Integrating Warm-Season Cover Crops Into Your Rotation

By Chris Benedict, Washington State University Whatcom County Extension


Weedy check plots 48 days after cover crop plantingBuckwheat 48 days after plantingUnderstory of Buckwheat plots (48 days after seeding)Flowering mustards ('Ida Gold' on left, 'Pacific Gold' on right)Rapeseed (48 days after planting)Sorghum-sudangrass 48 days after seeding6 - 6<>



Typically cover crops are fall planted, overwintered, and terminated in the spring time. These cold-tolerant cover crops are commonly small grains, but a number of legumes and brassicas work well in this region as well. Several producers have asked about what cover crops work well during the summer time to help manage weeds and can be easily integrated into a diversified vegetable rotation. This article should help to navigate through that decision making process.


Cover Crop Choice

This article is not an exhaustive list of what warm-season crops can be grown here, but is based on some field trials over the past five years. There are a number of different cover crop choices available locally and through seed catalogues, but the first thing you want to decide is-what is your end goal. Are you trying to suppress weeds? Are you trying to add nitrogen? Are you trying to add organic matter? Most fall-planted cover crops can be consider generalists, while many warm-season cover crops can be thought of as specialists. Buckwheat is a perfect example. It has the ability to suppress weeds while growing, but contributes very little in terms of long-term organic matter as it’s very fleshy and breaks down quickly. Seeding rates, general advantages and days to flowering (based on a July 1rst planting date) for a variety of crops are included in Table 1. Below is a description of each type that has been trialed.


Table 1. Seeding rates, advantages, and days to flowering, Whatcom County 2011-2013.


Great quick cover, suppresses weeds from going to seed while it’s growing. Will set seed before you know it so keep an eye on flowering to avoid it becoming a weed itself. Recommend to terminate within 50 days of seeding for western Washington. Trials in Puyallup showed 28 days of weed suppression after mowing and tilling. Probably best warm-season cover crop to cover ground and suppress weeds between cash crops.



There are many different types and each has associated cultural traits (annual vs perennial). Not overly competitive with weeds early on, but once established can shade out weeds. Biggest benefit is nitrogen addition and quick breakdown. Maximizing nitrogen contribution does take time though.



Typically grown in warmer regions, this cover crop does alright in this region. Soil temperatures should be above 58°F at planting. It’s one of the rare warm-season legumes and if the stand gets good establishment can shade out weeds. In one of our trials, there was weak stand establishment which may indicate that growing this cover crop here can be hit or miss.



Also known as oilseed rape or rape, this brassica establishes more quickly than other brassicas particularly under cooler soil temperatures. Once established it can create a dense over story to shade out weeds and showed weed suppression post incorporation in trials at Puyallup.



This hybrid cross of sorghum and sudangrass produces large amounts of biomass without going to seed. It does need warmer soil temperatures (>65°F), but it has been planted in trials as early as late May and did just fine. Great weed suppressor, but with the abundance of biomass it does tend to lock up nitrogen post-incorporation.



Typically these are fall sown, but our trials have shown great promise for use during the summer months. Biomass accumulation was rapid once established in trials in Whatcom County. Nitrogen addition is a plus.


Mustards (e.g. ‘Pacific Gold’, ‘Ida Gold’)

This group will become established very quickly, shade out weeds, and can suppress weed emergence for up to 20 days after incorporation. It is recommended to terminate them before flowering (around 50 days). While they don’t produce as much biomass as buckwheat or sorghum-sudangrass, they are very effective tools to manage weeds for organic growers.

NOTE: These crops need to be certified disease free to be planted in the Brassica Seed Production district in most of Northwestern Washington ( OR


Integrating Into Your Rotation

This is often a complicated matter of timing, but can be simplified by thinking about a.) Goals and b.) Windows of opportunity (timing). As previously mentioned, warm-season cover crops should be thought of as specialists so matching their attributes with your goals is a good starting point. Next think about timing; how much time between crops within a bed or field? Clearly the longer you can wait the more cover crop biomass you’ll accumulate and the more likely the benefits will play out on your farm. However, some crops described above provide benefits sooner than others so it’s possible to select an appropriate variety to fit your needs if you plan ahead. You also want to consider soil-borne diseases and arthropods as cover crops can harbor these pests.


Why Use Warm-Season Cover Crops

In trials in Puyallup and Whatcom County we’ve found that even 30-40 days can bring about benefits resulting from weed suppression once established.  Additionally, once mowed and tilled in, emerged weeds were significantly lower in plots with buckwheat, mustards, or rapeseed when compared to fallow ground. This suppression lasted for approximately 28 days after incorporation (Figure 1). Plots were left for 48 days at which point weed biomass was significantly lower in nine of the 12 cover crops tested (Figure 2).


Figure 1. Number of emerged weeds 28 days after cover crop incorporation, Puyallup, WA 2011-2012.

*Indicates a cover crop treatment that had significantly lower number of weeds as compared to the weedy check.


Figure 2. Weed biomass grams/m2 48 days after cover crop planting, Puyallup, WA 2011-2012.

*Indicates a cover crop treatment that had significantly lower weed biomass as compared to the weedy check.



In a 2014 trial in Whatcom County, buckwheat and sorghum-sudangrass (grown alone and in combination for 40 days) were compared to evaluate cover crop biomass accumulation and weed emergence at 14 days after incorporation. The planting included: 1.) Buckwheat (BW) alone, 2.) Sorghum-sudangrass (SS) lone, 3.) BW 25%/SS 75%, 4.) BW 50%/SS 50%, 5.) BW 75%/SS 25%. We wanted to directly compare these two cover crops and also get a sense of their ability to be blended.  Across all of the blends, buckwheat tended to dominant sorghum-sudangrass in biomass production regardless of the mix ratio. Interestingly, weed suppression was lowest (at 14 days after incorporation) in the 25% and 50% buckwheat when compared to no cover crop (Figure 3).


Figure 3. Number of emerged weeds 14 days after incoporation opf cover crops, Whatcom County, WA 2012-2014


Take Home Message

Integrating warm-season cover crops, even for short period, can provide many benefits for diversified vegetable farmers. Their ability to shade out weeds and inhibit weed establishment after being incorporated can buy time and provide planting flexibility. Identifying the type that is suitable for your farm is dependent on your end goal and open windows you may have in a bed or an entire field.