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Cultivating Questions Winterkilled Cover Crops for a Mild Climate

Cultivating Questions Concerning the Bioextensive Market Garden

by Anne and Eric Nordell of Trout Run, PA

I bought your video several years ago at a Washington Tilth conference; and I’ve changed my farming for the better because of it. Thank you. While I hope to transition to horses in the future, I currently farm using a Farmall Super A tractor. My hope is that I’m approximating horsedrawn tillage with this small tractor – it’s something like a motorized (and very heavy) riding cultivator. Alas, I can’t keep it from producing all that noise and noxious gas. Its single bottom plow even allows me to skim plow (albeit a little sloppily).

As Anne well knows, our mild climate makes it too easy to overwinter cover crops. Then the typically wet springs (and, on our farm, wet soils) let the cover put on loads of topgrowth before getting on the soil (the Super A is no help here). Buckwheat is the only crop that I can be certain will winterkill. Field peas, oats, annual rye and crimson clover have all overwintered here. Any suggestions?

My wife and I have just purchased a farm (I use the word “purchase” lightly as it is not paid for – Western Washington farmland is fast going the way of estates and golf courses and is priced accordingly). We’ll be working out rotational cover cropping for these new soils – silt loam and peat muck. At times they’re good and wet, and the weed pressures will be new to us also. It seems that a three mile move has us learning from the beginning again.

Best wishes for the coming year, and thank you for your fine work. Your articles are among my important resources.

Jeff Canaan

Lynden, Washington

Winterkilled Cover Crops For A Mild Climate

We feel a little bit more prepared to answer Jeff’s question about selecting winterkilled cover crops for a mild climate now that we have been in touch with several growers located in the coastal Northwest and the warmer parts of the central Midwest. All of these growers rely on cover crops that die back overwinter.

Each one agreed that buckwheat is the most dependable winterkilled cover for their area because it is so susceptible to a light frost. However, they warned, the timing for planting this frost-sensitive green manure can be tricky. Establishing buckwheat early enough to maximize biomass runs the risk of this quick-maturing cover setting viable seed before the first killing frost. These farmers also noted that the succulent buckwheat residues breakdown very quickly in the spring: a real advantage for early planting, but a drawback if a lot of winterkilled mulch is desired to hold in soil moisture and suppress weed germination.

Another favorite of these mild climate market gardeners is sorghum-sudangrass. They claimed this warm season grass is almost as frost-sensitive as buckwheat but produces a lot more coarse biomass. For this reason, we have trailed sorghum-sudangrass mixed with forage soybeans the last two years, comparing it with our usual winterkilled mix of oats and Canadian field peas.

We were pleased that the sorghum-sudangrass (“sudex” for short) could be planted a month earlier than the oats and peas without any danger of it going to seed before it died with the first fall frost. The sudex also produced significantly more topgrowth. However, the slow-to-decompose bamboo like stalks did not blanket the ground as completely as the mat of winterkilled oat-and-pea residues the next spring, allowing weeds to germinate in the patches of exposed soil which were also susceptible to the drying effects of the sun and wind. Several participants at our small group tours last year mentioned that they prefer Japanese Millet because its finer, leafier stalks produce a denser stand than the coarse sudex, resulting in much better weed suppression and moisture preservation.

If the seeds for these warm season annual grasses are not available in your area, both the sudex and Japanese Millet can be ordered from the Albert Lea Seed House in Albert Lea, Minnesota (1-800- 352-5247). Also, Seedway of Hall, New York (1-800-836-3710) offers several lines of sorghum-sudangrass as well as a new forage soybean. (Make sure to specify you want untreated seeds if your farm is certified organic. Also, be forewarned that shipping charges may double the costs of these reasonably priced cover crops.)

Seedway’s Donegal forage soybean variety grew 3-4’ tall in our trials and is supposed to get even taller where the summers are hot. Mixed with the sudex, or planted alone, this warm season legume should add some nitrogen to the soil system before it dies back with the first hard frost of the fall. It might also serve as a more manageable alternative to buckwheat or sudex where the winters are mild.

In addition to selecting cover crops with the appropriate degree of frost sensitivity for the area, the maturity of the plants at the onset of winter can make a big difference in their winter hardiness. Even in our relatively cold climate, we have noticed that the short and succulent topgrowth of late planted oats does not start dying back until the soil is frozen deeply and then thaws again. On the other hand, oats planted early enough to begin making seedheads before the first hard frosts will begin yellowing as soon as the ground starts to freeze. This principle may also hold true for more frost-sensitive covers, like sudex and soybeans, in milder climates.

Likewise, if the winter freezes are not strong enough to kill even the least winterhardy cover crops, then mowing the plants just before they reach maturity may do the job. We often use this mowkilled method for cover crops that we expect to overwinter in our area. For example, the old sicklebar mower is very effective at killing rye once this winterhardy cover has formed seedheads. We are guessing that oats and peas planted early enough to initiate seedheads by late winter in the Pacific Northwest could be successfully mowkilled before spring planted cash crops.

If none of these cover crop options prove dependable, then possibly the techniques we have developed for shallowly tilling winterhardy covers in northern Pennsylvania could be adapted to overwintering cover crops where the off-season is milder. The following photo essay documents two of the cover crop-tillage combinations we now use before LATE planted cash crops in the bio-extensive market garden. Maybe they would also work for earlier planted cash crops in coastal Washington. Mechanically speaking, we feel pretty certain that the horsedrawn tools shown in the photos would make a good match for the old Super A tractor.

We must end with one warning. The combination of winter cover crops and minimum tillage creates a wonderful habitat for slugs. Mulch-tilled cover crop residues provide both shelter and sustenance for these slimy critters, especially during the long periods of wet, cool conditions so common to the west side of the Cascades.

If slugs are already a problem, we suggest starting out with the most slug-resistant crops – such as garlic, potatoes, dill, cilantro, and transplanted tomatoes – for your first experiments with mulch-tilled cover crops. In the next column we plan on highlighting a couple of the makeshift ways we have tried to keep the slugs at bay, hoping this will encourage others to send us by snail mail their strategies for outsmarting these slow moving bugs for us to share in these pages.

Cover Crop-Tillage Combinations – 2002 Field Notes

Field 1 – Oats surface tilled before late May, zukes, cukes, lettuce, peas and spinach interseeded with buckwheat followed by a cover crop of rye late September; peppers and carrots on north side interseeded with string beans (for slug control), the pathways mulched with old hay after picking the beans

Field 2 – Rye surface tilled before summer cover of buckwheat and sudex followed by oats planted on ridges second week of September

Field 3 – Rye on ridges in middle of field surface tilled before late June and July, zukes, cukes, lettuce, peas and beans interseeded with buckwheat followed by cover crop of rye and spelt late October; surface tilled sudex, forage soybeans and field peas before May, spinach and peas interseeded with buckwheat followed by rye mid-September on north side of field; surface tilled sudex, beans and peas before April planting of strawberries interseeded with vetch on south side of field, berries mulched with wheat straw in December

Field 4 – Rye surface tilled/chicken tilled late June followed by cover crop of sudex and field peas seeded late July on south side of field and sudex, peas and oats mid-August in middle of field; rye on north side surface tilled before July broccoli, kale, pak choy, carrots and beets interseeded with rye

Field 5 – No-till garlic, onions, strawberries, leeks and radicchio early April on oat and pea ridges, surface tilled/chicken tilled late August followed by rye late September; ridge-tilled oats on north half of field seeded to April carrots, spinach, larkspur, beets and peas interseeded with buckwheat or hairy vetch, followed by rye mid-September

Field 6 – Rye surface tilled before June cover crop of oats and peas surface tilled before planting oats and peas on ridges late August

Field 7 – Rye and vetch surface tilled in June before July and August plantings of carrots, broccoli, kale, collards, lettuce, spinach beets and salad turnips interseeded with rye

Field 8 – Rye surface tilled late June replanted to rye and vetch early August; strawberries on south side surface tilled/chicken tilled after harvest before planting rye on ridges early September

XP (Experimental Patch) – Surface tilled sudex, peas and beans before planting alternating rows of tomatoes, onions and cut flowers interseeded with vetch followed by rye and spelt late October

Field 9 – Surface tilled oats and peas before May plantings of onions, leeks, winter squash, lettuce and peas; north half interseeded with vetch; south half interseeded buckwheat followed by rye and spelt mid-October

Field 10 – Rye and Italian ryegrass cover crop clipped repeatedly all summer, providing chicken pasture in October

Field 11 – Skim plowed Italian ryegrass clover mix before May planted potatoes interseeded with rye and vetch and followed by rye cover crop mid-September

Field 12 – Rye plowed early June replanted to vetch and clover mix early July

CS (Contour Strip) – Deep plowed bare ground on north side before April, carrots, beets, spinach and lettuce followed by rye late September; surface tilled oats and peas before early potatoes followed by oats and peas late August

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

We include our 2002 Crop Map and Field Notes in this column for serious students of cropping systems who really want to figure out how the cover crop-tillage combinations illustrated in the photo essay fit into the overall crop rotation. For example, a quick glance between the 2002 Crop Map and the 2001 version, found in the Spring 2002 SFJ, might reveal how the rotation alternates between cash crops and fallow lands, the fields fallowed in 2001 reducing weed pressure and rebuilding soil structure for the cash crops planted in 2002.

On top of this annual rhythm from fallow fields to cash crops and back again, the cash crops themselves rotate between those planted EARLY and LATE. This seasonal distinction determines the choice of fall cover crops in the fallow lands. Oats and peas, which reliably winterkill in our climate, precede EARLY cash crops in the rotation while overwintering cover crops, like the rye and hairy vetch or ridged rye featured in the photo essay, precede LATE crops in the market garden.

A close look at the Field Notes might indicate how we have tailored the cover crop-tillage combinations to the monthly planting windows within the broader designation of EARLY and LATE planted crops. For instance, in 2002, we planted:

  • the earliest cash crops on the ridges of winterkilled oats and peas in field 5 during April and early May
  • the next succession of early transplanted and direct seeded crops into the surface-tilled oats and peas in fields 1 and 9 throughout the rest of May and early June
  • storage potatoes in late May following a skimplowed sod of Italian ryegrass and clover in field 11 (more details in the next issue)
  • midsummer successions of lettuce, beans and zucchini the end of June/early July in field 3 following several discings of ridged rye
  • fall crops of broccoli, carrots, lettuce, spinach, kale, collards and napa into the surface-tilled rye and vetch in field 7 which held enough soil moisture to get these crops started during the dry, hot stretches in July and August.

If this list of cover crop-tillage combinations seems mystifying at first, consider that the guiding principle we used to develop them is really pretty straightforward. We simply try to find the cover crop-tillage combo that makes it easy to kill and shallowly till the cover crop six to eight weeks before the designated planting window.

This lead-time allows for adequate decomposition of the cover crop residues before planting the market garden crops. More importantly, this two-month delay between killing the cover and planting the cash crop usually provides an adequate opportunity to build up and preserve enough soil moisture from natural rainfall to get the cash crops off to a good start. In areas where no rain is likely to fall for more than two months, then the cover crop-tillage combinations would have to be tailored to provide an even longer moisture-rebuilding period before the planned planting window.

The handful of cover crop-tillage combinations we now use to try to insure a moist, fertile seedbed for each monthly planting window requires some rather specialized tools. We hope no one thinks that the “coultervator,” “ridger,” and “ridge-till cultivator” featured in recent issues are necessary to grow a good crop of produce. We got by just fine with a more limited selection of implements during our first years on the farm as described in “A Few Long Furrows” and the 52-minute video (see below).

On the other hand, we can’t help feeling that increasing the diversity of cover crop management options with the use of a few more antiquated tools has helped to make it possible to keep up with the growing demand for our fresh market vegetables despite the recent extremes in the weather. For those that do not have access to a wide selection of old horsedrawn or tractor equipment, we conclude the photo essay with a brief spotlight on a brand new disc suitable for tackling almost all of the cover crop-tillage combinations we now employ if it was equipped with a hydraulic lift.

For more background on the development of the bio-extensive rotation and each of its components, we have made available for $10.00 each ($15.00 in U.S. Funds outside of the country) a video and two booklets of articles (like the one mentioned above) about our farm system. The packaging and presentation may not be snazzy, which is why we can offer these information- dense materials so reasonably. Please send check or money order to Anne and Eric Nordell, 3410 Route 184, Trout Run, PA 17771.

Balancing Nitrogen And Carbon In The Cover Crop Mix

During the first ten years on the farm we really emphasized legumes in the cover crop mix in order to supply enough nitrogen for the cash crops in the rotation. Fortunately, the nitrogen-fixing legumes really thrived once we applied compost and lime. In our minds, using small amounts of farm-produced compost and purchased rock minerals to promote the growth of leguminous cover crops seemed like a more sustainable way to fertilize the cash crops with nitrogen than importing a lot of manure and bagged fertilizer.

We pushed this cheap nitrogen philosophy to the max before heavy feeding crops by planting two nitrogen-fixing cover crops back-to-back. Those of you who have seen the video, or read our early articles, may remember that, in the fallow year before heavy feeding LATE crops, we often followed a cover crop of leguminous sweet clover with an overwintering cover of rye and nitrogen-fixing hairy vetch. Using back-to-back legumes during those early years resulted in vigorous mid-summer and fall crops with a minimum of inputs.

After a decade of this practice, we observed a real difference in how the rye-vetch cover crop performed following the yellow blossom sweet clover. Instead of nice, balanced growth between the small grain and legume, the rye now completely dominated the hairy vetch. At the same time, we noticed that the nodules on the roots of the vetch were no longer pink inside, indicating that the bacteria in the nodules were not actively fixing free nitrogen from the air in the soil.

We assumed that the lush growth of the rye and the lazy nodules on the vetch meant that there was now plenty of nitrogen cycling in our biologically revived soils. In fact, the nitrogen-fixing sweet clover may have been producing a surplus of nitrate, causing the rye to grow rank and the n-fixing nodules to shut down on the vetch. At this point in the development of our soil, planting back-to-back legumes no longer seemed necessary, or even desirable.

Instead, we began planting high carbon, nitrogen consuming cover crops before the rye and vetch in the fallow year preceding LATE crops in the market garden to intentionally draw down the available nitrogen in the soil and to encourage the leguminous vetch to start fixing nitrogen again. For example, the 2001 Field Notes in Spring 2002 column indicate that we used a mix of rye, spelt and Italian ryegrass for the first set of fallow year cover crops in field 7. This mix produced several heavy clippings of high carbon biomass. By the time we plowed down this fallow field in mid-July, the Italian ryegrass looked pale green and nitrogen starved.

When we planted the usual overwintering cover crop of rye and vetch in field 7 in early August, the rye dominated the mix initially, but by the end of September the vetch was actually outgrowing the small grain. To the eye, it looked like a perfect 50-50 stand without changing the seeding rate. Using the mix of rye, spelt and ryegrass to intentionally deplete soil nitrogen during the first half of the fallow year seemed to give the leguminous vetch the upper hand in the fall.

As you may be able to see in the first two photos, this rye-vetch cover went on to produce a lot of biomass in the spring of 2002, the vetch vines twining almost to the top of the shoulder-high rye at the time of the first discing. What you can’t see underground is that the vetch’s root nodules were deep pink inside and actively fixing nitrogen.

By growing just one legume in this fallow year cover crop sequence, we have been able to bring the nitrogen status of the soil back into balance and reinvigorate the nitrogen-fixing capability of the hairy vetch once again. No doubt, the carbon-nitrogen balance in the cover crop mix will change again in the future along with the development of the soil’s metabolism. The important lesson for us is to realize that the soil system is dynamic and always changing, and that we need to keep adapting our management practices regarding carbon and nitrogen based on visible changes in the vigor of both the cash crops and cover crops.

Surface-Tilling Rye & Vetch Before Fall Broccoli & Carrots

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

1a. Discing rye and vetch in field 7 on May 24, 2002. Usually we like to start knocking back this live cover earlier in May when it is only waist-high and just starting to head out. However, we received some much needed rain at that time so tilling this overwintering cover crop did not get started until it had grown well over the team’s shoulders. To completely knock back the extra heavy topgrowth, we disced the half-acre field two times, in opposite directions, overlapping half of the disc’s width with each pass – a two-hour work session at a steady, but very manageable pace.

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

2a. The five foot disc, weighted down with a heavy wood platform and one skinny teamster, did a good job of flattening and killing the rye and hairy vetch. Keep in mind that the ability of this two-horse tool to kill the live cover is more a matter of biology than mechanics. That is, the tough rye is much more willing to die when fully headed out and approaching the end of its annual life cycle. Whatever the method or timing, the idea is to stop the cover crop from pulling more moisture out of the soil and begin mulching in the rainfall occurring over the 6-8 week period before planting the cash crops.

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

3a. To contend with the extra amount of cover crop biomass due to our late start, we decided to get out the “coultervator” introduced in the Winter 2000 issue of SFJ. This extra step with a specialized tool is not usually necessary if we make the initial discing a week or two earlier. However, using this modified subsoiler speeded up decomposition and made the discing more effective by throwing the soil and residues into low ridges.

A wide potato shovel on the coultervator’s single shank provides adequate ridging action despite working the ground only 4” deep. The coulter mounted at the front of the beam slices through the residue ahead of the shovel, preventing the long stems of rye and tangly vetch from hanging up and dragging on the coultervator’s shank.

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

4a. We followed the coultervator with the disc and the “ridger” a week or two later, then leveled field 7 with the spring tooth harrow and roller before spreading a superlite application of compost and lime the last week in June. Assuming that the rank stand of vetch would supply most of the nitrogen required by the crops, we decided to apply this light dusting of compost to serve as innoculant for the sheet composting process rather than putting it on by the ton as a fertility source for the cash crop.

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

5a. and 6a. After much trial and error, we discovered we could use the ground driven fertilizer spreader (also highlighted in the Winter 2000 issue) for spinning on a superlite coating of compost if we made a couple of modifications to both the machine and the well composted horse manure. First, we installed a lime agitator attachment in the hopper. This spinning, corkscrew-like device prevented the finished compost from clumping up and bridging in the hopper.

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

Next, we mixed the organic material with granular calcitic lime, five parts to one, by volume. Since our poorly buffered soils benefit from light applications of limestone, using this gritty aggregate to improve the flowability of the compost seemed like a win-win solution. At least, we learned the hard way, that without the agitator and granular lime, the compost would plug up the wide-open gates which regulate the flow of fertilizer from the bottom of the hopper on to the spinner.

If our calculations are right, we put down about 1-1/2 cubic yards (less than a ton) of compost to the acre and approximately 600 lbs. of granular limestone with this method. We then worked in this dusting of compost with another pass of the spring tooth harrow before forming the planting beds.

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

7a. We seeded fall carrots on July 6, three weeks after we had received the last soaking shower. We used the lister planting technique described in the Summer 2002 Journal to direct seed the carrots into the moist soil preserved under the surface-tilled rye and vetch. In this close-up you can see how the extra large sweep moves the surface layer of dry soil and cover crop residues to the sides while the narrow tooth marks a planting furrow for direct seeding the carrots in the exposed strip of moist earth.

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

8a. We received a good bit of rain beginning July 27, darkening the soil surface so you can see the uniform good stand of carrots well established during the previous dry, hot period. The broccoli transplanted directly into the mulch-tilled residues on the right also got off to a good healthy start and survived the next stretch of unusually hot, dry weather which lasted until the middle of September.

Cultivating Questions Winterkilled Cover Crops for a Mild Climate

9a. We cultivated the carrots twice before interseeding the pathways with a single row of rye on August 29. No hand weeding except for removing a smattering of sheep sorrel threatening to go to seed in the rows. Although the fall crops showed definite signs of stress during the extended heat wave, the surface-tilled rye and vetch held in enough moisture to keep the carrots and broccoli growing until the temperatures finally moderated the latter half of September. We harvested consistently nice heads of broccoli for the farmer’s market in September and October, and a good yield of November carrots sold mostly right-from-the-farm to individuals for winter storage.

To be honest, we ended up with a higher percentage of carrot seconds in field 7 than usual due to slug damage. The combination of the mulch-tilled cover crop and eleven inches of rain over the six weeks before the carrots were dug really brought in the slugs. Fortunately, there is a high demand for our No. 2 carrots, especially among the juicing crowd who rate sweetness over cosmetic perfection.

Also on the plus side, there was no observable washing in field 7 despite the abundant rainfall during October and November. We assume the lack of erosion was due to the good soil structure created by the rye-and-vetch root system and all the earthworms feeding on the surface mulch of residues. In fact, when a Cornell researcher, collecting data on our farm for the Northeast Organic Network (NEON), measured the infiltration rate of the soil in field 7 using a tool called the infiltrometer, he did not observe any runoff even when he applied simulated rainfall at a rate of 30” per hour for 45 minutes!