-filled pores. A mix with fewer coarse particles and many fine ones has fewer air-filled pores and many water-filled (micro) pores. This relationship between particle size and pore size determines the watering properties for each mix.
9. Media pH management via liquid fertilization
Liquid fertilizer is a tool to manage pH. Water-soluble fertilizer can affect media pH. Fertilizers containing ammonium or urea (e.g. 20-20-20) can have an acidifying effect, which can cause a decrease in the media pH after repeated use.
Fertilizers containing little or no ammonium or urea (e.g. 20-10-20) are typically not strong acidifiers. Some fertilizers such as 15-0-15 have a basic effect, and can result in a pH increase.
The label on soluble-fertilizer bags should always show the fertilizer’s potential acidity or potential basicity. This measure can be used to judge a fertilizer’s potential to affect or change the media pH.
The amount of lime in a medium may not always be the primary factor controlling its pH. Irrigation water alkalinity and the type/concentration of fertilizer applied can also have a significant influence on the medium pH. When managing the medium pH, all three factors should be considered.
10. Fertilizer selection to complement water quality
You should have a fertilizer tool kit on hand to make adjustments to the medium pH. Growers should have acidic and basic fertilizers on site and treat them like tools to address water alkalinity and manage media pH.
Water alkalinity level can have a major influence on medium pH. Alkalinity, a measure of the water’s ability to neutralize acid, is influenced by the bicarbonate ion.
Water high in bicarbonates can cause the medium pH to increase over time, with higher levels causing a greater degree of increase than lower levels. While moderately alkaline water may not change the medium pH, low alkalinity water can actually cause the medium pH to decrease.
Most growers with low alkalinity water use basic fertilizers that usually contain calcium and magnesium. On occasion, an application of 20-10-20 is used to increase the ammonium and micronutrient concentration in the medium.
Growers with high alkalinity water normally adjust the alkalinity with acid injection and then use a near-neutral fertilizer like 17-5-17. They can then apply acidic or basic fertilizers to manage the medium pH if values fall out of the acceptable range.
Growers with moderate alkalinity water or ideal alkalinity levels of 60 to 120 parts per million bicarbonate (HCO3) can apply 20-10-20 and may alternate with a Cal-Mag fertilizer or use a low-phosphorus basic fertilizer like 15-0-15 or 13-2-13.
Jamie Gibson is director of research and development, Bob Steinkamp and Michael Tilley are technical services managers, and Hugh Poole is director of technical services, Conrad Fafard Inc., Fafard Technical Services, (864) 224-7989, Ext. 2382; firstname.lastname@example.org; www.fafard.com.