The two reference scales
University of Maryland Extension (UMD) sorts crops into size classes by minimum volume of growing media and depth. UW–Madison Extension (UW) uses two simpler classes. Their figures overlap but are not identical, mostly because UMD's "large" class assumes one plant per container.
| Class | UMD minimum | UW–Madison minimum |
|---|---|---|
| Small crops (greens, radish, herbs) | 1–3 gal, 4–6 in deep | At least 2 gal, 4–6 in deep |
| Medium crops (dwarf fruiting crops, beans, roots, cole crops) | 4–6 gal, 8–12 in deep | — |
| Large crops (tomato, pepper, eggplant, cucumber, squash) | 8–10 gal, 12–16 in deep, one plant per container | At least 5 gal, 12–18 in deep |
Crop-by-crop table
| Crop | UMD class | UW–Madison | Notes |
|---|---|---|---|
| Tomato (full size) | Large: 8–10 gal, 12–16 in | 5 gal, 12–18 in | Clemson: 14–20 in container or a drilled 5-gal bucket for compact cultivars |
| Tomato (dwarf) | Medium: 4–6 gal, 8–12 in | — | See tomatoes in containers |
| Pepper | Large (dwarf types: medium) | 5 gal, 12–18 in | |
| Eggplant | Large (dwarf types: medium) | 5 gal, 12–18 in | |
| Cucumber | Large: 8–10 gal | 5 gal, 12–18 in | Trellis to save space (Virginia Extension) |
| Winter squash | Large: 8–10 gal | 5 gal, 12–18 in | Heavy fruit may need cages or slings |
| Summer squash | Medium: 4–6 gal | 5 gal, 12–18 in | |
| Broccoli | Medium (cole crops) | 5 gal, 12–18 in | |
| Cabbage | Medium: 4–6 gal, 8–12 in | — | |
| Bush beans | Medium or small (listed in both) | 5 gal, 12–18 in | |
| Beets | Medium: 4–6 gal, 8–12 in | — | |
| Carrots | Medium: 4–6 gal, 8–12 in | — | NC State Extension Gardener Handbook: at least 10 in deep (10–12 in of mix preferred), plants 2–3 in apart |
| Swiss chard | Medium or small (listed in both) | — | |
| Peas | Small: 1–3 gal, 4–6 in | 2 gal, 4–6 in | |
| Leaf lettuce, salad greens | Small: 1–3 gal, 4–6 in | 2 gal, 4–6 in | |
| Spinach | Small: 1–3 gal, 4–6 in | 2 gal, 4–6 in | |
| Kale | Medium (as a cole crop; kale not named): 4–6 gal, 8–12 in | — | NC State Extension Gardener Handbook: at least 3 gal, 8 in deep, plants 6 in apart |
| Asian and mustard greens | Small: 1–3 gal, 4–6 in | — | |
| Radish | Small: 1–3 gal, 4–6 in | 2 gal, 4–6 in | |
| Scallions / green onions | Small: 1–3 gal, 4–6 in | 2 gal, 4–6 in | |
| Basil, cilantro, thyme, mint, marjoram, garlic | Small: 1–3 gal, 4–6 in | Cilantro: 2 gal | |
| Rosemary, parsley, lavender, fennel | Medium: 4–6 gal, 8–12 in | — | UMD calls these "larger herbs" |
| Potatoes | 30 gal of potting mix | — | |
| Strawberries | 8 in deep | — | Volume not stated |
| Dwarf fruit trees | 25–30 gal of potting mix | — |
- Small: 1–3 gal, 4–6 in deep (greens, radish, herbs)
- Medium: 4–6 gal, 8–12 in deep (roots, beans, cole crops)
- Large: 8–10 gal, 12–16 in deep (tomato, pepper, squash)
- Potatoes: about 30 gal
Choosing the container itself
- Drainage. UMD: every container needs drainage holes or slits unless it is a self-watering type. UW–Madison advises filling the whole container with mix and not adding rocks or pot shards at the bottom, because layering slows drainage.
- Color and heat. UMD warns that dark containers can get hot enough to injure young roots. NMSU's Southwest Yard & Garden suggests light-colored or shaded pots to reduce soil heating and evaporation.
- Material. Porous clay, ceramic, concrete and wood dry out faster than plastic or metal (UMD). Plastics not made for outdoor use can turn brittle. Fabric grow bags keep roots cooler than black plastic but need more water.
- Reused containers. UMD lists 5-gallon buckets, half whiskey barrels, wooden crates, nursery pots, kids' wading pools and storage bins among workable options.
- A child's project. Nebraska Extension describes miniature gardens as small container plantings decorated to suit a child's interests; any container works as long as it has drainage holes. See miniature gardens with kids.
- Bigger is easier. Clemson notes that containers that are too small "create the need for constant watering", and NMSU recommends moving wilting container tomatoes into a larger volume of soil.
Soil, sun and water
UW–Madison recommends a light, organic-rich, well-drained commercial potting mix (pasteurized when possible), a slow-release fertilizer if the mix has none, at least six hours of direct sun, and daily watering once plants are large and fruiting. How to check the mix and how much to apply are covered in watering containers. For converting pot volume into bags of mix, use the soil calculator: one cubic foot is about 7.5 gallons. Growing on a balcony adds weight, wind and railing-box questions; see the balcony gardening guide.
Sources
- University of Maryland Extension — Types of Containers for Growing Vegetables
- University of Wisconsin–Madison Extension — Growing Vegetables in Containers
- Clemson Cooperative Extension Home & Garden Information Center — Tomatoes in Containers
- New Mexico State University, Southwest Yard & Garden — Blossom end rot in container grown tomatoes
- Virginia Cooperative Extension — Vertical Gardening Using Trellises, Stakes, and Cages (HORT-189NP)
- NC State Extension Gardener Handbook — Chapter 18, Plants Grown in Containers (Table 18-1)
- Nebraska Extension in Lancaster County — Miniature Gardens: A fun way to get your child interested in gardening
All sources were opened and checked in October 2026. Rules and figures change; confirm with the original before acting.
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- Growing tomatoes in containers
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