Humans have been cultivating plants in containers for thousands of years. Horticultural pottery fragments from Egyptian settlements date back to 10,000 years ago, and more recent Roman remains have shown that they were likely the first to grow perennials in containers for the purpose of bringing them indoors during colder weather. As gardeners in 2015, the only things that differentiates us is the technology.
Unglazed terracotta containers were the OGs of container gardening – these porous pots are able to insulate the root zone from hot and cold temperatures as well as allow the root zones to “breathe” with the very small amount of air that is able to pass through the clay walls.
The downside of these pots is their price and weight. Plastic pots came along in the 1950s and, being less expensive to produce and much lighter and more durable, these pots quickly took over. Plastic pots are far from perfect, though.
When roots grow to the edge of a plastic pot, there is a small layer of water and nutrient between the medium and the walls. As roots’ sole purpose is to hunt out food and water, the plant is tricked into thinking there is still space for it to grow even while its wrapping around the edges inside your container. So what’s wrong with living on the fringe? Roots like cool and moist conditions and plastic pots can get fairly warm (especially under grow lights or in direct sun). The roots that are circling around the inside of the edge are more susceptible to excess heat (with no media for insulation), drought (the edges of growing media tend to dry out first), and disease (unhealthy roots suffering in these regions are more prone to disease).
Leave a plant in a plastic pot for too long and inevitably you get a few large roots wrapping and circling around the edges of the pot over and over and eventually choking itself out.
So how do we get the roots to concentrate their growth within the growing media? Enter air pruning!
Air pruning is a technique that exposes root tips to relatively dry air, thus stopping their growth. The plant now realizes its boundaries and focuses on creating secondary roots that branch out from the originals within the growing media. These secondary roots then spread throughout the media until they reach the thin layer of air at the walls, and the cycle is then repeated. Over and over your plant is producing smaller, more fibrous roots, filling up its container. More root hairs = more root tips, and more root tips = more water and nutrient uptake. In short, the overall effect of this growth and pruning cycle is the creation of a well-developed root system in a relatively short amount of time.
Benefits of Aeration Containers:
- Stronger, healthier plants and starts
- Less growing media needed*
- Improved overall root structure (more root tips)
- Decreased risk of transplant shock
- Better use of water and nutrients
- Promotes beneficial biology
- Quicker Growing times
- Elimination of root wrapping (circling) in pots
- Fewer transplants required
*Plants grown in root-pruning containers demonstrate the ability to cope with being containerized for longer — I personally love them for container fruit trees. This means you could potentially decrease the size of your container without the risk of a plant becoming root bound. Just note, you may have to water more frequently.
A few types of root pruning containers:
- Smart Pots – Made from custom black (or tan) non-woven polypropylene material. The black fabric helps warm growing media (outdoors in early season) and allows heat to escape via breathable material. These pots are highly durable (think 5+ years of continuous use) and many sizes come with handles!
- Root Pots– Made from 100% recycled materials (often jean fabric), they have a blue-ish recycled look to them. These also help with thermal insulation during cold temperatures and evaporative cooling during hot conditions. Root pots are biodegradable and reusable for 2.5 – 5 years.
- Air Pots – A black plastic container with an unusual design – inward and outward-pointing cones resemble egg carton. The bottom is a plastic mesh. These containers are highly durable and can be taken apart for storage.
- Winstrip Trays – 72 cell propagation trays with an open design that enables air pruning in the early stages of plant life. Highly durable polypropylene plastic is UV resistant and usable for 10-15 years. (https://fifthseasongardening.com/winstrip-trays-soil-blocks-without-the-mess)
Here are some photos from our experiments growing plants in plastic containers vs. Smart Pots.