Armed with a Japanese weeding knife and my favorite trowel, I’m currently shopping in my own garden, as I do each spring, carefully scouting for the raw material of more cohesive, eye-catching beds and borders — all at no charge.

I’m after what I call self-sowns and overgrowns: small seedlings that a previous generation of columbine or cosmos spawned, plus established perennials bulked up enough to yield divisions. As much as I enjoy nursery shopping, adopting some new impulse-buy plant will just add one more note to the mix.

The plants we multiply, instead, can become our garden signatures, creating a tighter palette we can echo in multiple spots.

Volunteer plants are no-budget unifiers, and the garden is generous and spontaneous when producing them. Anyone who has grown dill to the seed-head stage, perhaps for pickling, knows about next year’s self-sowns.

But nature rarely plants volunteers in the right spot; that’s our job. The cracks between paving stones or driveway gravel are favorite propagation ranges. Or an entire colony will erupt in one spot along a bed edge, as if every seed from a poppy pod successfully germinated in six square inches. So we pop these out, moving some and composting others — we edit, making pleasing pictures.

Putting the Volunteers to Work

It’s like a treasure hunt, equipped with an empty nursery flat, the plastic saucer meant to fit beneath a large pot or even a discarded baking sheet from the kitchen to hold what you find. Restraint is necessary, though, because who can accommodate the hundreds of nicotiana seedlings that surface if you grow flowering tobacco long enough and don’t remove every seed head in the fall?

You also need to learn identify who’s who when plants are young, which can be confusing. A desired self-sower, celandine poppy (Stylophorum diphyllum), is a near look-alike for a nasty Eurasian weed, greater celandine (Chelidonium majus), something I definitely don’t want to move artfully around.

Gradually, you learn: Nicotiana gets its start as tiny round leaves of bright green, growing flat to the ground. Poppy leaves are blue-green ruffles; larkspur’s are fine and hairlike.

What you grew last year that will sow itself, whether annuals, biennials or perennials, will depend on your region and soil conditions — and also how you clean up this spring, and how you did last fall. Deadheading or yanking every seed-laden thing will mean your volunteers may sprout only from the compost heap, if at all.

How to Manage Them

One strategy for managing volunteers: Remove seed-laden, faded plants closest to any cracks and crevices, said Louis Bauer, the director of horticulture at Wave Hill in New York City, where the gardeners edit many self-sowers — including larkspur, poppies and foxglove — to dramatic effect.

But in places where volunteering would be welcome, he told me recently, be sure not to clean up too roughly or to mulch beneath the parent plant. Leave strategic pockets of open ground for volunteers to sow into.

“Because some of these need a little space and light,” Mr. Bauer said, “in the fall, we do a little extra cleanup, so that there is bare ground, and don’t load it up with mulch. And if you’re careful to do that, they’ll come up in the middle of the bed, where you want them. Then you really do just edit out the extra ones in spring.”

Some of Wave Hill’s more zealous volunteers that need sterner management include purple-leaved Perilla (shiso), Atriplex hortensis (red orach) and Nigella (love-in-a-mist). In my garden, dill and nicotiana take top honors for excess.

Except for the biennial foxglove, all of those are annuals — plants that arise not from their roots the following year (as perennials and biennials do) but deposit some of their seed to sprout when conditions are favorable. Other annuals that self-sow are Verbena bonariensis (tall verbena), nicotiana, calendula and various poppies (including the breadseed or opium poppy, Papaver somniferum). Cleome (spider flowers) and cosmos can be prolific, too.

Tuck your finds into gaps in between perennials, or into new spaces on their own.

The Real Prize

Plants that behave as biennials — sending up only foliage their first year, then flowering the second — are the real prize, as even if you bought a packet of seeds you would have to wait until the second year for a show, and one flowering-size plant could cost upward of $25. No, thank you.

Angelica gigas, with big, wine-colored umbels in late summer following arresting, plum-sized buds, is one. Scoop up three or five to position on one side of a path and five or seven to stagger on the other side.

Perennials that reliably volunteer in my Northeastern garden include Pulmonaria, European wild ginger (Asarum europaeum), various Corydalis, dwarf goatsbeard (Aruncus aethusifolius), certain spurges (Euphorbia), hellebores and even the small-stature woodland peonies that go by the botanical name Paeonia obovata or Paeonia japonica.

Each of us will have a different palette, sometimes varying from spring to spring. At Wave Hill this year, Mr. Bauer said, Great Solomon’s seal (Polygonatum biflorum var. commutatum) and tall garden phlox (Phlox paniculata) are growing madly.

A few of those, and many others, will size up in time to offer up divisions. The Pulmonaria and gingers are easy to tease apart, but with the phlox or a hosta, another kitchen discard — an old serrated bread knife — is indispensable for cutting apart dense, unearthed root masses. Even little treasures like trillium are possible targets.

To make more, I dig an occasional good-size clump when in flower and separate the knobby underground rhizomes, maybe using that knife again, before transplanting the divisions.

A Word on Timing

Time your backyard shopping for overcast days, ideally before rain. Never move seedlings or divide perennials when it’s dry. If the weather doesn’t cooperate, soak the transplants-to-be the day before, and again once in their new spots.

The process with seedlings is simply to pry them out of moist soil gently, to get as much of the root system as possible. Many come out bare-root, but don’t panic. Just get them into their new spots promptly.

And while you’re out there: Don’t turn a blind eye to emerging weeds. Pull them out before they get a foothold. They slip out of the ground as easily in moist soil as the new, desired residents will slip right in.

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