Fabric pots (aka grow bags) are becoming very popular in home gardens as some gardeners are moving away from plastic pots. Manufacturers of fabric pots claim a number of benefits like better root growth, and cooler conditions for roots, but are these claims valid? The plants you buy at nurseries are still being sold in plastic pots and that industry is always looking for ways to grow plants faster. If fabric was better for plants would the industry not use them?
Will your plants grow better in grow bags? Should you stick with plastic or move on to fabric?
What are Fabric Pots
Fabric pots are a new form of pot for growing plants. They are called fabric, but they are really plastic and most are made from BPA-free polypropylene which is very similar to landscape fabric. The material has small holes in it that allow water and air to flow through.
They come in a variety of sizes including very large ones that are now being promoted as instant raised beds.
The Claimed Pros and Cons of Grow Bags
The following is a list of the claimed pros and cons.
- Prevent circulating roots by air pruning them.
- Keep the soil cooler on hot summer days.
- Cheaper than plastic.
- Provide superior drainage.
- Easier to move around.
- Pots are short lived.
- Dry out faster.
- Wasted soil that roots can’t use.
Grow Bags Are Cheaper
If you are shopping at a place like Amazon, fabric pots are cheaper than plastic and the difference is even more noticeable as the size goes up. Plastic pots last longer making fabric pots more expensive long term.
Plastic pots can be recycled for free using old nursery pots. I use a lot of plastic pots but have not bought any in 30 years.
Fabric Pots Provide Superior Drainage
Provided a pot has drainage holes in the bottom, drainage is dictated mostly by the soil, not the pot itself.
Grow Bags Are Easier to Move Around
Many grow bags come with handles, making it slightly easier to lift a larger pot. Without handles they are quite cumbersome to move.
Fabric Pots Are Short Lived
Manufacturers claim that the pots will last for 2 to 4 years before they need to be replaced. A good quality plastic pot lasts much longer.
Grow Bags Dry Out Faster
The fabric pot breathes better allowing moisture to leave the pot on all sides. Therefore the soil inside the pot dries out faster.
This is a major reason for not using them. In hot weather, containers need to be watered almost daily and I take steps to modify the soil to reduce the frequency of watering. The last thing I want are pots that dry out faster.
Fabric Pots Keep Soil Cooler
This is a common claim by manufacturers of these pots and many articles mention it. But is it really true?
A leading manufacturer of such pots claimed, “the temperature inside a plastic pot can easily exceed 120 degrees Fahrenheit (49°C) on the hot side of the pot. This causes root damage, stunts growth, and stresses the plant.” However …. they failed to provide the equivalent temperature in a fabric pot! It might have been just as hot?
They also failed to provide the temperature in the center of the pot.
I set up an experiment to compare 20 gal black fabric pots to 20 gal black plastic pots, using these pots. Three of each were filled with the exact same amount of soil and received equal sun in a location that got direct sun all day until about 7:00 pm. Temperatures were measured in the center of the pot, two inches below the surface of the soil. The difference in temperature between fabric and plastic was minor. The soil temperature was influenced much more by the ambient temperature, than the material of the pot. Sun shinning on the pots did not increase the temperature of the soil above the air temperature.
Fabric Pots Air Prune Roots
One of the main claims for fabric pots is that they air prune roots and thereby prevent root-bound plants. Roots in a plastic pot grow towards the pot and once they touch the edge they tend to grow between the soil and pot, circulating around inside of the pot. After a while you get a thick layer of roots on the inside of the pot, a condition known as being root-bound. Roots behave differently in fabric pots. As they near the pot, the soil is too dry and they stop growing. Instead new roots start farther inside the pot forming a more normal-shaped root ball.
Fabric pots do seem to prevent circulating roots.
Are Root-Bound Plants a Problem?
Gardeners have been led to believe that root-bound plants are a problem, but is it? There are certainly cases where it is a problem, but there are others where it’s not.
The biggest concern for circulating roots is for woody plants: shrubs and trees. In woody plants the roots keep getting thicker as the plant ages. When circulating roots get thick enough they can strangle the trunk thereby killing the tree. If fabric pots prevent this condition they would be a good choice for growing woody plants.
Transplanting perennial herbaceous plants from pots to gardens is less of a concern because non-woody roots tend to stay thinner or are replaced by new roots as the plant ages. There is still a short term problem because the circulating roots may not enter the native soil as quickly as we would like, but there is usually no long term effect. Therefore the root pruning characteristics of fabric pots are much less valuable for non-woody plants.
Most gardeners use fabric pots for annuals such as flowers and vegetables. Are circulating roots even a problem with these?
Most houseplants develop circulating roots and grow root-bound. The roots of these plants are quite healthy. They are thick and white with limited rotting because they are getting enough water, air and nutrients. The plants also grow well. Circulating roots are not a significant problem for most houseplants.
You can find numerous pictures online that compare fabric to plastic, like the picture above, but they rarely indicate how long the plant has been in the pot. The above picture shows circulating roots for both pots but is this the result of one season or two?
I wanted to see how quickly annual plants developed a root-bound condition. The pots that were used for the temperature measurements, described above, grew most of the summer. Originally they contained bush beans and then weeds took over and I just let them grow. At the end of the summer I carefully removed the pots so I could examine the root ball. The plastic pots were showing some circulating roots but the condition was far from severe. In fact it was better than most of the plants I purchase from nurseries. The fabric pots had almost circulating roots.
Fabric pots did reduce circulating roots, but plastic pots were also not very root-bound. Given the size of pot, there was not enough time in one season to create a root-bound condition and this would be true of many annuals grown in pots. And even if circulating roots do develop, it is quite possible they do not affect growth over one season.
Fabric Pots are Effectively Smaller
The outer inch or two of soil that is next to the fabric pot is much drier which prevents roots growing in this space. That doesn’t sound like much but consider a 5 gal pot that is 12″ in diameter and 10″ high. It has a volume of 1131. If you now remove a 1″ layer of soil from around the vertical sides, the volume drops to 785 – a 30% reduction in effective pot size. A loss of 2″ results in a loss of 56% of the volume.
Which condition inhibits plant growth more; some circulating roots or a pot that is 30% to 56% smaller?
Air Pruning Happens Mostly On the Sides
Roots grow through the bottom of fabric pots in both directions, if they are sitting on soil. Plant roots grow down and weed roots grow up into the pot. Weed roots rarely grow up into a plastic pot.
What Does Science Say About Fabric Pots?
Four tree species were grown in both plastic and fabric pots: live oak (Quercus virginiana), red maple (Acer rubrum), sweet gum (Liquidambar styraciflua) and golden rain tree (Koelreuteria
paniculata). Tree growth was the same in both types of pots. Average monthly high and low soil temperatures were the same in both types of pots. There were less circulating roots in fabric pots.
The popular Knock Out™ rose grew better in fabric pots than plastic pots, but root growth was better in plastic. Soil temperatures measured right at the edge of the pot, on the southern sunny side, showed higher temperatures in plastic than fabric.
The winter survival rate, in zone 5, for Viburnum plicatum tomentosum ‘Mariesii’ was greater in fabric than plastic for unprotected plants. Plants in fabric containers had less root and shoot damage. The minimum winter lows were 2-3°C higher in fabric pots during cold events.
Some growers are now using fabric bags for trees and planting the whole tree and bag in the field. Dr. Gilman has reviewed these and commented that, “because the root ball is smaller, there is less water storage capacity in the fabric container root ball than in the larger-sized root ball of the B&B field-grown tree. Combined with a dense root system, this lesser reserve makes trees produced in fabric containers more sensitive to desiccation immediately after digging than trees grown directly in field soil.” Root balls of field-grown trees in fabric containers are very fragile and must be handled carefully. The method is being used successfully.
The interest for fabric pots is mostly for woody plants because they have the problem with circulating roots. I found no research that compared annual plants in the two pot types.
Fabric Pots vs Plastic Pots – Which is Better?
If you are growing trees for several years in the same pot, fabric pots may be a better option because they reduce root problems. But most gardeners don’t do that. Most gardeners are using them for annual plants and then the benefits are less clear.
Pros for annual plants:
- Prevent circulating roots by air pruning them – not really a benefit.
- Keep the soil cooler on hot summer days – value seems to be overstated.
- Cheaper than plastic, but only if you buy something – plastic can be free and lasts longer.
- Provide superior drainage – not true.
- Easier to move around – valid point if the bag has handles.
Cons for annual plants:
- Pots are short lived.
- Dry out faster – major problem with annuals.
- Reduces the useable soil in the pot.
Fabric pots and plastic pots both work. For annual plants, plastic works better because it requires less watering.
Far better than either type of pot is growing in the ground. Use pots and raised beds only when you can’t use the ground soil or you don’t have any.