What is yellow sweetclover?

Yellow sweetclover identification is easiest in summer, when the plants are covered in yellow spikes and the air around them smells like fresh hay. It’s a biennial legume from Europe and Asia that was planted for years as livestock forage, soil-building cover crop and honey plant. It escaped and now grows wild along roads and in open ground across Pennsylvania.

It isn’t on the state noxious weed list, and it does feed bees. But in prairies, meadows, pastures and reclaimed land it can grow into dense stands that crowd out other plants, and its seed lasts a very long time in the soil.

Like other legumes, it partners with soil bacteria to pull nitrogen from the air, which lets it thrive on thin, poor ground where other plants struggle. That’s a big part of why it shows up on gravel shoulders and fill banks long before grass does.

Yellow sweetclover identification: what to look for

In its first year, sweetclover is a short, bushy plant you might not notice. In its second year it shoots up, blooms and dies after setting seed.

• Leaves: three narrow oval leaflets with fine teeth all the way around. The middle leaflet sits on a short stalk.
• Height: second-year plants are usually 2 to 6 feet tall, with many branches.
• Flowers: small yellow pea-shaped flowers packed along long, slender spikes from late spring through summer.
• Smell: crushed leaves and flowers smell sweet, like fresh-cut hay. That comes from a natural compound called coumarin.
• Root: a deep, thick white taproot.

White sweetclover is a close relative with white flowers and the same habits. It’s handled the same way.

First-year plants can be tricky. They look like a small, leafy, branching clover a foot or so tall, with no flowers. Crush a leaf and check for the sweet smell, and look for the fine teeth all around the leaflet edge.

Where it grows in Pennsylvania

Sweetclover loves dry, open, disturbed ground. In central PA you’ll find it on roadsides and railroad embankments, gravel lots, reclaimed strip-mine sites, old fields and pasture edges. It tolerates poor soils and does well where the ground is limestone-based or close to neutral.

It’s less common in maintained lawns, because regular mowing keeps it from flowering. The places it takes over are the ones nobody mows often: banks, back lots, construction fill and field margins.

Wind doesn’t move its seed far, but mowers, road graders, fill dirt and hay do. A stand along a township road can follow the shoulder for miles, and a load of contaminated fill can start a new one in a backyard.

Yellow sweetclover look-alikes

The plant it’s most often confused with is alfalfa. Alfalfa’s leaflets have teeth only near the tip and its flowers are purple in short, rounded clusters. Sweetclover’s leaflets are toothed all around and its yellow flowers run in long spikes.

Black medic is another yellow-flowered legume, but it stays low in lawns with small round flower clusters. Bird’s-foot trefoil has five leaflets per leaf and bigger flowers in small fans.

Crown vetch also lines PA roadsides, but it’s a sprawling vine-like plant with many small leaflets per leaf and pink-and-white flower clusters, so it’s easy to separate once you look closely.

How to get rid of yellow sweetclover

Sweetclover lives two years and spreads only by seed, so the key is simple: kill plants before they flower, and stay at it, since the seed bank can last for decades.

• Pull or dig: first-year plants and young second-year plants pull out best from moist soil in spring or fall. Get the top of the taproot, or it may resprout.
• Cut low: cut or mow second-year plants close to the ground before they bloom. Cutting too high leaves buds on the stem that can sprout and flower again, so check back a few weeks later.
• Bag flowering plants: if plants are already flowering or setting seed, cut them and bag the tops.
• Plant competition: reseed bare banks and field edges with grass or native mixes so seedlings have less room.

Herbicides: broadleaf products work best on rosettes and young plants in spring or fall, before bolting. Extension programs commonly list ingredients such as 2,4-D, dicamba, triclopyr and clopyralid for sweetclover in pastures and rough areas, with choices depending on the site. Read and follow the label, since the label is the law, and check grazing and haying restrictions. Applying herbicide for hire in PA requires a PA Department of Agriculture pesticide applicator certification, so larger fields or roadsides go to a licensed vegetation-management company.

Keeping it from coming back

Expect new seedlings for years, especially after any soil disturbance brings buried seed to the surface. Check cleared areas every spring and fall and pull young plants while they’re easy.

If you cut sweetclover for hay, cure it thoroughly. Moldy sweetclover hay can produce dicoumarol, a blood thinner that causes dangerous bleeding in cattle, so talk to your vet or extension office before feeding it.

On a small property, a few minutes of pulling in spring for two or three years usually wins. On larger ground, a steady mowing schedule plus thick grass is what keeps it in check.

Questions we get about yellow sweetclover

Similar weeds

Black MedicMedicago lupulinaBird's-foot TrefoilLotus corniculatusCrown VetchSecurigera varia