How to Use Fabric Grow Bags: Healthier Roots and a Wider Plant

A fabric grow bag looks like a simple thing: a soft pot with handles. But the fabric changes how roots grow, how warm the mix gets and how fast it dries. Learn those three things and a grow bag is easy. This guide covers choosing a size, filling and planting, watering and feeding, tying stems out to the rim so the plant grows wide instead of tall and floppy, and packing the bags away at the end of the season.

What the fabric does for roots

In a hard plastic pot, a root that reaches the wall turns and keeps growing along it. Over a season those roots can wind round and round the inside of the pot. In a fabric container, a root tip that reaches the porous wall meets air, and the fabric prunes it there. University of Florida extension describes the result as less root circling around the container wall [1].

Researchers have measured this. In a trial of root-pruning containers, the fabric pots had little to no root growth along their sides, and all the root-pruning containers grew more fine roots than standard plastic pots [2]. In another study, trees grown in fabric containers had fewer circling roots, and three years after planting out, more of their roots had grown beyond the original root ball [3]. A third study notes that the porous fabric encourages a fibrous root system and keeps root circling down [4].

The fabric also keeps the mix cooler. Water evaporates through the sides, and in one study the hottest daily temperature in the mix averaged about 11°F (6°C) lower in fabric containers than in black plastic ones [4]. The same paper notes that very hot container mix is known to injure roots and slow plant growth [4].

What the research does not say

We called this guide "Healthier Roots", not "Bigger Roots", on purpose. The studies above found roots that branch more and circle less, not more root by weight. In one, the root balls of plastic-grown trees weighed more [3]. In another, the plants in fabric pots were the smallest above ground, likely because they ran short of water later in the day [2]. Most of this work was done on nursery trees and shrubs, not tomatoes, and a 2002 extension fact sheet noted how little had been published on growth in fabric containers [1]. The fair summary: fabric gives you better-branched roots and a cooler root zone, and it asks you to water more. The watering section below shows how.

Choose the right size

Our fabric grow bags come in packs of five, one size per pack, from a 1 gallon transplant pot to a 25 gallon bag (height x width):

• 1 gallon (3.8 L), 7 x 7 in: a transplant pot. Move a seedling up into it from its seed cup, then on to a bigger bag or the garden bed once the roots fill it.
• 2 gallon (7.6 L), 9 x 9 in: herbs, lettuce or strawberries.
• 3 gallon (11.4 L), 10 x 10 in: a compact pepper, kale or chard.
• 5 gallon (18.9 L), 12 x 12 in: one pepper, one bush tomato or a few lettuces. The most popular size.
• 7 gallon (26.5 L), 13 x 13 in: a vining tomato or an eggplant.
• 10 gallon (37.8 L), 15 x 15 in: big tomatoes and potatoes.
• 15 gallon (56.8 L), 17 x 17 in: squash, big tomatoes or a crop of potatoes.
• 25 gallon (94.6 L), 20 x 20 in: a dwarf fruit tree or several plants together.

Iowa State recommends at least a 2 gallon container for a cherry tomato and at least 4 gallons for a standard tomato, and says that in general the larger the container, the better [7]. Kansas State puts peppers and dwarf tomatoes in 1 to 3 gallon containers [8]. Size also sets how often you water: small containers dry out faster than large ones [5]. If you are often away on summer days, go up a size.

Fill it with potting mix, not garden soil

Garden soil packs down in a container, so water drains poorly and roots get too little air, and it pulls away from the sides as it dries [6]. Use a bagged potting mix made for containers. Iowa State says a good one is lightweight, drains well, holds moisture and nutrients, and does not compact readily [6].

1. Set the bag where it will grow. A full bag is heavy, so fill it in place. Fruiting vegetables need at least five hours of direct sun a day and do better with eight to ten [5].
2. Dampen the mix first. Clemson suggests filling with a slightly damp mix: peat in the mix takes up water much more readily if it is wetted before it goes in the container [5].
3. Fill to just under the rim. Clemson's guide is to within half an inch of the top [5]. The welded steel rim keeps the bag standing open while you scoop, so you can fill with both hands free.
4. Settle it. Water the mix once, let it drain, and top it up if it has sunk.

Saucer, pavers or nothing?

A grow bag drains through its base and sides, which is the point. Where that water lands is up to you:

• Garden bed or gravel: nothing needed.
• Hot patio or deck: raise the bag on a couple of pavers, bricks or a pot stand. Clemson suggests lifting containers off the cement so air moves underneath and the base does not sit on hot pavement [5].
• Balcony or indoors: use a wide saucer or tray so the water does not reach the neighbors or the floor [5]. Empty it after watering: the mix should never stay soggy [5].

Move a seedling in

Grow bags are the natural next step after seed cups. If you started your plants in our seed starting cups, push the bottom plate up and the whole root ball lifts out in one piece. Our guide How to Use Seed Starting Cups covers starting seeds and moving them on.

1. Dig a hole in the filled bag about the size of the root ball.
2. Set the root ball in, firm the mix around it and water it in gently [5].
3. If the plant will need a stake, put it in now. Clemson advises adding stakes and cages while plants are very small, to avoid damaging roots later [5].

Water until it drains, and more often than plastic

This is the one habit to learn. Clemson notes that porous containers lose water through their sides and need watering more often [5]. In a Florida study, fabric containers needed 66% more irrigation water than plastic ones to make up for that loss [4].

• Check every day, and twice on hot, dry or windy days [5]. Push a finger into the top inch or so of mix: if it feels dry, water [8].
• Water until it drains. Keep going until a little water runs out of the bottom; that tells you the whole bag is wet [8].
• Use a gentle flow, a watering can or a soft spray, so you do not wash the mix out [8].
• Do not let it dry out completely. On tomatoes, uneven watering leads to cracked fruit and blossom end rot [7].
• Help it hold water. Group containers so their leaves shade each other's mix, and add mulch: both help on hot days [5].

Feed container plants regularly

A container holds a small amount of mix, and every watering washes some fertilizer out [8]. If your potting mix came with fertilizer, Clemson says it feeds plants for eight to ten weeks; after that, add a water-soluble fertilizer at the label rate every two to three weeks [5]. Iowa State's container tomato guide uses a soluble fertilizer every week or two [7]. Never go above the label rate: container plants have no big volume of soil to buffer them, and too much fertilizer can burn and kill plants [5].

Grow a wider plant: tie stems out to the rim

Peppers get top-heavy in August. Tomatoes lean. Left alone, a plant in a bag can get dense in the middle, and heavy stems can bend or break [9]. Around the rim of each bag sit aluminum grommets rated to 10 lb, there to anchor ties and supports. Tie a few stems outward to them and the plant spreads across the bag instead of shooting straight up.

Why open it up? Georgia's extension guide recommends letting more air and sunlight into dense tomato plants, which can help with disease control; it does that by pruning [9]. Tying stems outward opens the middle without cutting anything. Keep the leaves, though: tomato fruit likes some shade and can scald in full sun [9].

1. Wait for a stem that bends easily. Start once a side stem is long enough to bend gently toward the rim without forcing it.
2. Cut a length of soft garden wire. Its stainless steel core sits under 3.2 mm of soft rubber, so it holds the bend and spares the stem. One 50 ft coil makes about 100 ties of a few inches.
3. Loop it loosely around the stem. Penn State's method: fix the tie firmly to the support, then make a loose figure-eight around the stem, so it never pinches or chokes the stem as it grows [10]. Here, the grommet is the support.
4. Thread the other end through a grommet and bend it to hold. Draw the stem outward gently and stop where it stops bending easily.
5. Repeat around the bag. Three or four stems tied out to grommets on different sides give an even, open shape.
6. Check every couple of weeks. Loosen ties as stems thicken, and add a new tie higher up as the plant grows [10].

For a branch that needs to bend at a sharp angle, a plant training clip holds it open with a rounded 110-degree bend instead of a kink. Our guide to the clips shows how to use them alongside the wire.

Support heavy plants

Standard tomatoes need a stake or cage to stay upright, with stems tied loosely to it [7]. In a bag, push the stake into the mix at planting time and tie it to two or three grommets to brace it. Bags are light, and a tall plant in a light mix can blow over in strong wind [5]: a stake braced to the rim, or a sheltered spot, prevents that. If you are buying bags and wire together, the Grow Bag and Garden Wire Set pairs five 5 gallon bags with a 50 ft coil of yellow soft garden wire for less than buying them separately.

Move it with the handles

One perk of a container is that it can move: into the sun, out of a storm, or under cover before the first fall frost [5]. Large containers full of mix are heavy, though [8]. Lift a full bag by both handles, not by the rim or the grommets, which are there for ties. For the biggest bags, get a second pair of hands or a plant dolly [5].

End of season: empty, wash, dry, store

1. Empty the bag. Pull out the old plant and roots and tip out the mix.
2. Brush and wash it. Brush off the loose mix, then wash the bag with soap and water and rinse. Iowa State recommends cleaning used containers before you replant them, because disease organisms and insects can survive in old pots and the debris left in them [11].
3. Dry it fully in the air before you put it away.
4. Store it indoors in a dry place until spring. The bags are washable and reusable, season after season.

FAQ

Do fabric grow bags stop roots circling?

They reduce it. In studies, fabric containers had little to no root growth along their sides and fewer circling roots than plastic pots [2][3]. The research was done mostly on nursery trees and shrubs, so treat it as a strong hint for vegetables, not a promise.

How often should I water a fabric grow bag?

Check every day, and twice on hot, dry or windy days [5]. Water when the top inch feels dry, and keep going until a little runs out of the bottom [8]. Fabric loses water through its sides, so expect to water more often than you would a plastic pot [4][5].

Can I use garden soil?

No. Garden soil compacts in a container and drains poorly. Use a bagged potting mix made for containers [6].

What size grow bag for tomatoes?

A 5 gallon bag for a bush tomato, 7 gallons for a vining one, and 10 or 15 gallons for the biggest plants. Iowa State suggests at least 2 gallons for a cherry tomato and at least 4 for a standard tomato, and bigger is better [7].

Do I need a saucer?

Indoors and on balconies, yes, and empty it after watering [5]. Outside on a patio, raise the bag on pavers or a stand instead [5]. On soil or gravel, you need nothing.

Can I lift the bag by the grommets?

No. Use the handles. The grommets are rated to 10 lb for anchoring ties and supports.

Can I reuse the bags next year?

Yes. Empty them, wash with soap and water, dry them fully and store them indoors [11]. They are washable and reusable.

Questions? Email hello@onlinegardenco.com. Fastest customer service: get responses in minutes, not hours or days.

References

1. Gilman, E. F. (2002). Fabric-Container-Grown Trees (ENH870). University of Florida IFAS Extension, EDIS.
2. Simshaw, K., Woodward, T., Saulsbury, L., and Hoch, W. A. (2016). Effect of four root-pruning nursery containers on biomass, root architecture and media temperature. Acta Horticulturae 1140: 191-192.
3. O'Connor, A. S., Klett, J. E., and Koski, A. J. (2018). Container type affects root development of Chanticleer pear during landscape establishment. Arboriculture and Urban Forestry 44(4): 165-173.
4. Million, J. B., and Yeager, T. H. (2022). Fabric containers increased irrigation demand but decreased leachate loss of nitrogen and phosphorus compared with conventional plastic containers during production of dwarf Burford holly. HortScience 57(7): 743-749.
5. Polomski, R. F., and Shaughnessy, D. Container Vegetable Gardening (HGIC 1251). Clemson Cooperative Extension, Home and Garden Information Center.
6. Iowa State University Extension and Outreach. I would like to grow a few vegetables in containers. Can I use garden soil or should I purchase a commercial potting mix? Yard and Garden.
7. Iowa State University Extension and Outreach. Growing Tomatoes in Containers. Yard and Garden.
8. Marr, C. W. (1998). Container Gardening: Vegetable Gardens (EP-31). Kansas State University Research and Extension.
9. Westerfield, B. (2022). Staking and Pruning Tomatoes in the Home Garden (Circular 1150). University of Georgia Cooperative Extension.
10. Lamont, P., and Sellmer, J. C. Supporting Herbaceous Plants in a Flower Garden. Penn State Extension.
11. Iowa State University Extension and Outreach. How to Clean and Disinfect Plant Containers. Yard and Garden.