Growing Media
Tree seedlings differ from most ornamental container crops because conservation and reforestation seedlings are essentially root crop. Meaning, the quality of a tree seedling is determined by its outplanting performance – both initial survival and subsequent growth.
Tree seedling survival and growth are directly related to the ability of the root system to promptly regenerate new roots (known as root growth potential or RPG) and grow out into the surrounding soil. For this reason, tree seedling container and medium are designed to encourage the seedling to form a good root system in the nursery and to protect these roots until the seedling is outplanted.
The relative health and vigor of the root system is also reflected in the morphology and growth of the seedling shoot and for this reason containers and media should be designed to enhance the root-shoot relationship.
1. Physical properties
A good growing media should be composed of lightweight organic and mineral components that are typically 20-30% solid by volume. That means that each container has 70-80% pore space.
Blends of organic and mineral substances provide plant support, air, and a water/nutrient reservoir to make available nutrients and gases to the plant as it develops a root system for exchanging the solutes.
Growing media can be comprised of many things such as peat moss, bark, perlite, vermiculite, mineral soil, and muck peat soils, and most recently coconut coir, sawdust, and rice hull.
Most mixes are either peat- or bark/sawdust/coir-based. Bark, sawdust or coir-based mixes decompose more slowly and therefore, shrink less over time making them well suited for crops lasting longer than five months.
Most media have 60-70% organic matter such as peat moss, bark, sawdust or coconut coir and the balance can be vermiculite, or perlite. The blend depends on availability, cost of components and the needs of the project
Media texture is important. Texture refers to the particle size and the air porosity level. Texture can affect drainage. The less it drains, the less air space is in the mix. Compacting the mix will decrease the porosity. Once the medium is compacted in the container, it cannot be changed without repotting the plant.
A medium that is too compact will be slow to dry out, decreasing the ability to feed the plant correctly and increasing the potential for root rot such as Pythium. Compact medium also increases the potential for algae to develop in the medium surface.
Algae growth attracts fungus gnats and shore flies that can infect the plant with Pythium. If the mix is too compact, there will be less air and more water in the mix. This reduces young roots ability to penetrate deep into the mix.
Generally, if a mix has the proper aeration, you should see roots at the bottom of the container rather quickly after transplanting (5-7 days).
Having and maintaining porosity is one of the most critical aspects of plant production. You can always adjust your watering to increase the amount of water needed, but you can not increase the amount of air present in the mix.
Air porosity is fixed at the time of potting and only decreases over time. Fortunately, good root growth can maintain or even slightly increase aeration.
A healthy root system maintains porosity and aids in water penetration and wettability. At the time of transplant, the physical properties of the mix “dominate” the young roots, dictating how fast and far they can grow. In three to five weeks, the root system should be sufficiently robust to dominate the container. High porosity causes rapid growth and allows the root system to take over the container quickly.
Poor aeration produces slow root growth and can delay or prevent the roots from complete exploration of the container. Growers have a saying: “Spare the roots, they can make or break your crop”.
