Plant pots. What a beautifully simple growing system. Fill a receptacle with potting soil and voila—you’re good to go. No wonder then that this technique dates back to ancient times. In fact, fragments of Egyptian horticultural pottery have been dated as far back as 10,000 years, although they are thought to be simply for the purpose of portability between settlements. The Romans are thought to be the first to have grown perennial plants in pots with the intention of bringing the plants inside during periods of cold weather.

Until the 1950s, most plant pots you could buy at your local gardening store were made of terracotta clay—then along came plastic—a substantially cheaper more durable and lightweight product. However, not all gardeners were convinced, and for good reason. Unglazed terracotta pots insulate the root zone from hot and cold temperatures and, because clay is porous, the pots are able to ‘breath’—albeit with only a small amount of air that’s able to pass through the clay pot, many old school gardeners claim this as the main reason they can produce better plant growth than with plastic pots.

It’s pretty obvious why plastic pots with solid sides are now the norm; they’re cheap, more practical and durable than clay and, of course, featherweight in comparison. Also, although they restrict air, there’s no arguing that they work, and in most cases they work pretty well. However, as we can see, plastic pots are far from perfect which means, of course, that there’s room for improvement. It turns out that, with a few design tweaks, many of the fundamental problems associated with standard plastic pots can be reduced, if not eliminated.

So what’s the problem with standard containers?

Remember, Mother Nature didn’t create plant pots. Humans did! In normal plastic plant pots, roots grow until they hit the inner edge—invariably the roots start circling around the pot. Now, in this case, being at the fringe of the action isn’t such a good thing. Those plastic pots can get fairly warm, especially under grow lights or if exposed to direct sunlight. Remember, roots like cool and moist conditions. Roots that are circling around a pot are more susceptible to the negative effects of excess heat (because there’s no media to provide insulation,) drought (the edge of your growing media tends to dry out first) and disease (unhealthy roots that are suffering in these conditions are more prone to disease.) One thing’s for sure—these outer roots are certainly not taking advantage of all the water, nutrients and beneficial biology that may be present within the growing media.

So how do you get the roots to concentrate their growth within the growing media? The answer actually lies in providing a consistent restrictive environment for root growth at the pots edges. This can be done in a few ways, but the most common techniques are air pruning and root trapping. Understand how these techniques work, and you’ll understand how plant pot design can be vastly improved!

Air Pruning

Essentially air pruning is a physical technique that inhibits root growth from extending beyond its growing media, or circling around its container, by exposing the root tips to relatively dry air and stopping their growth. As these roots now have nowhere else to grow, it stimulates further growth of secondary roots that branch out from them within the growing media. These secondary roots will spread throughout the pot until they reach the sides and bottom and get air pruned again, stimulating more root growth and more root hairs. More root hairs = more root tips. More root tips = more water and nutrient uptake. The overall effect of this growth and pruning cycle is the establishment of a well-developed root system in a relatively short space of time throughout the entirety of the growing media.

Root Trapping

This may be a new concept to some growers; it involves a more mechanical approach in comparison to air pruning. With root trapping the root tips become physically trapped in the pot material, which is made from coarse felt or other fabric materials. When a root tip grows in to the material at the edge of the pot it becomes trapped, preventing horizontal and vertical growth around the edge of the pot. I know, I know, words like “trapped” and “preventing” don’t sound good. But, because the root tip is unable to grow any further at the pots edge, it’s forced to branch and grow secondary roots within the growing media. This all adds up to a more effective root zone.

Parallels with the above techniques can be made with the plant growth we observe above the ground. Most gardeners know that pruning a lateral branch or terminal (uppermost growth tip) can create a more productive plant. You cut and remove the tip to encourage lateral branching and in this way create a fuller, bushy plant with more branches. The same can be done for the root structure, air pruning and root trapping is all about encouraging lateral root branching.

Clever Containers in Action

This illustration shows how air pruning works in a breathable fabric pot.

1 – Shortly after the cutting/seedling is planted into the pot, primary roots grow towards the sides and bottom.

2 – The fabric sides of the pot allow to breath, allowing air to get to the growing media. This air is drier than the root zone environment within the pot, so as the roots start to grow out of the growing media the relatively dry air prevents further growth, and stimulates secondary roots to branch out within the pot.

3 – The root branching and pruning cycle keeps occurring, creating a plant with an abundance of healthy well branched roots within the growing media.

Key Benefits

• Stronger, healthier starts.
• Elimination of root circling in pots.
• Less growing media needed.
• Decreased risk of transplant shock.
• Improved overall root structure (more root tips.)
• Better use of water and nutrients.
• Promotes the activity of beneficial biology.
• Quicker growing times.
• Fewer transplants required.

There are many containers in grow stores and garden centers that have adopted air pruning, root trapping or a combination of both into their designs to help improve results. These can be classified to two general categories, fabric and plastic containers. Here’s a list of clever containers with and brief description of how they work.

Fabric Containers

Generally, fabric containers are relatively inexpensive, can be used with a variety of growing media, and are suitable for hydroponics systems (ebb & flow and drip irrigation), as well as basic hand-watered application.

High Caliper – Smart Pot

Made from black custom non-woven, polypropylene material, Smart Pots use a combination of root trapping and air pruning; the root tips grow into the fuzzy fabric of the pot and become trapped, the fabric is breathable causing the root tip to become air pruned. The black fabric helps to warm the growing media (outdoors in the early season) and also helps heat escape via the breathable material and evaporative cooling action in summer. Smart Pots are highly durable—see if you can rip one—and can be re-used over several grows.

Sizes Available: 1 2 3 5 7 10 15 20 25 30 45 65 100 200 300 400 (Gallon)

10 Gallon MSRP: £5.95

Aurora innovations – Root Pots

Made from 100% recycled materials, the Root Pots certainly have an authentic recycled look to them. The breathable fabric allows air pruning of the roots to occur, preventing root circling. This fabric pot helps with thermal insulation during cold temperatures, but also allows evaporative cooling during hot conditions. Root Pots are re-useable for between three and five years, but are also bio-degradable.

Sizes Available: 1 2 3 5 7 10 15 20 30 45 65 100 150 200 250 300 400 500 600 (Gallon)

5 Gallon MSRP: £4.00

RootMaker – RootTrapper II

These containers are made using thick felt, and as the name suggests, utilize root trapping to encourage root branching. The felt is lined with an outer white plastic layer that is designed to keep the internal temperature of the container cool as well as prevent evaporative moisture loss. The white outer coating also reduces the risk of roots completely dying on sides that are exposed to direct sunshine, which can happen with black containers. Water use can be reduced because there are no large drain holes at the bottom or evaporative sides. The base is made from a slightly different material that allows for adequate drainage. They are reusable, but trapped root tips will remain in the fabric.

Sizes Available: 1 2 3 5 7 10 15 30 45 60 100 150 400 500 (Gallon)

5 Gallon MSRP: £7.25

Square Root Aeration Containers

Utilizing a slightly different approach, theses containers have a space saving square design. The soft breathable fabric provides adequate drainage at the base and aeration through the sides, allowing air pruning to occur. The black fabric helps to keep the substrate warmer during cold temperatures and reduces substrate temperatures during high temperatures.

Sizes Available: ½ 1 3 5 7 10 20 30 100 (Gallon)

5 Gallon MSRP: £5.95

Plastic Containers

These high tech plastic containers are more expensive than the fabric, but are more durable and can be reused time and time again. Like the fabric pots, they are suitable for use with many types of growing media and indoor and outdoor gardening applications.

Air-Pots

Air-Pots have a somewhat unusual design, the sides of the pot are made up of inward and outward pointing cones, much like an egg box, and the bottom is plastic mesh. The inward pointing cones are solid whereas the outward pointing cones are open ended. The purpose of these cones is to firstly deflect the roots and stop them from circling within the pot, but more importantly, to direct the roots towards the outward pointing cones and allow the root tips some direct exposure to air, causing air pruning.

Sizes Available: 0.3, 0.8, 1.2, 1.5, 2, 2.4, 3.4, 4.3, 5.2, 6.5, 8, 10, 12, 45(gallon)

5 Gallon MSRP: £10.00

RootMaker II – Propagation Containers

These plastic propagation cell trays have either 32 or 60 cells. Within each cell there are tree tiers and in the corner and sides of these tiers are small holes. These holes allow air pruning to occur during the initial propagation stages, preventing root circling.

Available Sizes: 32 Cell Tray & 60 Cell Tray

RootMaker II – Injection Molded Containers

Larger versions of the propagation cells, these plastic pots have a tiered design with multiple small holes on each tier to allow air pruning to occur and stop root circling around the pots sides. These are available in square and round variations in the 1 gallon size, but are only available with a round design for the 3 and 5 gallon containers. The Root Makers are made from largely recycled plastic.

Sizes Available: 1 3 5 (gallon)

5 Gallon MSRP: £8.95

RootBuilder II

These pots have a similar design to Air-Pots. The walls are assembled around a bottom disk to create either a 3 or 5 gallon container. The walls of the containers have outwardly projecting funnels with a hole at the tip. Root growth is directed toward the holes at the end of these funnels, where the roots get air-pruned.  The shape of the containers is designed to direct the roots to the air holes but also creates shade keeping containers and root zone cooler which will also help to improve plant growth. The Root builder II containers also have solid root directing bottoms which direct the roots towards the edges of the pots. .

Sizes Available: 1 2 3 5 7 (gallon)

5 Gallon MSRP: £12.95

Clever Containers – Grower Feedback and Tips

Quick Establishment

The quickest way to see the benefits of air pruning containers is to use them in the early stages of propagation and vegetative growth when plants are establishing their root systems. Growers using air pruning containers observe quicker establishment into new pots (whether these are larger sized air pruning containers or larger normal pots,) improved root growth and overall plant vigor.

Reducing the Chore of Transplanting

Whilst it’s important to match pot size to plant size, transplanting can be a chore! One of the great benefits most growers can achieve with root pruning containers is the reduced amount of necessary transplants, particularly when growing with potting soil. Many commercial nurseries have experienced time and labor saving benefit for years, and now many indoor growers are taking advantage of some of these same benefits.

When growing in standard pots and potting soil, most growers would go through three to five (or in many cases more) potting stages, for example; propagation cube > 0.3 gal > 1 gal > 3 gal > 5 gal.

With root pruning containers this can be significantly reduced, with most growers only having to use two potting stages, for example; propagation cube > 0.5 gal > 3 gal.

Smaller volume

Plants grown in root pruning containers have shown the ability to cope with being containerized for longer, in comparison to plants of a similar age in the same volume standard pots. This means you could potentially step down the size of your containers without the risk of the plant becoming root bound, saving floor space and reducing the need for so much growing media. However, if you do reduce the volume of your pot, an increase in water requirements should be expected.

Water Consumption

Of the majority of growers we spoke to, all found the growing media to dry slightly quicker than usual. Most said this was not as significant as they thought it would be, with the average reporting a 10-20% faster drying time.

When growing in standard pots if the growing media is allowed dry more than usual, it can become difficult to re-saturate and water can run straight down the sides. The same goes for Air-Pots and RootBuilders, but instead of water running down the sides, it will run out the outward pointing cones. To get around this instant run-off when the growing media becomes too dry, you need to re-saturate with small amounts, and water slowly.

Tip: Consider installing a drip irrigation system to water your plants. These are an excellent way to deliver water slowly in order to achieve an even and steady re-saturation.

Moving Pots

One of the advantages of growing in any pots is their ability to be moved around the garden to suit your needs. Obviously, this is still possible with all root pruning containers but the fabric pots need some extra care. Lifting any fabric pot using the rim while they are wet and heavy will cause the sides to stretch, this often causes the root tips to rip, which can be felt as the pot is lifted. So, while lifting or moving fabric pots its a good idea to support the bottom.

Filling Technique

A few growers we spoke to using Air-Pots and RootBuilders reported that they had to spend more time during the potting up stages adding the growing media to the pot. The growing media in these pots needs to be slightly more compacted than in normal pots, and frequent tapping down of the during filling is absolutely crucial to ensure all that growing media settles in all of the outward pointing cones/funnels. Some growers tip the pot on its side whilst filling with growth media to ensure that it fills every nook and cranny—very important!

Growing Media

Most growers using root pruning containers are growing with potting soil and hand watering. However, other growers are choosing to use clever containers for automated growing applications, particularly drip irrigation systems. Growing media that is more prone to drying out faster, like rockwool and clay pebbles are much more suited to automated irrigations.

Nearly all types of growing media are suitable for use in root pruning containers. One minor issue we heard about is from growers using clay pebbles in Air-Pots in automated drip systems. These growers are happy with the improved growth of their plants, but found that some of the outward pointing cones can become blocked with pebbles when filling. To get around this they found that taping the sides of the pot and poking the obviously blocked holes sorted things out.

General Air-Pruning Tips

Air pruning can be applied to any stage of plant growth, however, most growers avoid air pruning roots of newly rooted cuttings or seedling grown in small propagation cubes or plugs. Cuttings and seedlings are quite a tender stage of a plants life, so a sensible approach would be to wait until the next propagation stage to begin air-pruning, where plants are grown on in larger rockwool blocks or low volume root pruning containers.

Season variations in climate should be taken into consideration when choosing the growing media or potting soil mix for short cycle plants in air-pruning pots to help you deal with over- and under-watering issues. During winter when average temperatures are lower, the growing media can be lightened up with less absorbent media like perlite, clay pebbles or coarse coco chips. During summer when water requirements increase, the mix can be made heavier by reducing the previously mentioned amendments. These seasonal variations will help you achieve the maximum potential when using air-pruning techniques.

The relative humidity of the air in the growing environment will determine the severity of the air-pruning. In humid conditions, roots may stop growing at the surface of the media and produce secondary roots with the media, but the tips may not completely dehydrate. Although we do want the roots to stop growing in response to contact with drier air, we don’t want to stress our plants by trying to grow them in an environment with low humidity (below 50%). Experimenting with different relative humidity for your species and variety of plants and growing media is highly recommended. Try varying the humidity between 55-80% to see what effect is has on your air-pruning.

Got an air-pruning experience you want to share? Email us at: rant@urbangardenmagazine.com