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Sudex: Summer soil builder

June 1, 2021
A woman stands in front of a green field of sudex, which is almost 6 feet tall.
Sudex can grow very tall in agricultural settings.

Early June is a great time to consider planting summer cover crops. Lettuce rows or other early spring vegetable beds might be prime locations to sow these plants. Unproductive areas or a newly created vegetable bed could also benefit from a cover crop tune-up.

Sudex, a grass cover crop, is a fast-growing option that can build your soil while suppressing weeds over the season. But, unlike other types of cover crops, some extra work will be required to take full advantage of this plant species.

The term “sudex” refers to several different types of grass species and their crosses. They all belong to the Sorghum genus and look similar to corn.

Sudex is primarily bred as food for livestock. Today, Minnesota farmers often use sudex as an emergency source of food for their animals or plant them when springs are too wet for corn or soybeans.

Garden catalogs that sell cover crop seed will often have sudex listed as sudangrass or sorghum-sudangrass. Make sure you do not accidentally buy grain sorghum (aka milo or broom-corn), which is not well suited for cover crop use.

Sudex benefits

There are few things that can take off like sudex, given ample moisture and heat. Over the summer, the roots and leaves of sudex add an impressive amount of organic matter. Sudex’s root system is also good at breaking up hard, compacted soils.

As an annual plant, sudex is not better at fixing compaction than perennial crops like alfalfa. But if you do not have several years to take the area out of vegetable production, sudex can be the next best option.

Aside from healing soil, sudex can also be a powerful ally against weeds. Sudex can combat weeds in two different ways, by shading them out as they grow, and by sending out allelopathic chemicals from its leaves and roots. The term “allelopathic” means that it hurts plants—black walnut is probably the most famous example of an allelopathic species.

Sudex can interfere with the germination of small-seeded weeds, but be aware that these chemicals could hurt young transplants too. A 2009 University of California Davis study found broccoli and lettuce transplants mulched with fresh sudex were stunted and had lower yields. The scientists recommended waiting 6 to 8 weeks before planting anything into dead sudex. By then, the weather should have broken down the allelopathic chemicals.

A person is swinging a double-bladed weed cutter in a sudex patch, with mustard in the background.
A double-blade weed cutter can help manage sudex in gardens. Photo: Diane Bugeja

Planting sudex

Unlike other cover crop grasses such as rye or oats, sudex is a warm-season grass. This means the plant does most of its growth during the summer rather than the spring or fall. This makes early to mid-June a good time to plant sudex in most parts of Minnesota. At my family’s garden in Iowa, I usually plant sudex right after garlic. In Minnesota, planting sudex after garlic can be tricky, as garlic will likely be ready until the end of June to early July.

You can check when to sow sudex by selecting your state and county on the Midwest Cover Crop Decision Tool. Look for sudangrass or sorghum-sudangrass on the chart, as the term “sudex” is not used by the website.

While you can spread seed across the area (called broadcasting), I plant sudex in rows about 6 inches apart.

  • Seed between 1 and 1.5 inches deep.
  • In sandy areas, plant a bit deeper to 2 inches so that the seeds can be closer to moisture.
  • The spacing within the row does not matter as much but plan on using about one-half to a full pound of seed per 1,000 square feet.
  • For weedier plots aim at a higher seeding rate, especially if you decide to broadcast.

Managing sudex

Three rows of sudex grow in a sandy garden patch.
Sudex patch in late September: after two cuttings during the year these plants were still two feet tall.

Unlike most other cover crops, sudex needs to be periodically cut down to make it manageable. A good rule of thumb is to cut the plant down to about six inches once it gets 3 to 4 feet tall. Unchecked sudex can reach almost 12 feet tall in the right conditions, and be overwhelming for a gardener to tackle.

Depending on planting time and weather, cutting sudex may need to be done a few times a year. I used a double-blade weed cutter on my plot, but other tools such as a grass trimmer or scythe could also work well.

  • Maintain sudex from June to late fall and allow the winter to kill the plants.
  • To avoid any possible allelopathic effects on other plants, rake up the cut sudex during the summer and pile it up away from the garden or lay it between their rows.
  • Over a few months, the cut material should be OK to use as mulch.
  • To make things easier for spring planting and tillage, mow the sudex one last time, usually around fall frost.

Sudex does not have many serious pests or diseases of note. Still, remember that sudex is part of the grass family (Poaceae), so avoid rotating sudex areas into its relative, corn.

While I have observed Japanese beetle feeding damage on leaves, the damage was light and did not need any treatment aside from plopping beetles into a soapy bucket.

Is sudex right for your garden?

Overall, for poor soils that need some repair quickly, sudex is an attractive option. For most of the summer and fall, it helps add more organic matter while suppressing weeds.

While sudex does die over the winter, don't forget to maintain this cover crop. You need to have a plan to keep the height of sudex to a manageable level and deal with freshly cut residue.

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