Grow bags: fabric vs plastic pots, sizes and potatoes

Fabric bag or plastic pot at a glance

What a grow bag is

University of Maryland Extension (UMD) describes fabric bags, also called grow bags, as relatively inexpensive, reusable, lightweight and portable, made from natural fibers, synthetic (petroleum-based) fabric or a blend, in many sizes and styles. Oklahoma State University Extension suggests bags of durable polypropylene or heavy-duty landscape fabric. Like any container, a bag needs a mix that drains well but still holds moisture (Oklahoma State), and UMD's rule for all containers applies: drainage is critical.

Drying rate and root temperature: what the trials measured

The difference is the side wall: water evaporates through fabric, which cools the mix and empties it faster. The measurements come from nursery research on shrubs, not vegetables.

• Four-state field trial (Nambuthiri and others, HortTechnology, 2015). One-gallon containers in Kentucky, Michigan, Mississippi and Texas, May to October 2012. Mix temperature averaged about 6 °C higher in black plastic (about 36 °C, roughly 97 °F) than in wood pulp, fabric and coir containers. Fabric had the greatest sidewall water loss. The authors conclude that porous containers raised water use but reduced heat stress in hot summers.
• Florida holly trial (Million and Yeager, HortScience, 2022). 36-cm fabric versus plastic containers. Peak daily mix temperatures averaged 6 °C lower in fabric, but fabric needed about 66% more irrigation water and produced about 31% more drainage water. Fabric still lost 30% less nitrogen and 47% less phosphorus to leaching. Plants in fabric were 8% larger in one experiment and no different in the other.

For a home gardener, a bag can protect roots on a hot patio if someone checks moisture daily. UMD advises checking containers daily in summer rather than watering on a set schedule, notes that small containers dry out faster than large ones, and shows straw mulch on a large grow bag to reduce evaporation. Oklahoma State adds that frequent watering can leach nutrients, so plants in bags may need extra fertilizer, applied as the product label directs.

Air pruning: a claim with thin vegetable evidence

UMD explains the idea: a root that grows to the edge of a bag meets open air, stops lengthening and branches into a denser, fibrous root system that stays in the mix, while in a solid pot the root hits the wall and keeps going. UMD says bags improve plant growth this way. The published container trials found in research for this guide are on nursery shrubs and trees, and they show smaller effects:

• In five-state trials (Wang and others, 2015), plants in alternative containers, including fabric, generally grew about as well as in plastic, while fabric and other fiber containers used more water.
• In the Florida holly trial, plants in fabric were larger in one of two experiments (Million and Yeager).
• In a red maple study of eight container types, including a fabric type, container type had only a small effect on root architecture; shaving the outside of the root ball at each move to a bigger container had a large one (Gilman and others, 2016).

None of these measured vegetable yields, so the size of any benefit for a tomato or pepper in a bag is not established.

Bag sizes by crop

Grow bags are sold by volume, like pots, so the usual container minimums apply. Two UMD pages give different numbers for the same crops; the larger figure is the safer choice. UMD also notes that a 2-cubic-foot bag of potting mix fills three 5-gallon buckets or grow bags.

Oklahoma State finds shallow-rooted crops such as salad greens and radishes do well in bags, while sprawling crops such as melons, parsnips and winter squash usually do not, though dwarf cultivars can work. For all 25 crops in the site's table, see container sizes for vegetables; for a full tomato method, see growing tomatoes in containers.

Potatoes in grow bags

UMD lists potatoes among the more challenging container vegetables. In 2025 its Home and Garden Information Center (HGIC) answered a gardener with golf-ball-sized harvests: "We have yet to see yields from containers that equal those from potatoes planted in the ground or in raised beds." UNH Extension likewise says a container may not yield as many potatoes as garden soil, and UMD says potato "towers" don't usually work.

For comparison, UMD gives 6 to 15 lb per 10-foot row for potatoes in the ground. Maryland Grows harvests a week or two after the plants yellow and the stems wilt, by emptying the bag. Planting dates, seed pieces and pests are on the potatoes page.

Where sources disagree

Grow bag checklist

• Pick the bag by crop: the larger UMD figure when the two pages differ; on a balcony, count the weight of the full, wet bag (see balcony gardening).
• Fill with a potting mix that drains but holds moisture; moisten peat- or coir-based mix before filling (Maryland Grows). The soil calculator converts volumes into bags of mix.
• Check moisture daily in summer; expect to water more often than a plastic pot, possibly daily.
• Mulch the surface of large bags to slow evaporation (UMD).
• Feed according to the label, since frequent watering leaches nutrients (Oklahoma State).
• Potatoes: expect smaller harvests than in the ground; add mix as plants grow.
• After the season, empty, wash with soapy water, dry completely and store, so soil-borne problems do not carry over (Oklahoma State).