The Hawaii Cover Crop Handbook

  1. Cover Crop Cocktails
  2. Cover Cropping in Hawaii: The Benefits and Costs
  3. Where to Buy Cover Crop Seed in Hawaii
  4. Additional Resources

Cover crops are non-cash crop plants that are planted to cover and improve the soil. Many different species of grasses, legumes, and forbs are used as cover crops and, depending on the species, they offer a wide range of benefits:

  • Excellent weed control
  • Prevents erosion of topsoil
  • “Green manure” – nitrogen-fixation and nutrient scavenging
  • Attracts beneficial insects
  • Builds soil organic matter
  • Bio-tilling
  • Nematode suppression

In 2001, Oahu RC&D produced a Cover Crop Handbook that featured information on three of the most common cover crop species used in Hawaii at that time: oats, buckwheat and sunn hemp.  In the past eight years, cover crop use has increased dramatically, and producers are seeking ways to maximize the benefits of cover cropping.  One way to achieve this is by utilizing multi-species plantings (aka Cover Crop Cocktails). 

On this webpage, we’ll provide “nuts and bolts” know-how that will get you onboard with cover cropping. If you’re new to cover cropping, we encourage you to start by reviewing our 2011 Cover Crop Handbook, and the guidelines it contains for implementation of a sunn hemp, buckwheat, and black oats cover crop mix (available in English, Korean, Ilocano, Tagalog, Mandarin, Khmer).

If you’re an experienced cover crop user and want to use cover crop cocktails, this addendum will help you grow soil – not problems.  Information is based largely on Oahu RC&D’s collaboration with five farms to research the costs and benefits of utilizing cover crop cocktails and includes: 

  • cover crop cocktails (multi-species or polyculture plantings): the benefits, drawbacks and how to make a mix for your farm,
  • a recap of the best cover crop resources available for farmers in Hawaii.

Cover Crop Cocktails

Mixed-species plantings (aka Cover Crop Cocktails) utilize up to six or more different plant species to combine benefits of individual species and generate increased biomass and nitrogen production, increased habitat for a variety of beneficial insects, improved tolerance of adverse conditions, and a prolonged duration of active growing period. Multiple studies have been conducted on the mainland to evaluate cover crop cocktails and there is growing interest in their use, as well as recognition among farmers and agricultural professionals that they can provide increased benefits.

Conversely, mixed-species plantings may result in higher seed costs and more complicated management needs (e.g. best time to kill one crop may not be the best for a different crop in the same mix).

The five farmer-researchers that participated in our 2017-2018 on-farm trials gravitated towards two kinds of cover crop cocktails.

A blanket of quick-to-establish buckwheat flowers in a field recently planted with a multi-species cover crop cocktail.
  • Simple cocktails using a succession of two species, starting usually with buckwheat due to its rapid establishment and weed suppression, followed by a vigorous grass (e.g. sorghum-sudan) or leguminous cover crop such as sunn hemp or lablab. All of these species produce substantial biomass and have additional useful traits (nitrogen fixation, nematode suppression, etc.) The main advantage of this type of mix is that they are simple and they WORK! Because the cover crops are seeded together, but develop at different times, full seeding rates are used for each species. Therefore, seed cost is double that of using a single species. Essentially this type of cover crop cocktail allows you to grow two cover crops – buckwheat and the second species that succeeds it – while only planting and terminating once.
A field day crowd takes in the wisdom of speaker Gabe Brown and a field planted with a high-diversity cover crop cocktail (6+ species).
  • High-diversity cocktails popular with our farmer-researchers included six or more different species from distinct plant families and with varying growth habit (above and below ground) and beneficial traits. In general, two or more species each of forbs, grasses, and legumes were selected in an effort to emulate the diversity in natural ecosystems. Seeding rates were lowered for each species, though total seeding rates are still substantially higher (and of course more expensive) than single-species cover crop plantings. One advantage of this approach is that if one or two species fare poorly, the planting can still thrive. This approach is experiencing some success in the mainland US and generated beautiful fields and positive results for some of our farmer-researchers. However, it did have several drawbacks worth noting, including high seed cost, time-consuming seeding calculations, and the challenges of successfully planting a variety of different sized seeds. In the next paragraph, we’ll provide some more detail on the experience.

The 2018 trial plantings allowed each participating farmer-researcher to compare a “standard” three species cocktail of sunn hemp, buckwheat, and oats with a custom cocktail designed with input from the project’s technical advisor to address their specific objectives and growing conditions. The 2018 plantings suggest that high-diversity custom cocktails can successfully generate biomass and contribute plant available nitrogen to the soil, and can do so at a comparable or in some cases improved rate in comparison to a “standard” cocktail. However, the customized cocktails required higher seeding rates and included more costly seed, especially for farms that required organic-certified seed. An additional cost posed by the customized cocktail was the planning involved in designing the cocktail and sourcing the seed.

The farms that participated in the trial are young farms that are evolving their production system and crops as they grow. For this type of farm, cover crops are a relatively small piece of the puzzle they must put together to find and sustain success. As such, the additional time and effort involved in designing and using a custom cover crop cocktail may not be merited; simply put, a simple single species or standard cocktail cover crop will suffice and management resources will be better spent refining other aspects of their operation, such as identifying successful cash crops. Conversely, farms with well-established production systems that wish to optimize their cover cropping practices to maximize soil health gains may find that the incremental benefits obtained from customizing a cover crop cocktail justify the effort involved. Based on our experience working together with farmer-researchers, multiple seasons of cover crop planting can be required to dial in a cover cocktail recipe, both in terms of identifying appropriate species, adjusting the balance of the different species, and setting overall seeding rates. We collected data from each farm on the cost and labor required to manage the cover crop cocktail, but unfortunately the small field sizes and variation in equipment used and how each farm logged this data do not provide sufficient accuracy to calculate reliable cost per acre benchmarks for other growers at this time.   The Cover Crop Calculator developed for Hawaii was a useful protocol and farmers appreciated the information it provided on the cover crops contribution of biomass and plant available nitrogen. The logistics of drying the cover crop samples and getting them submitted for lab analysis were sometimes challenging. Though farmers appreciated the information on plant available nitrogen (PAN), the small acreages (<10) of the farms that participated in our trial did not add up to significant fertilizer savings (see Table 1). Even when the customized cocktails generated more PAN, these additional contributions did not compensate for the higher seed costs.  

Table 1. 2018 On-Farm Trial Results

Site 1Site 2Site 3Site 4Site 5
Seeding Rate (lb/ac) 92 125 92 125 92 128 92 126 92 115
Cover Crop SH/BW/O1 CC2 SH/BW/O1 CC3 SH/BW/O1 CC4 SH/BW/O1 CC5 SH/BW/O1 CC6
Dry Matter (lb / ac) 3,045 4,636 2,462 2,140 4,621 4,146 2,869 3,874
C:N 16 13 12 12 21 13 17 13
PAN (lb/ac) 28 51 34 33 64 101 33 60
Seed Cost ($/ac) $205 $328 $333 $458 $333 $558 $205 $451 $205 $326
Fertilizer Savings ($/ac) $9 $17 $11 $11 $21 $33 $11 $20  –   – 

SH/BW/O: Sunn Hump, Buckwheat and Oats; objective: good general performance in a wide variety of conditions to generate biomass, fix nitrogen and suppress weeds.

Site 1 Custom Cocktail (CC): Sunn hemp, black oats, lablab, sorghum-sudan, pearl millet and graza radish; farm objective: reduce soil compaction, improve soil texture and suppress weeds.

Site 2 Custom Cocktail (CC): Buckwheat, lablab, alfalfa, sunn hemp; farm objective: weed suppression, increase soil organic matter, fast break-down of crop residue, and attract beneficial insects.

Site 3 Custom Cocktail (CC): Sunn hemp, black oats, lablab, sorghum-sudan, tillage radish, cow pea; farm objective: contribute nitrogen and organic matter to soil.

Site 4 Custom Cocktail (CC): Sunn hemp, lablab, sorghum-sudan pearl millet, graza radish; objective: weed suppression, organic matter, transition to perennial cover for orchard crop.

6 Site 5 Custom Cocktail (CC): Sorghum-sudan, buckwheat with a small amounts of basil and cilantro; objective: add organic matter and have a flexible termination date with the option to mow and let cover crop regrow to prolong cover.

How to Mix a Cover Crop Cocktail for Your Farm or Garden

  1. Create a list of potential cover crop species for your farm that are well-adapted to your site, growing conditions and the season. Getting advice from experienced farmers that cover crop in your area is a great way to learn, if they are willing and have time to share. We always recommend this super useful chart put together by the UH CTAHRʻs Dr. Koon-Hui Wang, which lists good cover crops and recommended seeding rates for different altitudes in Hawaii.
  2. Eliminate problematic cover crops that can harbor pests or disease that affect your cash crops. For example, farmers growing cabbage, broccoli, or other brassicas for the market typically avoid mustard cover crops that can harbor pests and disease.
  3. Plan how you will seed and terminate your cover crop and eliminate any species from your list that you don’t have the proper equipment to deal with. Many farmers love using sorghum-sudan or sunn hemp to generate tons of biomass, but following either of these quickly with a cash crop requires a flail mower to mulch the cover crop and allow the residue to breakdown quickly. If you don’t have access to a flail mower and can’t afford to leave chunky residue in your field for months, avoid planting these species. We’ve heard lots of complaints from first time cover croppers who learned this the hard way – you don’t have to join their ranks. Remember, the goal is to build soil, not problems!
  4. Choose three or more of the remaining species on your list, including at least one grass, one legume, and one forb. This is a good time to review info sheets on each of the cover crop species you’ve selected, to learn more about growing tips and their beneficial attributes. You can prioritize traits such as nematode suppression, if that is important, but don’t get bogged down in the details. Emphasize species that you expect to grow on your farm and save the fine-tuning for later plantings when you’ve gotten more experience.
  5. Calculate your seeding rates. We recommend to start seeding high, to ensure good coverage, and subsequently to dial down the seeding rate by 5-10% every planting until you find the lowest amount of seed that achieves adequate coverage – the sweet spot. For small farms that are broadcasting seed, we’ve frequently seen total seeding rates that are 120 lb/acre or higher for cocktails, even though farms using seed drills can coax successful cover crop plantings from rates lower than 50 lb/acre. Of course, low germination due to seed quality or seedbed preparation, uneven irrigation and predation by birds all increase the seeding rate required for success. No one likes to waste money on seed. But you’ll suffer greater losses if you underestimate the amount of seed needed and end up with a sparsely vegetated field that exposes bare soil to sun, rain, and wind while leaving plenty of room to encourage nuisance weeds.
  6. Trial, observe, adjust and repeat. This is a prototype planting. It’s an opportunity to learn and adapt the cocktail to your conditions and cropping system. Don’t expect it to work perfectly the first time around. If this sounds like a lot to manage, consider doing single species cover crop plantings in different fields to observe and incorporate them into a cocktail next season.

Cover Cropping in Hawaii: the Benefits and Costs

Cover Crop SpeciesAdvantagesDisadvantages
BuckwheatNo to low maintenanceNematode-susceptible
Weed suppression
Quick establishment
Attracts beneficial insects
Good topsoil conditioner
Reduces soil erosion
Oil, Tillage or Forage Radish No to low maintenance
Nutrient scavenger
Reduces soil erosion
Weed suppression

Other forbs commonly used in Hawaii include sesame and marigold.

Cover Crop SpeciesAdvantagesDisadvantages
Sunn HempNo to low maintenanceFibrous when mature
Weed suppressionReduction in tuber crop yield
Quick establishment
Reduces soil erosion
Adds nitrogen to soil
Good windbreak
Reduces pests

Other legumes commonly used in Hawaii include lablab, pigeon pea, and cow pea.

Cover Crop SpeciesAdvantagesDisadvantages
OatsNo to low maintenanceAllelopathy delays cash crop
Weed suppressionReduction in tuber crop yield
Quick establishment
Reduces soil erosion
Biomass / Organic matter
Adapted to many soil types

Other grass cover crop commonly-used on Hawaii include sorghum-sudan grass, black oats, pearl millet, and rye.

Direct costs of cover cropping:

  • Cover crop seed cost
  • Cover crop seed shipping cost
  • Cover crop planting costs
  • Cover crop seed harvesting costs
  • Possible cash crop yield decrease

Indirect costs of cover cropping:

  • Cover crop competition if grown with cash crop

Other challenges

Several of our farmer-researchers on diversified, organic farms opted for cocktails of six species or more. They couldn’t have been happier with the results. A question that came up was whether planting diverse cover crops (and the additional cost) provides as much value in terms of soil health for their operations, compared to the large-scale commodity cropping systems where cocktails are being adopted on the mainland. Large commodity-crop farms have specialized equipment that facilitates planting and the low levels of diversity in their crop fields may make planting diverse cover crops much more valuable. In contrast, small diversified farms in Hawaii are confronted with a more difficult management process to plant a diverse cover crop and potentially lower value to soil health, since they already have a high level of crop and landscape diversity on their farms due to the mix of perennial and long-term annual crops and conservation species in their farm design and cropping system.

Where to Buy Cover Crop Seed in Hawaii

This UH publication walks you through the process of sourcing seed with some of the special considerations for invasive species and disease that pertain to Hawaii. We recommend giving it a read before you purchase cover crop seed for the first time. Three important takeaways are:

This UH publication walks you through the process of sourcing seed with some of the special considerations for invasive species and disease that pertain to Hawaii. We recommend giving it a read before you purchase cover crop seed (or source other plant material). The two important takeaways are:

Here are a short list of cover crop seed suppliers in Hawaii and online. We encourage you to support local agricultural businesses by buying local (if you sell cover crop seed and would like to be listed on this page, please send us an email).

Local (Hawaii) Seed Suppliers

  • Fukuda Seed Store, Inc. (808) 841-6719 1287 Kalani St # 103, Honolulu, HI 96817 A retail seed store located in Honolulu’s Kalihi neighborhood.
  • Koolau Seed & Supply Inc. – Phone 808-239-1280, A wholesale provider of cover crop, pasture and turf seed located on Oahu. Will ship to neighboring islands.
  • Moloka’i Seed Company – Phone (808) 658-9979, Sells retail and wholesale quantities of cover crop and native plant seeds, some of which is produced on Molokai.
  • Oahu RC&D – We produce a crop of sunn hemp every year on Oahu, which sells out quickly. To be placed on our waitlist, please send us an email and you can also learn more from our brochure.

Online (mainland) Cover Crop Seed Suppliers

  • Green Cover Seed – A farmer-owned seed company that specialized exclusively in cover crop seeds. Their website has a built-in cover crop cocktail calculator that is informative, providing numbers on biomass generation, seeding rates, and quantified indicators of cover crop performance for different species and custom mixes.
  • Johnny’s Selected Seeds – An employee-owned seed producer and vendor that has expanded to offer supplies and tools.
  • Peaceful Valley Farm and Garden Supply – Phone: 888-784-1722. A well-known provider of seeds, supplies, and tools with an emphasis on organic production.

Additional Resources

Western SARE’s’ “Managing cover crops profitably” is one of the most referenced publications on using cover crops and for good reason. Read the book in its entirety if you’re looking for a comprehensive overview, or pick and choose individual chapters to address your interests.

Farming in Hawaii’s tropical conditions is a different proposition than farming in the continental US, from the year-round growing season to the tropical crops being produced. So its no surprise that cover cropping resources targeting mainland farmers can be a poor fit for Hawaii. Fortunately, the University of Hawaii’s CTAHR extension service has got you covered with an extensive list of publications on cover cropping, all tailored to Hawaii’s growing conditions:

Sustainable and Organic Agriculture Program includes links to most if not all of CTAHR’s recent publications on the topic. I find the info sheets on different cover crop species particularly useful, especially when planning a new planting or mix.

Want help choosing the best cover crop species for your farm in Hawaii? This 2010 article by Dr. Radovich walks you through the process, asks the right questions and tells you why there’s no one-size-fits-all recommendation.

This 2012 article in HanaʻAi explores the ways that cover crops can promote beneficial insects and soil organisms. Dr. Wang, the author, is an agroecology professor and researcher, whose research spans a variety of fascinating interactions between cover crops and the insects and soil microbes that impact agricultural systems. She makes many of her research publications available on her webpage.

This 1988 UH-CTAHR publication covers the use of cover crops for Hawaii’s many orchard crops with specific recommendations for avocado, banana, coffee, and macadamia plantings.

Another way to dig up cover crop information relevant to Hawaii’s tropical growing conditions is to seek out extension publications from other origins in the tropics or subtropics. One advantage here is that many of these areas, whether states in the southeast US or other countries, have important agricultural sectors, which are supported by active extension agencies producing great publications:

  • This webpage addresses the challenges of cover cropping in hot and humid areas and though it was developed for the subtropical part of Texas, its explanations of environmental challenges, such as the fast break-down of organic matter in hot, humid condition, and its chart of cover crop species that perform in such conditions are definitely worth a read.
  • University of Florida’s IFAS extension service has a website on cover crops that can be a good place to look when you don’t find the info youʻre looking for closer to home. Southern Florida’s vegetable growers often cover crop in the summer season, similar to practices of low altitude, organic vegetable producers in Hawaii.