Crate Expectations: Growing More Dahlia Seedlings in Less Space

Experienced dahlia growers are familiar with “sink pots.” These are dahlia seedlings—or less often, cuttings—grown in small plastic pots to the point of blooming, without ever being transplanted into open soil. The miniature tuber clumps these dahlias grow can be saved and regrown to produce genetic duplicates of the plant that produced them.

Most often, sink pots are used to grow large numbers of dahlia seedlings until they produce their first bloom. This allows the hybridizer to assess whether each plant possesses the bloom characteristics they’re seeking. If not, the seedling is culled (also referred to as “rogued”) and removed from further growing. If the bloom shows promise, the plant is allowed to mature in hopes of producing tubers that can be saved for future propagation.

In her book Dahlias: Seed to Bloom: The Dahlia Grower’s Companion, noted hybridizer Kristine Albrecht describes using 4-inch plastic pots placed in tight rows, with each pot literally touching the next. As each dahlia blooms, undesired plants can be easily removed, leaving more room for nearby seedlings.

When we think about methods for growing dahlias outside of traditional garden beds, we usually picture pots, raised beds, or grow bags. These allow gardeners to control the growing medium and can help avoid issues caused by poor native soil or excessive moisture.

Although seedlings can certainly be grown out in prepared beds, using compact 4–5 inch pots instead makes it possible to fit far more plants into a relatively small footprint. Successful dahlia hybridizing is a numbers game—the more seedlings that can be brought to the bloom stage, the greater the odds of identifying worthwhile varieties to carry forward for further evaluation.

When a dahlia is grown in a pot of any size—small or large—the size of its root system and resulting tuber clump is determined by how much space it has. Left to grow freely in the garden, a clump will typically reach 12 to 18 inches across, with some as large as two feet. To produce tubers of that size, each plant needs 18 to 24 inches of space—far more than most home gardeners can spare when they have many hundreds of dahlia seedlings they wish to grow and evaluate through the blooming stage.

In contrast, small-pot seedlings can be placed just 4 to 6 inches apart, allowing you to trial vastly more plants in the same space. Since the majority of seedlings are culled after their first bloom, this approach maximizes the number of trial plants without overcommitting space or resources.

A dahlia grown for a full season will produce a restricted tuber clump called a “pot tuber.” These can be stored over the winter and either divided into 2–3 viable sections or left intact. Each tuber, if it has at least one eye, will grow into a genetically identical plant the following season.

British dahlia expert Philip Damp noted in his 1981 book Growing Dahlias that pot tubers were becoming a preferred method for propagating and exchanging dahlias internationally. Kept compact by the confines of a small pot, they were, in his words, “the main exchange unit for both the trade (who exchange for cash) and the amateur who swaps his stock with friends in every corner of the dahlia-growing world.”

While the older term “pot roots” has faded, the idea behind them holds up. In the UK, where intact dahlia tuber clumps are still common—especially among those who take cuttings—pot tubers remain a practical way to generate early shoots from compact, manageable plants.

Virtually all of my in-ground growing space is devoted to dahlias planted directly in the soil, spaced 12 to 18 inches apart. I didn’t want to give up that prime real estate for seedling trials, so I began looking for alternatives. That’s when it struck me: the plastic bulb crates I rely on during digging season sit idle all summer. Why not put them to work holding seedling pots instead?

I could have sunk the pots directly into the ground, but this part of the garden was already paved with more than a ton of concrete pavers I’d installed years ago. Digging trenches just to bury pots would have meant dismantling all of that hardscaping—an enormous amount of work for a temporary setup. Instead, I adapted the crates for above-ground use.

This approach also makes sense for anyone working with gravel, patios, old driveways, or any space where digging isn't practical. Rather than viewing those areas as wasted, they can be turned into high-efficiency seedling trial beds with a little imagination.

To prepare the site, I laid down a thick weed barrier and positioned the crates on top. For a cleaner look—and to reduce reflected heat on sunny days—I added a generous layer of wood chip mulch, which I can easily remove if I want to repurpose the space later on.

Each crate holds eight 5.25-inch square pots with about three-quarters of a gallon of potting soil. The crates keep the pots snugly in place, even during wind or watering. I arranged forty crates in four rows of ten, which gives me room to grow 320 dahlias in just 130 square feet.

After some trial and error, I assembled an irrigation system using half-inch mainline tubing and quarter-inch drip lines with emitters spaced every six inches. A ten-minute run each morning keeps the soil evenly moist, with minimal runoff.

Since these pots aren’t sunk into the ground, the roots stay fully contained. That makes potting mix quality—and regular fertilization—especially important throughout the growing season.

I will admit that the way the drip tubing runs across the tops of the pots prevents me from easily removing individual plants. In a traditional row setup, I could rogue out a disappointing seedling with little disruption. Here, it would take too much effort to extract just one pot from the crate.

For that reason, I’ve decided to grow all the seedlings through the full season—even those that fall short of my goals. In a breeding context, even attractive blooms may lack the traits I'm looking for, and must ultimately be discarded. This year, I’ll tag promising plants, mark those that don’t make the cut, and let them all continue growing until fall.

Growing dahlia seedlings in large numbers (versus planting a few around the garden for color and enjoyment) is best suited to breeders and serious evaluators. I already owned the bulb crates, and I was fortunate that the square pots I bought—a 1,500-count pallet on Marketplace—fit almost perfectly inside them. My only cash outlay was for irrigation supplies, most of which I already had on hand.

This method isn’t for everyone, but it’s an efficient way to grow hundreds of seedlings in a compact area without sacrificing garden beds or installing permanent infrastructure. I’ll post an update later in the season as the plants grow and begin to reveal their first flowers. Like any good trial, Crate Expectations is built on a bit of hope, a bit of tinkering, and the thrill of not knowing exactly what’s going to emerge. Whether this system lives up to its name or simply teaches me something useful along the way, I’m glad I gave it a try.