What’s the Best Size Perennial Plant to Buy?

Most garden centers typically carry one or two sizes—often quarts and gallons—of any given perennial; mail-order nurseries usually offer different sizes at checkout too, including small plugs. Learning the pros and cons of different size perennial pots can help you plan better and plant smarter.

To understand sizes, it’s helpful to know a little about plant production. With perennials, young or bareroot plants are stepped up into larger sizes until they reach the final “finished” size that’s offered to the retail shopper. The starter plants are supplied by wholesale growers who specialize in producing starter plant material. Further down the supply chain the “finished growers” take the starter plants and grow them into the larger sizes destined for retail sales or the landscape.

In this article we’ll focus on the perennial sizes that are most likely to be available to an end consumer—essentially, what you find when you’re plant shopping. Those sizes fall into two categories: containerized plants and plugs.

In a garden center or other brick-and-mortar retail setting, perennials are usually containerized, meaning they’re in nursery pots. Plugs are actually the starter plants I mentioned above, once a wholesale product that was typically sold only to those in the nursery trade. But they’re becoming increasingly available through retail and online sources.

You know that the look and shape of nursery pots varies widely. They’re square, they’re round, they’re short and fat, tall and thin and everything in between.

The variety may seem chaotic, but there are standards for nursery stock that arrange container sizes into classes based on volume. To simplify communication, we use terms like “quart” and “gallon.” The size classes cover a range, so these terms aren’t true measures of volume (i.e., a quart-sized nursery pot isn’t necessarily 32 fluid ounces). However, they’re helpful shorthand that’s widespread in the industry.

Quarts and gallons make up the bulk of perennial availability and are most relevant for our discussion, so let’s look closer at those. (Less common are pints and other transitional sizes used for starter plants and groundcovers, along with two-gallon and larger containers primarily used for woody plants.)

A quart container usually runs four inches wide. They are used for perennials that are smaller at maturity. With greater soil volume than pints, they can accommodate perennials with more extensive root systems. Plants that come in quart containers are appropriate for direct planting in the landscape—in other words, “landscape ready.”

A gallon container usually sits six-and-a-half inches wide. This is the most common size for perennials. The larger soil volume accommodates species produced from bareroot material, like Astilbe and Dicentra, as well as species with larger root systems. Gallon perennials are also landscape ready.

· Best option for immediate visual impact (and impatient gardeners).

· Larger size = higher price, and the difference between a quart and a gallon is often a big jump.

· But perennial gallons are often large enough to divide into several transplants, reducing cost per plant. Related: "Dividing Perennial Plants: Why, When, How and What"

· The larger the container, the bigger the hole to dig and the more soil disturbance.

· Larger soil volume buffers stressful conditions. Smaller containers and plugs often establish more quickly, but they must be checked more frequently.

· The root structure is more developed in gallons, but they are also more likely to be rootbound. If a gallon is rootbound, best practice is to bareroot it (wash away soil and tease out roots) before planting.

· That said, check the root systems before you buy, especially in spring. Gallons potted up that same spring may not have filled out inside the pot yet, so you’re purchasing mostly potting soil and a plant with a small root system.

Related: "How to Get the Most from End-of-the-Season Sale Plants"

The increased availability of plugs is most evident in the realm of native plants, where ecological design and naturalistic planting styles call for planting densely. Plugs make those projects more affordable. Mail-order sources offering a wide range of native perennials in plugs include Prairie Nursery and Izel Plants.

Related: "The Matrix Planting Approach to Garden Design"

Plugs are young plants, usually less than a year old, with vigorous root systems. They’re grown in plug trays, which are continuous sheets of individual cells filled with potting soil—something like large seed-starting trays. The tray size is standard across the industry, about 11 by 22 inches; the plug size is usually referred to by the number of cells in a tray: 72s, 50s, 32s and so on. The larger the individual cells (plugs), the fewer in a tray. Thus, 32s (32 plugs per 11-by-22-inch tray) are larger than 50s (50 plugs per 11-by-22-inch tray).

Like containers, plugs come in different shapes and dimensions. Some are particularly deep relative to their height, allowing for a more extensive root system. Referred to as “deeps,” they produce plants better suited for placing directly in the ground than shallower plugs. Small plugs, like 72s, can be direct planted, but they’re difficult to manage. Let’s look at the sizes most likely to be offered as landscape ready.

The 50s are usually two inches wide. These are starter plants that a finished grower will usually step up to gallons. Regular 50s can be tricky to manage when planted directly in the landscape; deep 50s fare much better.

The 32s are a quarter of an inch wider than 50s. These represent the starter plants for gallons and larger container sizes. They, too, are landscape ready and a bit easier, with 32 deeps being even better.

· Lowest cost option; great for large projects.

· Easier to plant, especially in established landscapes or difficult soils. Small hole, less soil disturbance, no need to bareroot or disentangle roots.

· Plugs establish quickly. Why? Plants continue to photosynthesize even as the plug cell limits their vegetative growth. With excess energy stored in the roots, they’re ready to go when freed from the tray. Their ratio of surface area to volume is higher than it is for larger sizes, so plugs have more energy to spend relative to their size. Most perennial plugs will catch up to a quart in a season. Fast-growing perennials, including grasses, can catch a gallon in a single season.

· Plugs need consistent attention with watering and care, whether in the tray or after planting. Soil volume is smaller, with a lower margin for error under stressful cultural conditions.

· Can be difficult to tell small perennials from weeds; young plants can look quite different from a mature, containerized one.

· Easier for deer to pull out of the ground.

· Plugs are almost never sold individually. You are usually limited to purchasing a full tray of the same plant, but as demand has increased, some sources are offering mixed trays.

Images courtesy of Izel Plants