Killing an established alfalfa stand requires either cutting the roots below the crown with tillage, applying the right herbicide at the right time, or combining both. Alfalfa is a deep-rooted perennial that stores energy in its taproot and will regrow aggressively if you don’t fully sever or poison that root system. The method you choose depends on your equipment, whether you’re growing Roundup Ready varieties, and what you plan to plant next.
Why Alfalfa Is Hard to Kill
Alfalfa survives through a thick taproot that can extend several feet into the soil. Even after cutting, the crown (the woody knob where stems meet the root) pushes out new growth using energy reserves stored underground. A single pass with a disk or field cultivator often leaves enough crown tissue intact for the plant to bounce back within weeks. Successful termination means either destroying that crown completely or using a systemic herbicide that travels down through the plant and kills the root.
Tillage: The Traditional Approach
A moldboard plow or a chisel plow fitted with overlapping sweeps will completely cut alfalfa roots below the crown and kill the stand without any herbicide. The key is full root severance. Equipment that doesn’t overlap its cutting path will leave strips of intact crowns, and those plants will regrow.
Moldboard plowing inverts the soil and buries the crowns deep enough that regrowth is virtually impossible. A chisel plow can do the same job as long as the sweeps overlap so no roots pass between them untouched. If you’re confident your tillage cuts every root, you can skip herbicides entirely. One pass is usually enough with a moldboard plow, though heavier or older stands may benefit from a second pass with a chisel plow to break up any remaining clumps.
Herbicide Termination
For no-till systems or fields where you want to minimize soil disturbance, herbicides are the primary tool. The most reliable chemical option is a combination of 2,4-D and dicamba, which consistently provides over 90% control across university trials. You need a total application rate of at least 1.0 pound of active ingredient per acre, which typically works out to about 2 pints per acre of a combined product. A common recipe is 1 pint per acre of 2,4-D LV ester plus 1 pint per acre of dicamba.
Products containing clopyralid (sold under names like Stinger or Hornet) also effectively kill alfalfa. These are worth considering if you have specific crop rotation plans that favor their residual activity profile.
Roundup Ready Alfalfa
If your stand is a glyphosate-tolerant variety, spraying glyphosate alone won’t work. The plant was bred to survive it. You’ll need to use the 2,4-D plus dicamba combination or a clopyralid-based product instead. The rates and timing are the same as for conventional alfalfa. Tillage remains fully effective against Roundup Ready varieties since the genetic modification has no effect on mechanical root destruction.
Fall Application Beats Spring
Timing matters more than most people expect. Fall herbicide applications are significantly more effective than spring applications. In the fall, alfalfa is actively pulling nutrients and energy down into its roots for winter storage, which means a systemic herbicide applied to the leaves gets carried deep into the root system along with those nutrients.
Apply herbicides when daytime temperatures are still above 50°F and the alfalfa has 4 to 6 inches of regrowth. This gives the plant enough leaf area to absorb the chemical. In northern states, early to mid-October is the typical window. Two applications spaced about two weeks apart can improve results: for example, a first spray in early October when the alfalfa is 4 to 6 inches tall, followed by a second application two weeks later when regrowth reaches 5 to 7 inches.
If you wait until spring, the plant is breaking dormancy and pushing energy outward from the roots to the new shoots. Herbicide uptake is less efficient because translocation is moving in the wrong direction. Spring applications can still work, but expect more survivors and potential regrowth into your next crop.
Temperature Thresholds for Spraying
Most postemergence herbicides work best when air temperatures are between 65°F and 85°F. Below 60°F, plants metabolize slowly and herbicide movement through the plant drops off considerably. Weeds and perennials sprayed in cool weather may eventually die, but the process is slow and less reliable.
If nights are dipping below freezing but daytime temperatures warm to at least 60°F, you can still spray during the warm part of the day. Wait until the plants have been in active growth for a few hours before applying. The critical factor is that the plant’s internal systems are moving fluids when the herbicide hits the leaf surface.
Non-Chemical Options
For organic operations or small acreage where tillage isn’t practical, silage tarps (heavy black plastic sheeting) can smother alfalfa by blocking all sunlight. The warm, moist environment under the tarp initially triggers growth, but the plants exhaust their root reserves trying to reach light that isn’t there. This approach works well for annual weeds in as little as two weeks, but alfalfa’s deep taproot means you should plan on tarping for a longer period, potentially several weeks to a couple of months depending on the time of year and how established the stand is.
Intensive grazing or repeated very close mowing can also weaken alfalfa over time. Each time the plant regrows from the crown, it draws down root reserves. If you cut or graze before it can replenish those reserves (roughly every 10 to 14 days during active growth), you can eventually starve the roots. This isn’t a one-pass solution. It takes multiple cycles over a growing season and works best combined with other methods.
What to Know Before Replanting
Alfalfa releases compounds into the soil that inhibit the germination and growth of new alfalfa seedlings. This autotoxicity effect is well documented, though researchers still haven’t pinpointed the exact chemical responsible. If you’re rotating to corn, soybeans, or another crop, this isn’t a concern. But if you’re reseeding alfalfa into the same field, timing matters.
The safest approach is to wait at least one full year before planting alfalfa again. If that’s not practical, the minimum wait is two weeks after tillage-based termination or three to four weeks after herbicide-only termination with no-till reseeding. Research from Purdue University found that seedling establishment and forage yields were unaffected by autotoxicity when replanting was delayed at least 21 days after the old stand was destroyed. With tillage, successful replanting was possible as early as 14 days, likely because tillage breaks up the root tissue and speeds the breakdown of the toxic compounds.
For rotation to corn, fall termination of alfalfa is especially valuable. It locks in the nitrogen credits from the alfalfa root system, gives the soil time to settle over winter, and eliminates any competition for moisture in the spring when the corn crop needs it most.