Most container failures trace back to a few measurable things. They are pot depth, hours of sun, and how often the pot is watered and fed.
The facts below come from university Extension services, the public gardening advice offices of state universities. Each one is worth checking against your own pots.
The numbers that decide it: pot size and sun, crop by crop
These two columns explain more failures than anything the gardener does later. They are the minimums published by Colorado State University Extension in its container vegetable guide, CMG GardenNotes #724.
Treat them as floors, not targets. A bigger pot holds more water and is more forgiving of a missed day.
| Crop | Smallest pot that works | Direct sun per day |
|---|---|---|
| Leaf lettuce | 8 inches deep | 6 hours |
| Chard | 8 inches deep | 6 hours |
| Green onions | 6 inches deep | 8 hours |
| Beets | 8 inches deep | 8 hours |
| Carrots | 12 inches deep | 8 hours |
| Bush beans | 8 inches deep | Full sun |
| Cucumbers | 8 inches deep, 3 gallons or more | Full sun |
| Peppers | 8 inches deep, 2 to 5 gallons | Full sun |
| Eggplant | 8 inches deep, 4 to 5 gallons | Full sun |
| Summer squash | 8 inches deep, 5 gallons | Full sun |
| Broccoli, cabbage, kale | 10 inches deep, 5 gallons | 8 hours |
| Tomatoes | 12 inches deep, 2 to 5 gallons | Full sun |
Minimum container size and daily sunlight, from CMG GardenNotes #724, Colorado State University Extension.
The one to read twice: a tomato needs a pot 12 inches deep. That is the single most common reason a beginner’s tomato sulks all summer in a pot that looked big enough in the shop.
For a longer crop-by-crop list, see best vegetables to grow in containers.
Containers Dry Out Faster Than Garden Beds
Containers need much more frequent watering than garden beds. How often depends on the size and type of pot, the mix, the plant and the weather.
The range is wide. Small pots in a hot wind may need a drink once or twice daily. In cool weather, a large pot gets by on once or twice a week.
Check pots daily in summer: water when the potting mix is dry 1 to 2 inches down. The daily finger check takes a few seconds and beats any schedule.
Use Potting Mix, Not Garden Soil
A pot only has the mix you put in it. Use packaged potting soil, which is lightweight and drains well, because topsoil can be too heavy.
Heavy soil holds water and leaves roots short of air. That is the usual route to rot in a pot.
Fill pots with bagged potting mix, never soil dug from the ground. The potting mix guide covers what to look for in a bag.
Containers Run Out of Nutrients Faster
Iowa State says plants in containers need regular fertilizer. Nutrient levels in potting mix decline quickly, because plants take them up and watering washes them out.
Many bagged mixes include slow-release fertilizer. Iowa State notes it usually does not last the whole growing season, so plan to feed again by mid-season.
Follow the rate on the product label, since more fertilizer does not mean more growth. Organic and synthetic types both work when applied as directed.
Cool-Season Crops and Fall Pots
Colorado State lists radishes, lettuce, spinach and beets as cool-season crops, grown in spring and fall rather than in summer heat. That suits a small space, where you can run a quick crop and then replant.
Counting back from your last frost date tells you when to sow. The seed-starting timing guide walks through it.
For fall color after summer flowers fade, ornamental cabbage is a good fit. It is a hardy annual about 12 to 15 inches tall.
Plant cool-season crops in spring and fall, and keep summer pots for tomatoes, peppers and herbs.
Leave Room Between Plants
Crowding costs quality. Colorado State says to thin lettuce to 9 inches apart, because competition for space, water and nutrients lowers quality.
Air movement matters too. For cucumbers and melons, Colorado State lists good air circulation as the way to minimize powdery mildew.
Give each plant its own pot or its own spacing, and do not pack a pot to the rim.
For first-year costs and what beginners can expect, see the container gardening statistics guide.
