Yellow Sweet Blossom Clover Cover Crop

A seedling killer caused by several different fungi working together. It hits vegetables, flowers, herbs, microgreens, and cover-crop seedlings the same way — seeds rot before they emerge, or young seedlings collapse right at the soil line. Wet seed-starting mix and poor airflow in seedling trays are the classic conditions.

Triggers: Wet soil or starting mix, poor drainage, seedlings packed too tightly, contaminated trays or media, and stagnant air all favor damping-off.

Risk fades when: Drying the soil surface and improving airflow slows new spread. Collapsed seedlings don't recover, but the rest of the tray can be saved.

The most common leaf disease of alfalfa and clover cover crops. It makes small, dark brown to black circular spots with slightly fringed edges, often with a tiny raised fruiting disk in the center. Spots start on the lower leaves and move up, and infected leaves yellow and drop, thinning the stand. It is favored by cool, moist weather and is worst in spring and fall.

Triggers: The fungus overwinters in leaf debris and spreads by spores during cool, moist weather. Dry, warm summer weather slows it; cool, moist fall conditions favor a resurgence. A dense canopy that traps humidity raises risk.

Risk fades when: Dry, warm weather slows it. Two days above 82°F or three dry days exits the active window; spotted leaves stay damaged.

Mississippi State Extension states the case in one line: plant establishment is the most critical aspect in any planting. A cover crop that comes up thin never catches up, and every benefit you sowed it for - nitrogen, biomass, erosion control, weed suppression - scales with the stand you got. Almost all of it is decided in the first week. Seed must reach mineral soil and stay in contact with it. Iowa State is emphatic that seedbed firmness is very important, recommending a cultipacker or roller on tilled seedbeds before planting, then broadcasting onto that firm bed and rolling again for shallow coverage. Depth matters as much as contact: small seed buried too deep never emerges, and Iowa State caps forage seeding depth at half an inch. Broadcast seed sitting loose on a dry surface simply desiccates, which is why MSU says to raise broadcast seeding rates by 25 to 50 percent over drilled rates to compensate for poor seed-to-soil contact and uneven distribution. Then there is temperature. Warm-season species and mixes sown into cool soil sit and rot rather than germinate. And weeds: a crop that emerges into competition thins before it ever closes canopy.

Triggers: Mississippi State Extension: plant establishment is the most critical aspect in any planting; poor seed quality results in uneven stands, stunted growth and limited benefits, so use seed with a recent test date, high germination and pure-seed percentages, and few weed seeds. Increase broadcast seeding rates by 25 to 50 percent over drilled rates to compensate for poor seed-to-soil contact and poor uniformity of distribution; a cultipacker is recommended when broadcast seeding. Broadcasting is not recommended when planting multiple species, because different species require different seeding depths - this is the characteristic failure of a mixed-species stand. Legumes require the correct rhizobium inoculant to fix nitrogen, and inoculant plus fungicide treatment can account for a large fraction of bag weight, reducing actual pure live seed. Iowa State Extension: seedbed firmness is very important; use a cultipacker or roller on tilled seedbeds before planting, broadcast onto a firm tilled seedbed and cultipack or roll for shallow seed coverage and seed-to-soil contact; control seeding depth to no deeper than half an inch; control weeds before planting, since weeds compete with young seedlings for moisture and light. NC State Extension: drill seeding is often the most consistent method, providing improved uniformity and seed-to-soil contact; planting by the recommended dates is crucial for proper establishment, stand and biomass. NRCS defines a weed-free seedbed as not exceeding one weed seedling per square foot at planting. Warm-season species and mixes will not germinate or thrive in cold soil.

On Yellow Sweet Blossom Clover: Seed sown too deep, too dry, or into loose cloddy soil fails to establish evenly.

Prevention: Use shallow seeding and firm seedbeds with good moisture.

Risk fades when: A thin stand does not thicken. Once emergence is over, the options are to overseed into the gaps or to accept reduced benefit and terminate on schedule. Fix it at sowing instead. Firm the seedbed before you sow - roll or cultipack a tilled bed. Sow shallow, no deeper than half an inch for small seed, and press or roll the seed in afterward so it contacts mineral soil rather than sitting in loose dust or on top of residue. If broadcasting rather than drilling, raise the seeding rate by 25 to 50 percent. Wait for the right soil temperature: warm-season species and warm-season mixes sown into cool soil will not germinate. Sow into moisture, or irrigate after sowing. Control weeds before planting, not after. Inoculate legume seed with the correct rhizobium. And do not broadcast a multi-species mix if you can drill it, because the species need different depths and one of them will always be wrong.

A rust of alfalfa and clover cover crops that makes small reddish-brown pustules on both leaf surfaces and sometimes on stems. The pustules rupture and release powdery spores, and heavy infection yellows and drops leaves, weakening the stand. It is favored by humid weather and extended leaf wetness.

Triggers: Favored by humidity and extended leaf wetness. Reddish-brown pustules on both leaf surfaces release powdery spores spread by wind. Often associated with nearby spurge as an alternate host.

Risk fades when: Hot or dry weather slows rust. Two days above 88°F or three dry days exits the active window; existing pustules remain.

Self-seeding (or volunteer reseeding) happens when a plant is allowed to flower, set seed, and drop those seeds in the garden. Many cool-season crops like cilantro, dill, fennel, arugula, lettuce, and many flowers (calendula, cosmos, sunflower, poppy, nigella) reseed reliably year after year — often more reliably than deliberate sowing. Hybrid varieties typically don't reseed true-to-type — the seedlings may show traits of the parent lines rather than the labeled hybrid. Open-pollinated and heirloom varieties reseed faithfully. If you don't want volunteers, remove flower heads before seed sets. If you want them, leave seed heads on through fall and they'll sprout when conditions are right next season.

Triggers: Self-seeding follows any time a plant flowers, sets seed, and is left in place long enough for seed to mature and drop. Open-pollinated varieties reseed true; hybrid varieties may not.

On Yellow Sweet Blossom Clover: Second-year plants can reseed if left too long.

Prevention: Terminate before viable seed forms if volunteers are unwanted.

Risk fades when: Self-seeding isn't a problem per se — it's the plant doing what plants do. Manage by deciding whether you want volunteers next season.

The sweetclover weevil is a small gray-brown weevil specializing on sweet clover (Melilotus spp.), with secondary feeding on alfalfa and ladino clover. Adults notch leaf edges; larvae feed on roots and nitrogen-fixing nodules. Most damage occurs on cotyledon-stage seedlings — established stands in the Canadian prairies typically tolerate populations without significant yield loss.

Triggers: Diapause as adult overwinter. Cultivar selection matters — Melilotus alba less preferred than M. officinalis; wild M. infesta least preferred (Soroka & Muir). Nitrate fertilizer levels don't affect feeding.

On Yellow Sweet Blossom Clover: Weevil feeding can weaken stands in some areas.

Prevention: Rotate sites and monitor stands if sweetclover is grown repeatedly.

Risk fades when: Damage is concentrated on cotyledon-stage seedlings in spring; once plants grow past the seedling stage they outgrow the notching injury, so risk fades as the stand establishes, and adults move off to oversummer and overwinter in sheltered sites (Craig 1978).