Choosing Pot Sizes for Annuals and Perennials

It’s fairly obvious that a large plant needs a large pot. However, we often buy our plants when they are young and small. That's why knowing the mature plant size is important when selecting a pot.

For annuals, it’s pretty straightforward because a petunia, pansy, or pepper plant will reach its full size by the end of the summer. Find that information on the plant tag or look up the variety online. Note that cultivars vary greatly in size. A Tidal Wave petunia reaches up to 2 feet in height and spreads 3 feet or more, whereas a Picobella petunia remains compact, reaching not more than 10 inches in height and width by the end of the summer.

If the tag doesn’t provide a number, it often provides recommended spacings that can serve as a guide. While plant spacing recommendations are normally for planting in the ground in traditional beds, they serve as a useful proxy for mature plant size. 

For perennial plants, the pot size should be based on the size the plant will reach in the coming year or growing season. A houseplant, fig tree, or rosemary plant takes several years to reach its mature size, and it will need to be repotted a few times as it grows. Planting a baby fig (or any other perennial plant) in a pot that's sized for the full-grown tree leads to problems like overwatering. Instead, choose a planter that's about one-third larger than the current pot and upsize as the plant grows.

Why Container Size Matters

There are two main ways that container size impacts the plants you want to grow in them. One is the volume of water they can hold for roots, and the other is stabilizing the plant so it can stand up to strong winds.

Water availability: The size of a container determines how much soil it fits, and the amount of soil is directly linked to the amount of water available to the plant. The more soil there is in the pot, the larger volume of water can be stored in the pot for the plant’s use. As a result, the pot size you select for your plants also impacts how often you need to water them.

Let's assume that a mature tomato plant uses 3 quarts of water per day, and it's planted in a large container that has enough soil to hold 1 gallon of plant-available water. You’ll need to water that tomato daily, and maybe skip a day now and then in cloudy weather. However, if the tomato is in a smaller pot with less soil that only holds 2 quarts of plant-available water, you’ll need to water the tomato plant twice a day.

Stability: Although big planters are heavy and they are hard to move, that isn’t always a bad thing for patio plants. Summer thunderstorms and winds easily topple large plants in small containers, breaking off plant parts and making a mess.

When planted in the ground, a plant sends roots out far and wide to provide structural support, but that doesn’t happen in a pot. For outdoor plants, it's crucial to choose a container that holds enough potting mix to anchor the plant and keep it from blowing over.

Problems with Improperly Sized Containers

Using containers that are too big or too small for the plants you want to grow in them are both problematic for different reasons.

Containers That Are Too Small

A container that is too small doesn't have adequate space for roots to grow. A plant only grows as large as its roots will support so a too-small pot can stunt the whole plant. Plus, the more limited potting mix volume holds less nutrients and water, which also limits growth. So that tomato plant in a 6-inch pot will not reach its full potential and you won't get as much fruit from it.

Plants need an appropriately sized pot for two main reasons. The first is practical. Large containers and the potting mix to fill them are expensive, heavy, and take up a lot of space. Planting a couple of French marigolds in a 20-inch diameter pot is overkill.

The second reason relates to soil moisture. An oversized pot holds more moisture than the plant can use, and the soil may not dry down adequately between waterings, leaving the plants more susceptible to root rot. It is important that the soil goes through periodic times of partial dryness because that makes it less hospitable to the fungal organisms that cause root rots, as fungi love continuously damp conditions. That's one of the reasons why watering instructions call for letting the soil dry out between waterings.

Ideally, the correct pot size provides room for roots to grow, and enough soil moisture to avoid twice-daily waterings, but still dries down adequately.

Pot Size Recommendations

Pot size recommendations for plants are given in gallons, but often, that’s not the measurement on the label when you purchase containers. Those measurements may be given in inches or quarts. To calculate how much soil you will need for a pot, you need three dimensions: the diameter at the top, the diameter at the bottom. and the height of the pot. Look for an online potting soil calculator.

As a familiar reference point, a typical round, 5-gallon pot is about 12 inches in diameter across the top, and about 14 to 15 inches tall. For large plants like patio tomatoes, a 14-inch-diameter pot is the minimum size; a 20-inch-wide pot is better.

Guidelines for Pot Diameters

• 12-inch pots are best for 3 to 4 smaller plants or one medium plant.
• 14- to 16-inch pots fit 4 to 6 smaller plants, 2 to 3 medium plants, or one large plant, such as a patio tomato.
• 18- to 20-inch pots support 3 to 4 medium plants or 1 to 2 larger plants.

Guidelines for Pot Depth

Container depth is equally as important as diameter, particularly for vegetables with deeper root systems. Use the general guide below regarding suitable depths for growing vegetables and larger ornamentals in containers. 

• 6- to 12-inch deep pots are for small, quick-growing crops like lettuces, round radishes, spinach, and smaller flowers.
• 12- to 18-inch deep pots work for larger vegetables, such as kale, root vegetables such as carrots, peppers, and most annual flowers.
• 18- to 24-inch deep pots should be saved for big plants like tomatoes, summer squash, and large ornamental plants.