Growing media for mums and perennials

Premier Tech Horticulture produces several Pro-Mix blends that are suitable for perennials, mums and small nursery containers, including peat/bark blends and peat/chunk coir blends. All Pro-Mix products contain a wetting agent to facilitate water absorption and distribution, limestone for pH adjustment and a starter nutrient charge to help plants acclimate after potting.

Pro-Mix BRK and BRK20 are made with processed pine bark. The bark is from red pine trees which is partially composted, aged and screened to ½” particle size. This bark is nitrogen stabilized with a carbon to nitrogen ratio of 35:1 (C:N). This is important since a higher C:N ratio can result in nitrogen drawdown for large containers during the crop cycle and a higher application rate of nitrogen would be required so that plants are not deprived. However, this is not the case for these products.

Pro-Mix HPCC and CC40 products are made with peat and coir chunks. The coir chunks are pieces of the coconut husk that have been cut to a specific particle size of 7 mm. What is unique about coir chunks is that these particles hold water, which is accessible to plant roots and provide structure to the growing medium. Since the particles are cut to a specific size, there is less variability in the physical characteristics of the coir chunk particles. Compared to chunk coir, the majority of the plant accessible water from bark particles is found on the surface of these particles as the moisture within the bark particles is inaccessible to plants. Therefore, when coir chunks are used in growing media in place of bark, the water holding capacity is increased.

While bark provides weight and structure, the shape of the bark particles is flat, irregular and can have fine particles. Because bark particles can pack together more tightly, it produces a growing medium with a lower air porosity and reduced water holding capacity compared to the products with coir chunks.

When selecting these products for perennials, mums and small nursery containers, there are several factors to take into consideration:
  • Container size
  • Crop time/duration
  • Growing environment
  • Crop type
Container size
Let’s take a fall garden mum crop for example. Choosing a shallow 10” diameter mum pan (8” tall) or a deep 10” diameter nursery container (12” tall) influences the water holding capacity of the growing medium selected. Even though the containers are the same diameter, the height of the container and container volume will influence the water holding capacity. The height is a factor because there is a saturated layer of growing medium at the bottom of the container. As long as the growing medium is the same, the height of this layer is the same, regardless of the depth of the container. In the case of the shallow mum pan, a higher percentage of the growing medium is within the saturated zone, so the growing medium retains more water. In a tall nursery container, a smaller percentage of the growing medium is within this saturation zone (due to the effects of gravity). So if you guessed that the taller container would dry out faster, you’re correct.

Crop time / Growing environment
Now let’s consider the crop time/duration and growing environment for the same mum crop. Initial planting of rooted cuttings into a large container can be easy to overwater. For the next 14-18 weeks, the rooted cutting will grow into a large potted plant and will require irrigation at least once a day. If you are located in an area that has low relative humidity and high temperature, the mums will tend to dry out quickly. For this application, Pro-Mix CC40 or HPCC may be the best choice. If you are located in an area that has high relative humidity and high temperature, Pro-Mix BRK or BRK 20 may be the right product. With either choice, attention must be given to irrigation frequency and volume of water. Too much water can reduce crop quality and increase root disease. Since the crop is grown outdoors, rainy, cloudy weather can impact the crop as well. Check moisture content by feeling the moisture of the growing medium in several containers.

Crop type
Perennials can be categorized into four general groups based on their root systems. Fibrous roots are many small, well branched roots (coreopsis). Tap root is the main descending root of a plant (daylily). A rhizome root is a specialized, thickened root that grows horizontally with branching close to the surface (iris). The fourth type of root system is a corm, which is bulb-shaped but with a solid structure and roots at the base (anemones). Fibrous root and tap root perennials tend to be more forgiving of overwatering and Pro-Mix HPCC or CC40 would work well for these root types. Rhizomes and corms can easily rot from too much water and would grow better with either Pro-Mix BRK or BRK20. For all of these blends, the addition of Mycorrhizae favors the development of perennials and improves plant quality.

For more information
Premier Tech Horticulture Office
1, avenue Premier
Rivière-du-Loup (Québec)
Canada G5R 6C1
T: +1 418 867-8883
Toll free: +1 855 867-5407

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