Crimson clover (Trifolium incarnatum )is a cool-season annual broadleaf legume with an upright growth habit adapted to uplands (mountains, piedmont) and coastal plain regions in North Carolina. As a legume cover crop, crimson clover can fix atmospheric nitrogen for usage by the subsequent main cash crop. It is suitable to grow in well-drained soils like sandy loam but not recommended for extremely acid/alkaline conditions, waterlogged, and heavy clay soils, making it less suitable for the tidewater region. A soil pH of 5.5 to 7 is appropriate, and a pH under five can hamper nodule formation and stop nitrogen fixation. Crimson clover has more acid tolerance than other clovers. It can be planted in late summer to early fall and terminated in spring. Crimson clover has the capacity to overwinter, with rare chances of winterkill in warmer regions of the southeast US. It is a popular roadside cover crop in the southeast US and has high forage quality.

Benefits:

Crimson clover can produce dry matter and fix nitrogen, second only to hairy vetch. Biomass production is 2,000 to 5,000 lbs./acre, and nitrogen fixation is up to 50 to 125 lbs./acre in North Carolina. It has lower carbon and higher nitrogen content, with a lower C: N ratio of 17 causing faster breakdown of residues than grasses, leading to more rapid N release. It is not that effective in causing organic matter increase and weed control unless it is grown along with a small grain cover crop. Crimson clover is sometimes grown along or overseeded into wheat or oats and is left to grow after the main crop harvest for forage purposes. When clover is left to grow longer on the field as a rotation crop, in addition to N fixation, it has demonstrated better weed control, organic matter addition, erosion resistance, and ability to break disease and insect cycles. Crimson clover can flower early compared to other legumes and has bright long red flowers with abundant nectar that can attract beneficial insects, helping control pests. 

Planting and Management:

Crimson clover can be seed drilled at 8 to 20 lbs./acre or broadcasted at 15 to 30 lbs./acre six to eight weeks before the frost date up to ¼ to ½ inches depth in North Carolina. Light disking or cultipacking after broadcasting ensures good seed-soil contact. Lower seeding rates can be used in cool soils (mountain/piedmont) and higher in warmer soils (coastal plains). The planting date varies with elevation, and in NC, the seeding date is three weeks ahead in the mountains compared to the coastal plain region (until mid-November). Crimson clover can be rotated with cash crops that have early harvest in the fall or late planting in spring. The seeds can be spread before defoliation using a highboy sprayer in cotton cultivation systems and broadcasted before the leaf drop stage in soybean. In corn, they are planted after harvest. Phosphorous and potassium deficiency and acidic conditions should be avoided to facilitate good production of nodules for nitrogen fixation. 

Crimson clover can be a host to root-knot nematodes in the mountains/piedmont region, and care should be taken when choosing this cover crop in nematode-prone soils. Crimson clover’s resistance to insects is poor, and it can harbor pests like corn earworm, cotton bollworm, flower thrips, and tarnished plant bugs, making it a challenge to include crimson clover in these pest-prone areas.

Termination:

Incorporating crimson clover residue with light tillage can cause quicker nutrient release for the subsequent cash crop; however, this practice can cause chances for soil erosion and affect the weed suppression effect. Therefore, tillage inclusion should be done according to your field requirements. Cultivating an early maturing variety that flowers before termination can enhance the quantity of nitrogen fixed. In areas where reseeded crimson clover can turn into weed, it should be made sure to kill them before seed set or use long-season cultivars. Crimson clover residue can be managed with a no-till system for efficient nitrogen release and increased corn yields or can be harvested prior to planting corn if used for silage. 

In late-planted tropical silage corn, grain sorghum, and midsummer vegetable crops like pumpkin that require late spring establishment, an innovative solution to reduce seed costs can be practiced wherein crimson clover is allowed to reseed naturally. In full-season corn production systems, using crimson clover as a cover crop can be a challenge as corn requires early planting, and clover matures after the corn planting date. However, it can work if the clover standing on future crop rows alone can be terminated using strip tillage, and the clover in between the tilled rows can be left standing. Suppose there is a chance of weed problem, and less nutrient availability for the succeeding corn. In that case, the clover can be terminated before the seed is set. Otherwise, clover can be left to set seeds that can germinate when moisture is adequate in summer. This innovative seed cost-reduction method is currently being practiced in other southeastern states and can be explored further in North Carolina. Research conducted in cotton produced on sandy loam soils of Rocky Mount, NC, also indicated that letting crimson clover (early flowering variety) go to hard seed prior to termination can be used as a cost-saving mechanism for cover crop seeding. The seeds of crimson clover left in field germinated in fall to grow in between cotton rows and caused no yield reduction. 

The mechanical killing of crimson clover is easy owing to simple taproots. It can also be terminated by mowing at the early bud stage. Care should be taken in soils with less nitrogen while mowing clover as it can cause them to decompose quickly, leading to nitrogen losses before the main crop can use them. Rolling/crimping might not be completely effective in killing this cover crop, as crimson clover doesn’t reach the minimum required height (14 inches) for crimping. Herbicide killing of legumes takes more time than small grains, and two and a half to three weeks should be given for clover to die. The highest N is available at the late bloom or early seed set stage. Earlier termination in the late vegetative stage (30 days before seed set) can cause a reduction in nitrogen fixation up to 50 lbs./acre. 

Cover crop mixtures:

Legume mixtures with grasses can help balance and enhance biomass production, nitrogen addition, N scavenging, weed, and erosion control. Crimson clover can establish well with other small grains, grasses, and clovers. The two-thirds seeding rate of crimson clover can be combined with the one-third to one-half rate of the other crop for cover crop mixtures. Mixing with oats can help crimson clover to establish well and facilitate high biomass production and nitrogen scavenging. Crimson clover and wheat are a good mixture to be included in cotton production areas of the southeast. Barley can be mixed with crimson clover as their heights match. Crimson clover can also be mixed well with annual ryegrass. Crimson clover is less competitive for growth with small grains than hairy vetch, ensuring good small grain production.

Recommended varieties for North Carolina:

AU Robin (early maturing by two weeks, suitable for corn), AU Sunrise (ideal for corn, early maturing by 1-3 weeks), AU Sunup (earliest flowering variety), and Dixie (full season standard, suitable for cotton) 

References:

Managing Cover Crops Profitably

Southern Cover crops council Cover crop Information sheet

Winter Annual Cover Crops

Virginia NRCS Cover Crop Planning Manual

USDA Crimson Clover Plant Guide

Biomass Production With Legume and Small Grain Cover Crop Mixtures in North Carolina: Research Summary

Investigating Cover Crop Mulches in North Carolina Cotton Production