What Size Grow Bag for Vegetables? Sizes and Dimensions

What Size Grow Bag for Vegetables?

Match the bag to the crop’s root system. Salad greens and small herbs need 1 to 3 gallons. Bush beans, carrots, beets and chard need 4 to 6 gallons. Tomatoes, peppers, eggplant and cucumbers need 8 to 10 gallons each, and potatoes need 10 to 15. Those ranges come from University of Maryland Extension’s container guidelines, which also give the depth of mix each group needs. The chart sets them beside the smallest volume the same source lists for each crop, so you can see how far a small bag can be pushed.

Crop	Bag size to buy	Depth of mix	Smallest that works
Leaf lettuce, spinach, salad greens, radishes, scallions	1 to 3 gallons	4 to 6 inches	1/2 gallon for leaf lettuce
Basil, cilantro	1 to 3 gallons	4 to 6 inches	Not listed
Bush beans, carrots, beets, Swiss chard	4 to 6 gallons	8 to 12 inches	2 gallons, 1/2 gallon for beets
Cabbage, broccoli and other cole crops	4 to 6 gallons	8 to 12 inches	5 gallons for cabbage
Peppers, eggplant, cucumbers	8 to 10 gallons	12 to 16 inches	5 gallons
Tomatoes	8 to 10 gallons	12 to 16 inches	5 gallons standard, 2 gallons cherry
Summer squash	10 gallons	8 to 12 inches	10 gallons
Winter squash	8 to 10 gallons	12 to 16 inches	Not listed
Potatoes	10 to 15 gallons	2 to 3 feet is ideal	5 gallons per plant

Read the last column as a floor. A pepper will live in 5 gallons, but Maryland Extension notes that small containers dry out faster than large ones. If you have the space and the potting mix, buy from the second column.

Grow Bag Sizes and Dimensions

Grow bags are sold by gallon, yet what decides whether a crop fits is the diameter and the filled depth. A bag is close to a cylinder, so you can check any listing with one calculation: multiply 3.14 by the radius squared and by the height, all in inches, then divide by 231, the number of cubic inches in a US gallon.

Diameter	Filled depth	Volume	Fits the depth for
8 inches	7 inches	1.5 gallons	Lettuce, spinach, basil
10 inches	8 inches	2.7 gallons	Salad greens, bush beans
12 inches	10 inches	4.9 gallons	Carrots, beets, chard, cabbage
14 inches	12 inches	8.0 gallons	One pepper, eggplant or tomato
16 inches	12 inches	10.4 gallons	One tomato or summer squash
16 inches	16 inches	13.9 gallons	Potatoes

The gallon printed on the label rarely matches this arithmetic exactly. One University of Maryland potato project uses bags sold as 12 gallons that measure 16 inches deep by 16 inches across, which works out to 13.9 gallons brimful. Bags are also filled to within an inch or so of the rim to leave room for water, and that lowers the working volume further. When two products carry the same gallon label, compare the inches.

Depth deserves a separate look for root crops. Carrots and beets fall in the group that needs 8 to 12 inches of mix, so a wide, shallow bag sold for salad greens will not do even when its gallon figure is large enough.

Best Grow Bag Size for Tomatoes

One standard tomato plant per 8 to 10 gallon bag is the target, with 12 to 16 inches of mix under it. Maryland’s crop table gives 5 gallons as the minimum for a standard tomato and 2 gallons for a cherry type. The gap between minimum and target matters most in July, when containers can need water daily or more often and the smallest ones dry first.

Variety choice changes the answer. Compact determinate tomatoes stay smaller and suit the low end of the range, and tall indeterminate vines belong at the top end or beyond. Install the cage or stake while the plant is young. The same sizing logic applies to rigid containers, which is covered in growing tomatoes in 5-gallon buckets.

Bag Sizes for Potatoes, Peppers and Greens

Potatoes. University of New Hampshire Extension advises 5 gallons of soil volume per plant and calls a container of 10 to 15 gallons, 2 to 3 feet tall, ideal. Height matters because you add mix as the stems grow. The Maryland project plants 3 to 5 seed potatoes in each 12 gallon bag and reports an average harvest of about 3 pounds per bag. Maryland’s general container page is far more generous and lists 30 gallons of potting mix for potatoes, so treat 10 gallons as the low end. The full method is in growing potatoes in grow bags.

Peppers, eggplant and cucumbers. Each belongs in the 8 to 10 gallon group, one plant per bag, with 5 gallons as the floor. Bush cucumber varieties such as ‘Salad Bush’ and ‘Spacemaster’, which University of Illinois Extension lists for containers, cope with the small end better than full-length vines.

Greens, herbs and roots. Leaf lettuce needs only half a gallon per plant, so one 3 gallon bag carries several plants. Basil and cilantro fit the same 1 to 3 gallon size with 4 to 6 inches of mix, and Maryland’s herb guidance says small herbs grow well in 2 to 5 gallon pots, so 2 or 3 gallons satisfies both. Rosemary, sage, parsley and other large herbs do better in 5 gallons or more.

What Makes a Good Grow Bag

Fabric bags are inexpensive, reusable, lightweight and portable, and the porous wall changes how roots grow. When a root tip reaches the fabric it meets open air and stops lengthening, and the plant branches new roots behind it. Maryland Extension calls this air pruning. It produces a denser, fibrous root system, while a root that hits the solid wall of a non-porous pot carries on as a single root.

A few features separate a bag that lasts from one that sags by August:

• Handles sewn into the wall. A filled 10 gallon bag holds about 1.3 cubic feet of wet mix, and that weight hangs from the handle stitching every time you move it.
• A size you can lift. Bags of 15 gallons and up are best filled where they will stay.
• Lighter color in hot sites. Maryland Extension warns that dark containers create higher temperatures that can injure tender young roots. Tan or gray fabric is the safer choice on a south-facing patio.
• A flat, wide base. A tomato in a cage catches wind, and a bag as wide as it is tall stays upright.

To work out how much potting mix to buy, divide the gallons by 7.5 to get cubic feet. A 10 gallon bag takes about 1.3 cubic feet, and three of them take about 4 cubic feet. Fill with a soilless potting mix of peat or coir, bark, perlite or vermiculite. Garden soil compacts in a container and can carry weed seeds and pathogens. More on blends is in potting mix for containers.

Watering and Feeding in Fabric

The porous wall that prunes roots also loses water. University of Illinois Extension notes that mix in fabric, clay and unglazed ceramic dries faster than in plastic or glazed pots, and that in warm, dry weather containers may need water daily or more often. Check every bag each day. Water when the top inch of mix feels dry, and keep going until water runs from the bottom so the whole root zone is wet.

Mulch on the surface reduces evaporation in containers as it does in beds. Size is the other lever, since smaller pots dry fastest, and that is the practical argument for choosing 10 gallons over 5 for a tomato. If your bags still dry out by mid-afternoon, container plants drying out fast lists the fixes, and how often to water container plants in summer gives a schedule by pot size.

A slow-release fertilizer mixed in at planting often supplies nutrients for 3 to 4 months. Long-season crops such as tomatoes, cucumbers, eggplant and peppers may also need a light liquid feed every two weeks to keep producing. Go lightly, because excess fertilizer builds up as salts in a container. Rates are covered in fertilizing container plants.

Quick recap

• Greens and small herbs: 1 to 3 gallons with 4 to 6 inches of mix.
• Bush beans, carrots, beets, chard and cole crops: 4 to 6 gallons with 8 to 12 inches of mix.
• Tomatoes, peppers, eggplant and cucumbers: 8 to 10 gallons with 12 to 16 inches of mix, one plant per bag.
• Potatoes: 10 to 15 gallons, at 5 gallons per plant.
• Check listed inches as well as gallons; a 16 by 12 inch bag holds about 10.4 gallons.
• Fabric dries faster than plastic, so check daily and water when the top inch is dry.