Alfalfa Cover Crop

Alfalfa poisons its own seedlings. UMN Extension states it plainly: autotoxicity occurs when a mature alfalfa plant excretes a chemical that inhibits the germination and growth of seeded alfalfa. Wisconsin Extension identifies medicarpin as the compound most strongly implicated - water-soluble, more concentrated in top growth than in roots, and released into the soil from both living plants and decomposing residue once a stand is killed, whether by tillage, herbicide or winterkill. The effect is not subtle at close range. A classic University of Missouri study sowed alfalfa seed in a wagon-wheel pattern around an established plant: within eight inches, new seedlings rarely survived; between eight and sixteen inches they established but yielded poorly; beyond sixteen inches there was no effect. That radius is why the instinct to thicken a thinning stand by drilling more alfalfa into it fails so reliably - seedlings germinate, look acceptable, then die out over summer. The trait probably evolved in the dry Mediterranean, where suppressing nearby seedlings reduced competition for scarce water. Note that the evidence is not unanimous: a 2002 Agronomy Journal study found no consistent autotoxicity in the reseeding year.

Triggers: UMN Extension: alfalfa autotoxicity occurs when a mature alfalfa plant excretes a chemical that inhibits the germination and growth of seeded alfalfa. Management guidelines: allow at least one year between terminating an old alfalfa stand and reseeding alfalfa; do not thicken thinning stands of alfalfa with alfalfa; rotate from alfalfa to a grass crop to use residual alfalfa nitrogen; delaying spring reseeding two to four weeks after tilling a killed stand is NOT an effective strategy to alleviate autotoxicity and remains risky to establishment and yield. Wisconsin Extension: several potentially autotoxic chemicals are produced, one of which is medicarpin, present throughout the plant but more concentrated in top growth than roots; it is water-soluble and leaches readily into the soil from both decomposing plant material and growing plants. Once a stand is killed - by plowing, spraying or winterkill - the autotoxic compounds are released. Persistence depends on soil type, temperature and rainfall: on sandy soils effects are more acute but less persistent because the toxins leach; high rainfall leaches them more rapidly. The toxins are not present in the first year of a new seeding, so a failed or winterkilled new seeding can be reseeded without an autotoxicity effect. A University of Missouri study found that within 8 inches of an established plant new seedlings rarely survived; from 8 to 16 inches they established but productivity was greatly diminished; beyond 16 inches there was no impact. NOTE ON EVIDENCE: the effect is real but its magnitude is disputed - a 2002 Agronomy Journal study of stand age, reseeding delay and cultivar found no consistent evidence for autotoxicity in the reseeding year.

On Alfalfa: New alfalfa sown into old alfalfa ground germinates poorly and forms weak roots

Prevention: Do not reseed directly into existing or recently terminated alfalfa

Risk fades when: There is nothing to apply and nothing to rescue - a seedling in the toxic zone dies, and the ones that survive yield less for the life of the stand. Plan around it. Allow at least one year between terminating an old alfalfa stand and reseeding alfalfa; UMN is explicit that delaying spring reseeding by two to four weeks after tilling does not work and simply costs yield. Sow the killed ground to another crop - a grass such as oats, corn or sorghum, which also uses the residual alfalfa nitrogen. Never interseed alfalfa into a thinning alfalfa stand: the zone of influence extends roughly sixteen inches from every established plant, so a stand thin enough to interseed safely would barely exist. Thicken thin spots with red clover or a grass instead. One useful exception: the toxins are absent in a first-year seeding, so a stand that failed or winterkilled in its establishment year can be reseeded to alfalfa without penalty.

Sclerotinia crown and stem rot on clovers, alfalfa, vetches, and related forage legumes. A cool, wet-season disease that can kill seedlings or wipe out patches after fall establishment, wet winters, or prolonged snow cover.

Triggers: Cool moist weather, fall seeding, wet soils, dense canopy, poor air circulation, and prolonged snow cover all favor disease.

Risk fades when: Drier conditions reduce active infection pressure, though stand damage may already be visible by spring.

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 Alfalfa: Slow canopy closure on weak seedings

Prevention: Prepare a clean firm seedbed and seed shallow at adequate density

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.