Vegetables That Don't Grow Well in Containers

Which vegetables aren't worth growing in a pot? University extension services keep naming the same short list: full-size corn, pumpkins, watermelons and other big vining melons and winter squash. Potatoes, sweet potatoes and asparagus come close behind. They all fail for one of two reasons. Either the plant needs more root room and more spread than a container holds, or, in corn's case, it needs neighbors.

Know this before you buy seed. A packet of standard pumpkin seed looks much like a packet of bush squash, and the difference shows only in midsummer, when one vine has left the pot behind. Rutgers Cooperative Extension says it plainly: "Certain vegetables, sweet corn and watermelons in particular, do not adapt to growing in containers."

A container limits two things at once. It limits the soil the roots can reach, and for sprawling crops it limits the room the top can use. The NC State Extension Gardener Handbook calls root restriction "a physical stress on the plant." It says that stress causes a pronounced decrease in both root and shoot growth and reduces flowering and fruiting. For a crop you grow for its fruit, that last effect costs you the harvest.

The soil in a pot is also the plant's entire supply of water and nutrients. A bigger plant uses that supply faster. The Royal Horticultural Society advises containers at least 18 inches deep and wide, "otherwise frequent watering and feeding will be needed," and warns that a lack of root space can cause wilting and signs of nutrient deficiency.

Yield follows root room. University of Maryland Extension notes that indeterminate tomatoes grow larger and produce more fruit as the container gets bigger. Its advice, from Jon Traunfeld, is short: "Don't squeeze large plants into small containers."

Corn has a second problem, and it has nothing to do with roots. Corn is pollinated by wind, so pollen has to drift from plant to plant. That works when many plants grow close together in a block, and it rarely works when you have only a few.

Corn. Plants grow 5 to 8 feet tall, which makes a pot top-heavy. Epic Gardening reports that one container plant yields only 1 to 2 ears. Harvest to Table works out that a 15-gallon container holds three standard plants and a 30-gallon container holds six. It also notes that good pollination generally calls for blocks of a dozen or more plants. None of these sources name a dwarf corn variety that solves the problem.

Pumpkins and vining winter squash. Epic Gardening puts pumpkin vines at 20 feet or more and suggests a 60-gallon grow bag as the minimum if you try at all. Bob Vila gives a minimum of 20 to 25 gallons, at least 4 feet of width, and no more than two plants per container. The workaround that NC State and Rutgers both recommend is a bush-type squash. Epic Gardening points to smaller pumpkin varieties. These sources don't name specific bush winter squash cultivars, so read the packet for the words "bush" or "compact."

Zucchini and other summer squash work as bush types. The sources disagree on pot size: NC State's Onslow County office says 3 gallons, Virginia Cooperative Extension says 5, and University of Maryland says 10. The larger figure is the safer bet.

Melons. Full-size vines need roughly 5 feet of spread, and Dave's Garden recommends at least 16 inches of soil depth. The same article recommends compact cultivars such as 'Minnesota Midget' and 'Sugar Cube' for pots. Bob Vila adds that melons can be trained up a trellis, with fabric slings to hold the fruit.

Sweet potatoes and potatoes. NC State's Onslow County Extension says sweet potatoes need a 25-gallon container or a wine cask to grow well. Potatoes need at least 12 inches of depth, and 18 to 24 inches is better. University of Maryland's guidance shows both crops succeeding in 5-gallon grow bags, so these two are hard rather than hopeless.

Asparagus. Epic Gardening reports that it produces only tiny spears in cramped containers. The brief for this piece also raised rhubarb, but none of the extension sources used here address it, so treat it with the same caution until a source says otherwise.

Full-size tomatoes. University of Maryland and NC State both give 5 gallons as the minimum, and NC State's handbook says 12 to 24 inches deep. That is the minimum, not the ideal, and indeterminate types keep producing more as the pot grows. Cherry tomatoes need far less: 1 gallon by Virginia's count and 2 by Maryland's. If you aren't sure which kind you have, the difference between determinate and indeterminate tomatoes decides how big a pot a plant can use.

The extension lists use two different words. University of Maryland calls melons, corn, potatoes and sweet potatoes "more challenging," which means possible with a big enough container and steady care. NC State and Rutgers say full-size corn, pumpkins and watermelons simply don't adapt to pots. The first group may reward effort. The second mostly won't.

A dwarf variety is also not a free substitute. Virginia Cooperative Extension notes that dwarf or miniature varieties "often mature and bear fruit early but do not produce as well overall as standard varieties." You get a harvest sooner, but you get a smaller one.

Read the packet before you look at the picture. NC State's handbook says the plants that do well in containers are labeled bush, dwarf or miniature, or are bred for container growing. A standard variety of a vining crop is the one to leave on the rack.

Then match the plant to a pot you actually own. The container size chart lists gallons and depth crop by crop. If the answer is 20 gallons or more, ask whether the space would serve you better as soil in the ground. Epic Gardening suggests compost-amended raised beds for squash, and a first raised bed gives corn its block and pumpkins their room.

Light is the last check. Rutgers says fruiting vegetables in containers need 8 hours of sun a day. A melon in a big pot on a shady patio fails for a reason the pot didn't cause.