Summer Cover Crops for California Gardens: Buckwheat, Cowpeas & More

Summer Cover Crops for California Gardens: Buckwheat, Cowpeas, and More

Summer cover crops are among the most underused tools in California home gardens, yet buckwheat alone can go from seed to full bloom in just 30 to 40 days, making it possible to squeeze in a complete soil-building cycle between spring harvest and fall planting. UC ANR research confirms that warm-season cover crops in California can produce 3,000 to 8,000 pounds of dry biomass per acre, depending on species and management (UC ANR Publication 8225).

Most California gardeners already know about winter cover crops. The idea of planting crimson clover or fava beans in October feels natural, since those beds would otherwise sit empty through the rainy season. But far fewer gardeners think about what to do with beds that are empty during summer, the period between pulling out spring lettuce and planting fall broccoli, or the season when a new bed is sitting idle waiting for October.

Leaving those beds bare through summer is a missed opportunity. Weeds will move in fast. Soil biology declines without living roots to feed it. And all that sunshine and warmth, which could be driving photosynthesis and building organic matter, goes to waste.

Summer cover crops fix those problems. They are fast, heat-tolerant, and in some cases stunningly beautiful. Here is what to grow, when to grow it, and how to manage it. If you are new to cover cropping, our guide to the best cover crops for home gardens covers the fundamentals.

When Would You Use a Summer Cover Crop?

Summer cover crops are not for every bed in every season. They serve specific situations in your garden calendar:

Gap between spring and fall crops. You pull your spring peas, lettuce, or garlic in May or June and will not plant fall vegetables until September or October. That is 3 to 4 months of bare soil. A summer cover crop fills the gap.

Resting a bed for a season. After years of intensive production, some beds benefit from a full season without vegetables. A summer cover crop followed by a winter cover crop gives the soil a complete year of regeneration.

New garden establishment. You built new raised beds or opened new ground in spring, but you do not plan to grow vegetables until fall. Rather than fighting weeds all summer, plant a cover crop.

Pollinator support. Buckwheat and cowpea flowers attract beneficial insects during the critical summer period when pest pressure is highest.

Weed bank reduction. A bed with persistent weed problems benefits from a dense, fast-growing cover crop that smothers weed seedlings and prevents them from setting seed.

The key difference between summer and winter cover cropping in California is water. Winter cover crops ride on natural rainfall. Summer cover crops need some irrigation. But their water needs are modest, often less than a vegetable crop in the same space, and the soil-building return on investment is significant.

What Are the Best Summer Cover Crops for California?

Buckwheat (Fagopyrum esculentum)

Buckwheat is the standout performer for summer cover cropping, and it deserves special attention.

What makes it special. Buckwheat is not a grass or a legume. It is a broadleaf plant in the smartweed family (Polygonaceae), which means it does not share disease or pest problems with any common vegetable crop. It germinates in 3 to 5 days, establishes rapidly, and reaches full bloom in 30 to 40 days. That speed is unmatched among cover crops.

Soil benefits. Buckwheat has an unusual ability to solubilize phosphorus in the soil. Its root exudates release phosphorus that is bound to calcium and iron compounds, making it available to subsequent crops. UC Davis researchers have confirmed this phosphorus-mobilizing effect in California soils (UC ANR, Buckwheat as a Summer Cover Crop). Buckwheat also adds moderate organic matter and provides excellent weed suppression through rapid canopy closure.

Pollinator paradise. The masses of small white flowers attract an extraordinary diversity of beneficial insects. UC Integrated Pest Management (UC IPM) resources identify buckwheat as one of the top insectary plants for California gardens, attracting hoverflies, parasitic wasps, tachinid flies, and native bees. These are the insects that eat aphids, parasitize caterpillars, and pollinate your vegetable crops.

How to grow it in Santa Cruz County:

• Sow from April through August, after soil temperatures reach 55 degrees Fahrenheit
• Broadcast seed at 2 to 3 ounces per 100 square feet
• Rake lightly to cover seeds
• Water to establish (germination needs consistent moisture for the first week)
• After establishment, water once or twice per week. Buckwheat needs moderate moisture but is not a heavy drinker.
• Cut at full bloom (30 to 40 days after sowing), before seeds form

Termination. Buckwheat is the easiest cover crop to terminate. Simply cut or mow at the base when flowers are fully open. The succulent stems collapse and begin decomposing within days. You can sow a second round immediately, or wait 1 to 2 weeks and plant vegetables.

One critical caution. Buckwheat sets seed quickly after peak bloom. If you let it go even 1 to 2 weeks past full bloom, mature seeds will drop and germinate everywhere. The self-sown plants are easy to pull, but preventing seed set in the first place is much less work.

Coastal Santa Cruz note. Buckwheat grows well in our coastal climate but may take slightly longer to mature in foggy areas. Expect 35 to 45 days to bloom near the coast versus 30 to 35 days in warmer inland areas.

Cowpeas / Black-Eyed Peas (Vigna unguiculata)

If you want nitrogen fixation during the warm season, cowpeas are your best option. They are the summer equivalent of winter favas: a legume that thrives in the conditions when other legumes struggle.

What makes them special. Cowpeas are tropical legumes native to West Africa. They are adapted to heat, tolerate drought, and fix nitrogen actively in warm soils (above 65 degrees Fahrenheit). Their Bradyrhizobium bacterial partners work best at soil temperatures that would shut down the Rhizobium species used by temperate legumes like clover and peas.

Soil benefits. UC ANR research rates cowpeas as capable of fixing 50 to 100 pounds of nitrogen per acre during a summer growing season. They also produce moderate biomass, though less than sorghum-sudan grass or sunn hemp. The plant residue has a low carbon-to-nitrogen ratio, meaning it decomposes quickly and releases nitrogen fast after termination.

Dual purpose. You can harvest some of the pods for eating (they are, after all, black-eyed peas) and still get cover crop benefits from the remaining plants. This makes cowpeas appealing for gardeners who dislike "wasting" garden space on a non-food crop.

How to grow them in Santa Cruz County:

• Sow from mid-May through July, when soil temperature is consistently above 65 degrees Fahrenheit
• Plant seeds 1 to 2 inches deep, 4 to 6 inches apart, or broadcast at 3 to 4 ounces per 100 square feet
• Water to establish, then irrigate deeply once every 7 to 10 days
• Plants will flower in 60 to 80 days depending on variety and conditions

Termination. Cut or mow at flowering, before pods mature (unless you want to harvest some beans first). Residue breaks down quickly because of its high nitrogen content. Wait 2 to 3 weeks before planting fall crops.

Coastal Santa Cruz note. Cowpeas need warmth more than any other cover crop on this list. In foggy coastal areas, they may grow slowly and produce less biomass than expected. Plant them in your sunniest, most sheltered bed. If your garden is fully coastal and fog-prone, buckwheat is a more reliable summer choice.

Sorghum-Sudan Grass (Sorghum bicolor x S. bicolor var. sudanense)

For sheer biomass production, nothing in the summer cover crop lineup comes close to sorghum-sudan grass. This hybrid grass can grow 5 to 8 feet tall in a single season and produce an enormous volume of organic matter.

What makes it special. Sorghum-sudan grass combines the heat tolerance and drought resistance of sorghum with the rapid regrowth ability of sudan grass. According to UC ANR, sorghum-sudan can produce 4,000 to 8,000 pounds of dry matter per acre in California conditions. Its deep, extensive root system (reaching 4 to 6 feet in some soils, per UC Davis research) is one of the best tools available for breaking compacted subsoil.

Soil benefits. Massive carbon-rich biomass that builds soil organic matter rapidly. Deep roots that break hardpan and improve water infiltration. Allelopathic root exudates (sorgoleone) that suppress certain weeds. The fibrous root system leaves behind an extensive network of channels that improves soil structure long after the plant is gone.

How to grow it in Santa Cruz County:

• Sow from mid-May through June, when soil temperature is above 65 degrees Fahrenheit
• Broadcast at 1 to 2 ounces per 100 square feet, or drill in rows 6 inches apart
• Water regularly. Sorghum-sudan needs consistent moisture for maximum production.
• Mow at 3 to 4 feet tall and it will regrow from the crown (this is called ratooning)
• You can get 2 to 3 cuttings in a Santa Cruz summer

Termination. Final mow at the base in late August or September. Allow 4 to 6 weeks for the heavy residue to decompose before planting fall or winter crops. This long decomposition period is the main drawback of sorghum-sudan for small gardens.

Important caution. Sorghum-sudan grass can produce cyanogenic compounds (prussic acid) when stressed by drought, frost, or mechanical damage. These compounds break down quickly as the plant tissue dries, and they are primarily a concern for livestock, not for gardeners handling plant residue. However, avoid grazing animals on sorghum-sudan and do not feed fresh clippings to chickens.

Santa Cruz County note. Sorghum-sudan is best suited for larger in-ground gardens (100 square feet or more per bed). In a small raised bed, a 7-foot wall of grass is impractical and may shade neighboring beds. Reserve this crop for serious soil rehabilitation projects.

Sunn Hemp (Crotalaria juncea)

Sunn hemp is a tropical legume that has gained significant attention in warm-climate agriculture for its remarkable combination of nitrogen fixation and biomass production.

What makes it special. Most cover crops offer either nitrogen fixation (legumes) or heavy biomass (grasses), but not both. Sunn hemp produces both. UC SAREP notes that sunn hemp can fix 100 to 150 pounds of nitrogen per acre while simultaneously producing 3,000 to 5,000 pounds of dry matter. It is also documented as a nematode suppressant, particularly effective against root-knot nematodes.

Soil benefits. Nitrogen fixation plus heavy biomass plus nematode suppression. The combination of benefits makes sunn hemp one of the highest-value summer cover crops available.

How to grow it in Santa Cruz County:

• Sow from late May through July (needs warm soil, above 65 degrees Fahrenheit)
• Broadcast at 2 to 3 ounces per 100 square feet
• Water to establish, then irrigate moderately
• Plants reach 4 to 6 feet tall in 60 to 90 days
• Yellow flowers appear in late summer

Termination. Mow or cut before seeds set. The tall, somewhat woody stems take 3 to 4 weeks to decompose.

Availability note. Sunn hemp seed can be harder to find than other cover crop species. Check farm supply catalogs or online seed sources. It is not typically stocked at local garden centers.

Santa Cruz County note. Like cowpeas, sunn hemp needs real warmth to perform well. It is best suited for the warmest inland parts of the county. Coastal gardeners will likely get more reliable results from buckwheat.

Quick-Reference Summer Cover Crop Options

For gardeners who want a fast answer:

• Short window (4-6 weeks available)? Buckwheat. Nothing else matures this fast.
• Nitrogen is your top priority? Cowpeas (warm areas) or sunn hemp (warmest areas).
• Maximum biomass and soil breaking? Sorghum-sudan grass.
• Pollinator support? Buckwheat, without question.
• Coastal/foggy garden? Buckwheat is the most reliable in cooler conditions.
• Hot inland garden? All four species will thrive. Cowpeas or sunn hemp for nitrogen, sorghum-sudan for biomass.

How Do You Manage Water for Summer Cover Crops?

The main difference between summer and winter cover cropping in California is irrigation. Winter cover crops ride free on rainfall. Summer cover crops need some water.

Here is what to expect:

Buckwheat: Water once to establish (a thorough soaking at planting). After germination, water 1 to 2 times per week depending on temperature and soil type. Sandy soils dry faster and need more frequent watering. In Santa Cruz's coastal climate, buckwheat may need less water than inland because of morning fog and cooler temperatures.

Cowpeas: Water to establish, then irrigate deeply every 7 to 10 days. Cowpeas are drought-tolerant once their roots are established. They will survive on less water than this, but biomass production drops.

Sorghum-sudan grass: The thirstiest summer cover crop. For maximum production, water 1 to 2 times per week. A bed of sorghum-sudan uses roughly the same amount of water as a bed of corn.

Sunn hemp: Moderate water needs. Similar to cowpeas. Water to establish, then irrigate weekly.

Context matters. If you are already watering a summer vegetable garden and have a bed sitting empty with the irrigation lines in place, adding a cover crop costs very little extra water. If you would need to set up a new irrigation system just for the cover crop, buckwheat (with its low water needs and short cycle) is the most practical choice.

How Do You Transition from Summer Cover Crop to Fall Garden?

Knowing how to terminate cover crops properly is essential for a smooth transition to your next planting.

Timing the transition from summer cover crop to fall planting is important. You need to terminate the cover crop early enough that residue can decompose before you plant.

For September fall planting (broccoli, kale, lettuce transplants): - Terminate summer cover crop by mid-August - Allow 2 to 4 weeks for decomposition - Prepare beds and plant by early to mid-September

For October fall planting (garlic, fava beans, cover crops): - Terminate summer cover crop by early September - Allow 2 to 4 weeks for decomposition - Plant by mid-October

For a year-round cover crop rotation: - Summer cover crop (buckwheat or cowpeas) May through August - Terminate August - Winter cover crop (fava beans, crimson clover, or rye) October through March - Terminate March - Vegetables April through August - Repeat

This rotation gives every bed one full year of cover crops for every year of vegetable production. Over time, this 50-50 rotation (or even a 1-in-3-years rotation) dramatically improves soil health.

How Do Summer Cover Crops Differ Between Coastal and Inland Santa Cruz?

This distinction matters more for summer than winter cover crops because temperature differences are much more pronounced during the warm months.

Coastal gardens (Westside Santa Cruz, Natural Bridges, Capitola, Aptos coast): Summer highs are often in the 60s and low 70s, with regular fog. Heat-loving cover crops (cowpeas, sunn hemp, sorghum-sudan) may underperform. Buckwheat is the most reliable summer choice because it does not require extreme heat, just consistent warmth above 55 degrees Fahrenheit and reasonable sun.

Mid-county and inland valley gardens (Soquel, upper Aptos, Live Oak): Warmer than the coast, with summer highs in the 70s to low 80s. All summer cover crops will work, though cowpeas and sorghum-sudan may grow slightly slower than in truly hot inland areas.

Inland and southern county gardens (Scotts Valley, San Lorenzo Valley, Watsonville, Pajaro Valley): Summer highs regularly reach the 80s and occasionally the 90s. All summer cover crops thrive here. This is where sorghum-sudan and sunn hemp really shine.

Frequently Asked Questions

Is it worth the water to grow a summer cover crop during a drought year?

This depends on the severity of the drought and your water situation. During moderate dry years, buckwheat uses modest water (roughly equivalent to keeping a bed of annual flowers alive) and provides real soil benefits. During severe drought or mandatory water restrictions, prioritizing water for food crops makes sense. A compromise approach: grow buckwheat only in beds with existing drip lines, and use dry mulch (straw, wood chips) on other bare beds. See our guide to composting from kitchen scraps to garden gold for mulch-based approaches to soil building that do not require irrigation.

Can I plant a summer cover crop in a bed that had a diseased crop?

It depends on the disease. Most summer cover crops are not susceptible to common vegetable diseases (tomato blight, powdery mildew, bacterial wilt). In fact, some summer cover crops can actively help. Sunn hemp suppresses root-knot nematodes. Mustard family crops (not listed here as summer options, but sometimes used in spring) have biofumigant properties. However, if the disease was a soil pathogen, solarization (covering the bed with clear plastic during summer heat) may be more effective than a cover crop.

Can I mix summer cover crops together?

Yes, though summer cover crop mixes are less common than winter mixes. A practical combination is buckwheat plus cowpeas: the buckwheat establishes fast and provides early coverage while the slower cowpeas fill in below. The buckwheat blooms and gets terminated while the cowpeas continue growing. This gives you both phosphorus mobilization and nitrogen fixation in one bed.

Will summer cover crops shade my vegetable beds?

Sorghum-sudan grass can definitely shade neighboring beds, since it reaches 5 to 8 feet tall. Plant it on the north side of your garden or in beds that are not adjacent to short-stature vegetables. Buckwheat and cowpeas, at 2 to 3 feet tall, are unlikely to cause shading problems in normal garden layouts.

How do I source cover crop seed for uncommon species like sunn hemp?

Local garden centers typically carry buckwheat and cowpea seed, but sunn hemp and sorghum-sudan may need to be ordered from farm supply companies. Peaceful Valley Farm Supply (Grass Valley, California, now part of True Leaf Market) has historically carried a wide range of cover crop seeds including warm-season species. Online seed companies that specialize in cover crops are another reliable source.

Can I plant summer cover crops in containers or raised beds?

Buckwheat works well in raised beds and even large containers (15 gallons or more). Its compact size and short growth cycle make it practical for small spaces. Cowpeas also work in raised beds, especially bush-type varieties. Sorghum-sudan grass is too tall and aggressive for containers or small raised beds. Reserve it for larger in-ground garden areas.

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Summer cover crops deserve a place in every California gardener's toolkit. They are not as well-known as their winter counterparts, but they fill a genuine need: building soil during the warm months when beds would otherwise sit empty and unproductive. Start with buckwheat. It is fast, easy, and beautiful. Once you see the bees swarming a bed of buckwheat flowers and watch the residue melt into your soil, you will wonder why you ever left a bed bare in summer.

For more seasonal strategies and soil-building tips, grab our free gardening toolkit at Your Garden Toolkit.