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Cultivating Questions Weed the Soil Not the Crop

Cultivating Questions: Weed the Soil Not the Crop

A Whole Farm Approach to the Weedfree Market Garden

by Eric and Anne Nordell

We really appreciated Eliot Coleman’s encouragement in the Spring 2005 SFJ to go beyond the “shallow” standards of the federal organic law and reclaim the original vision of organics. In his thought provoking article, “Can Organic Farming Save the Family Farm,” he coined the term “deep organic farming” to describe the from-the-soil-up basis for producing truly nutritious and delicious food.

Although we continue to certify our farm for educational and marketing purposes, we have always set our own standards for conscientious farming. Many of our personal guidelines for sustainable stewardship fall right in line with Eliot’s suggestions for deep organics. Whenever possible we use a “cause correction” approach to problem solving instead of treating the symptoms. We have also tried to design the bio-extensive market garden to “mimic the patterns of the natural world’s soil-plant economy.”

That’s our opinion. We fear that readers of this column would argue that we practice “off-the-deep-end” organics. For sure, the recent CQ topics on “nose culture, “remineralizing the web-of-life,” organic no-till garlic without irrigation, the “grace and beans” of farm economics, even “triple mulch vegetables,” must seem like the beyond-the-beyonds of organic agriculture.

Certainly, these off-the-deep-end ideas are not necessary for a successful vegetable operation. They may even be misleading or unrealistic for new and transitioning farmers. For example, this past year we talked to several market gardeners who had experimented with some of our recent innovations in reduced-tillage vegetable production before putting in place the basics of the bio-extensive system. Needless to say, the results were not entirely satisfactory, in a large part due to complications with weed management.

With this reality check in mind, we thought this might be a good time to join the SFJ back-to-the-basics movement by providing a summary of our whole farm approach to weed-free market gardening. The following mini-manual on “weeding the soil” we put together for MOFGA’s “Rotations and Weeds” seminar in February. We have also added this updated overview to our collection of SFJ articles on bio-extensive weed control listed in the Resources section at the end of the article.

For beginning farmers already overwhelmed by all the new terms and tools of the trade, we hope you can skip over the details and grasp the main principles. Really, our “cause correction” approach to weed management is just common sense: resting the land; thinking a year or two ahead; understanding the life cycle of the cover crops and the weeds; preventing weeds from going to seed. That’s the deep stuff. All the rest is fluff, which may or may not apply to the unique requirements and character of your farming adventure.

Many sustainable growers subscribe to the philosophy of “feed the soil, not the plant.” Our whole farm approach to weed management follows the same line of thinking – we call it, “weed the soil, not the crop.” Instead of relying on the cultivator or the hoe to save the crop from the weeds, we use cultural practices, including cover cropping, bare fallow periods, rotation and shallow tillage, to reduce the overall weed pressure in the soil.

One result of this proactive strategy is we no longer depend on the cultivator or the hoe to grow certified organic produce. “Weeding the soil” has also enabled us to use reduced tillage and living mulches without compromising weed management.

Weedfree conditions did not happen overnight. It took five years — and a good bit of patience and observation — to see a dramatic reduction in weed pressure. Successfully weeding the soil also required all five of the following principles and practices.

Deterring Certain Types of Weeds by Changing the Soil Environment

We fallow half of the market garden each year for the benefit of the vegetables and the detriment of the weeds. Giving the land a rest from vegetable production somehow changes the soil biology so that “cultivated” weeds are less likely to grow. We can enhance this “rotation effect” by utilizing mature cover crops in the fallowlands to discourage two major types of weeds.

For example, one set of weeds seems to flourish where the soil is in poor condition due to crusting, compaction or lack of organic matter. It is almost as if nature sends in the weeds to repair the damage. Cover crops can serve the same purpose by fiberizing the soil surface with fresh additions of organic matter and by rebuilding soil structure through the aggregating effect of their extensive root systems. The key is devoting a full fallow year to cover cropping in order to put these “soil building weeds” out of business before planting the vegetables.

Cultivating Questions Weed the Soil Not the Crop

By contrast, a soil management program that is based primarily on animal manure or compost may lead, over the years, to the excessive or imbalanced fertility levels, and the high rates of nutrient release, that favor “high fertility weeds.” In this case, nature seems to be using these well-adapted plants to mop up the surplus nutrients and prevent them from polluting the environment.

Overloading the soil with quickly available fertility is generally not a problem when relying on mature cover crops to maintain soil quality. That is because the well developed cover crops recycle nutrients already existing in the soil and release this fertility relatively slowly as their residues decompose. Conditioning the soil with a fallow year of cover crops discourages both soil building and high fertility weeds from growing.

Using the Bare Fallow to Reduce the Number of Weeds in the Soil

Our goal is to grow two mature cover crops in each fallow year preceding vegetable production. Typically, we grow one cover crop in the spring and one in the fall. Between the first cover crop and the second is a window of opportunity to use tillage to reduce the number of weeds in the soil. The length of this bare fallow period, and the type and intensity of tillage required, depends on the life cycle and growth characteristics of the most pressing weeds.

Perennial Weeds

We have found the most effective way to set back deep-rooted perennial weeds is to use the moldboard plow to sever the taproot deep in the ground and to completely bury the weed’s crown. Usually little follow up tillage is necessary to finish off taprooted weeds, like dock, dandelions and thistles, if we plow them down deeply at the weakest point in their life cycle, that is, when these perennials are approaching maturity but before they set viable seed.

For rhizomonous perennial weeds, like quackgrass, shallow plowing or chiseling is preferable to deep moldboard plowing. The idea is to bring the rhizomes up to the surface of the soil to dry out in the sun rather than burying the tenacious roots deeply in the earth out of reach of most tilling equipment.

Cultivating Questions Weed the Soil Not the Crop

Generally it is necessary to work the fallowlands every 10-14 days to completely dehydrate the rhizomes and prevent them from rerooting. Secondary tillage tools which have a lifting and shaking action are ideal for this job, such as a springtooth harrow with closely spaced teeth or a field cultivator equipped with wide sweeps. These tools will be more effective if a chain link style of flexible pasture harrow is attached to the back to thoroughly shake the soil off of the uprooted sod clumps and to lay the rhizomes on top of the soil. Cutting tools, like a disc or a rotovator, tend to multiply rather than kill the rhizomes.

Even in wet weather, “the summer fallow” can be used to weed the soil of rhizomonous perennials as long as cultivation is repeated frequently enough to prevent the weeds from growing. Slowly but surely, this method will deplete the energy reserves of the rhizomes.

Keep in mind that two to three months of tillage may be necessary to finish off the quackgrass or other rhizomonous weeds. For this reason, good soil conservation practices should be considered before attempting an extended bare fallow in order to prevent wind or water erosion. Likewise, it is also important to establish a thick cover crop at the end of the summer fallow to recondition the soil, provide protection overwinter and smother out any surviving rhizomes.

Annual Weeds

Eradicating perennial weeds may require long periods of tillage, but, if done properly, the extended summer fallow is usually a one-time affair. Weeding the soil of annual weeds, on the other hand, requires much less tillage but may take a number of years as their seeds remain viable in the soil for a long time.

The fallow year management makes it possible to speed up the process in two very different ways: first, the intensive use of cover crops increases biological activity and the natural decay rate of annual weed seeds in the soil; second, the bare fallow period can be used to intentionally germinate and kill several generations of annual broadleaf weeds.

Our tillage strategy for the annual weeds is just the opposite from the perennials. Instead of loosening and drying the soil to dehydrate the roots, our goal is to produce a firm, moist seedbed in order to sprout the small seeded annuals. This means settling the soil with a disc, harrow and/or roller as soon as possible after incorporating the first cover crop of the fallow year. If it is practical to work the cover crop shallowly into the surface of the soil, so much the better as this practice will help insure adequate moisture for germinating the small seeded weeds while creating a mulch to protect the soil.

Just as soon as the annual weeds sprout, we shallowly cultivate the fallowland, in order to kill the weeds before they have a chance to get well established. After making sure that the young weedlings have died, we refirm the soil with a roller or cultipacker to germinate another batch of weeds. We repeat this process every 10-14 days over the course of the six week bare fallow period which usually begins the end of June and concludes with the planting of the second fallow year cover the second week of August. This procedure will effectively remove several generations of warm season annual weeds, like pigweed and lambsquarter, from the top two inches of the soil.

Shallow Tillage

We take advantage of the bare fallow period midsummer to reduce the seed bank of annual weeds in the surface of the soil. If we till deeply the next year before planting the cash crops, new weed seeds will be brought to the surface and grow with the vegetables. Therefore, it is critical to limit tillage to the top 2-3″ of the soil when preparing a seedbed for the produce.

We facilitate shallow tillage by segregating EARLY planted crops and LATE planted crops in different fields. Rotating between EARLY and LATE vegetables transforms the every-other-year fallow system into the four year, four field rotation shown in the chart.

Before EARLY vegetables, established in April and May, we use a cover crop that reliably dies back over winter. The dead top growth and root system of a “winterkilled cover crop,” such as oats and Canadian field peas, makes it possible to use shallow tillage early in the spring to prepare a seedbed in time for planting the first market garden crops of the season.

We often use a lightweight disc to chop up the mulch of winterkilled cover crop residues into smaller pieces before undercutting the spring weeds and forming the planting beds with a field cultivator equipped with large, widely-spaced sweeps. Other tools which can handle a lot of residue may work just as well for this job as long as tillage is restricted to the surface of the soil to prevent bringing up new weed seeds to germinate with the EARLY planted vegetables.

Cultivating Questions Weed the Soil Not the Crop

In preparation for LATE vegetables, we can take advantage of the soil building attributes of an overwintering cover crop, such as rye and hairy vetch, because there will be plenty of time to kill and breakdown this live cover with shallow tillage before these LATE planted crops go in the ground. We actually use two different types of shallow tillage to accommodate the extra long planting window for LATE vegetables and the life cycle of the overwintering annual cover crops.

For LATE crops planted in June, we begin shallowly tilling the live cover as soon as conditions permit the first part of April. At this time of year the overwintering cover is just beginning its vigorous, vegetative phase and can be very challenging to kill with a disc or field cultivator. We find it more efficient and effective to completely undercut the overwintering cover crop by moldboard plowing as shallowly as possible, no more than 2-3″ deep.

Cultivating Questions Weed the Soil Not the Crop

Keep in mind that the “skim plowed” sod may require 5-6 weeks, and several passes with the disc or harrow, before it is sufficiently decomposed for planting the LATE vegetables. The six week decomposition period provides another opportunity to use tillage for weeding the soil by intentionally germinating and killing a generation or two of annual weeds. The six week delay also allows plenty of time to finish off any remnants of the live cover that survive skim plowing, and for the pathogenic and allelopathic byproducts of decomposition to subside before planting the June vegetables.

Cultivating Questions Weed the Soil Not the Crop

For LATE crops established in July and August, we let the overwintering cover crop grow until it is waist high the third week of May. At this point in the cover crop’s life cycle its energy has shifted from vegetative growth to reproduction by pushing seedheads and starting to flower. This close to maturity, we can kill the overwintering annual cover crop simply by knocking it down and crimping it with a disc or a roller.

Again, we plan on at least six weeks, and several passes with the disc or field cu1tivator, before planting the LATE vegetables in July and August. In this case, the six week delay is necessary for all the extra biomass to breakdown and to restore the soil moisture that the live cover crop has removed from the ground. While the primary objective of shallow tillage is to prevent bringing new weed seeds up to the surface, it also works against the nature of two different kinds of weeds. Restricting tillage to the top 2-3″ of the soil discourages soil building weeds by preserving the soil structure created by the cover crop’s root system and by improving soil tilth through the concentration of the cover crop’s above-ground biomass in the surface of the soil. Shallow tillage also deters weeds that prefer anaerobic conditions by keeping the decay process close to the surface where the soil is more likely to be warm and well aerated.

Cultivating Questions Weed the Soil Not the Crop

Rotational Cover Cropping

Tailoring the cover crops to the planting dates of the vegetables sets in motion the four year cover crop rotation depicted in The Cover Crop Clock. Note the significant amount of time devoted to soil building cover crops, high1ighted by the gray shading. We emphasize small grain cover crops because these inexpensive, weed competitive covers are easy to establish, grow well on minimal residual fertility and their fibrous root systems and topgrowth seem particularity well suited to conditioning our crust prone silty soils.

The dark shaded areas in the clock represent periods of tillage before planting the vegetables, after harvest and during the bare fallow. Alternating between the fallow year cover crops and EARLY and LATE vegetables creates a tillage rotation that interrupts the life cycle of many different kinds of market garden weeds on a regular basis.

The chart located below illustrates several variations on the original four year cover crop rotation, such as adding clover at different places in the rotation to provide more nitrogen and improve insect habitat management, or shifting the bare fallow period to the start of the fallow year to weed the soil of cool season weeds. We only mention these variations here to emphasize the importance of adapting the rotational cover cropping plan to the inevitable changes in insect and weed pressure that develop over time.

Cultivating Questions Weed the Soil Not the Crop
Variations on the Original Cover Crop Rotation

Keeping New Weeds Out Of The Soil

Weeding the soil with cultural practices will not be 100% successful if weeds are allowed to set seed or new weeds are introduced into the fields. For this reason, we back up the long term cultural strategies of cover cropping, rotation, bare fallow periods and shallow tillage, with short term preventative practices like cultivation, hand weeding, composting and mowing

For example, we make an effort to mow the headlands, farm lanes and waste areas bordering the market garden frequently enough to prevent the grasses and weeds from going to seed and blowing into the vegetable fields. We also mow the spring cover crops in the fallow fields two or three times before plowing them down to begin the bare fallow period midsummer. Clipping the topgrowth whenever it reaches 2 to 2 1/2′ tall makes it much more manageable to incorporate the biomass with a plow or a disc while preventing winter weeds from making viable seed before turning under the cover.

We have found that timely mowing of the spring cover crops is especially effective for controlling weeds in the mustard family. However, for low growing winter weeds, like chickweed or dandelions, plowing under the cover crop before the weeds are in full bloom is much more effective than repeated mowing.

Another important aspect of keeping the cover crops weedfree is using clean seed. We make sure to clean the small grains purchased direct from local farmers with our old, hand-cranked fanning mill. Cover crops purchased from commercial seedhouses should come with labels verifying no more than .05% weed seed and preferably less. Even with these precautions, it is important to scout the cover crops regularly for weed surprises, and to either remove the weeds by hand before they go to seed or to turn under the cover crop prematurely, whichever is more efficient in terms of labor and time management.

New weed seeds can also be introduced to the farm in mulching materials, feed, manure and bedding. Consequently, we make a point of finding weedfree sources of straw mulch and feed, and we compost the small quantities of horse manure we use to enhance the growth and decomposition of the fallow year cover crops

Actually, we think composting reduces weed pressure in two significant ways. First of all, aerobic composting kills most weeds passed through in the manure or bedding. Just as importantly, the composting process stabilizes the nutrients in the manure so that when spread in the fields the nutrients are released relatively slowly. In this respect, compost, in moderation, is much less likely to stimulate high fertility weeds to grow than raw manures or fast acting, water-soluble fertilizers.

Although we no longer need the cultivator or the hoe to grow certified organic produce, mechanical and manual weed control was essential for growing good crops during our first years on the farm. Cultivation and hand weeding were also critical for preventing weeds from going to seed in the vegetables. Looking back, it would appear that “weeding the crop” played a supporting role in “weeding the soil.”

To be truthful, we still cultivate most of the crops in the market garden at least once or twice before seeding a living mulch in the pathways. However, our primary objective for stirring the soil is moisture preservation, not weed management.

We also continue to walk the vegetable rows as necessary to remove any weeds that threaten to make viable seed even if they are not interfering with the growth of the crop. This proactive task becomes much easier and faster — even recreational — after reducing overall weed pressure through the use of rotational cover cropping and the bare fallow periods.

Implementing The System

When we started farming together here in the mountains of north-central Pennsylvania in 1983, we had to put together the weed-the-soil system from scratch. Neither one of us had any experience growing vegetables for market, although we both had worked on farms where cultural practices were employed for managing the weeds.

We used an extended summer fallow during the first year on the farm to get rid of the quackgrass and other perennial weeds that infested the market garden site. Then we implemented the rotation of cultural practices illustrated in The Cover Crop Clock. In one four year cycle, we virtually eliminated soil building weeds like plantain and Queen Anne’s lace as well as high fertility warm season annuals like lambsquarter and pigweed.

Depending on the condition of the soil and the number and types of weeds in the seed bank, it may take two or three times through the rotation to see such a dramatic reduction in weed pressure. On the other hand, we know of growers who have reduced handweeding in onions to just a few hours per acre within two to three years of implementing this system

We believe there are at least three reasons why these experienced farmers were so successful at using this whole farm approach to weed the soil. First, they had already acquired the skills and the equipment to execute the cultural practices effectively. That is, they had the means and the ability to establish thick, weed competitive cover crop stands, to manage the bare fallow periods properly to reduce the weed seed bank in the soil, and to create suitable seedbeds for vegetables using shallow tillage.

More importantly, these growers possessed the discipline to take land out of production, foregoing short term returns in order to realize the long term payback of a biologically efficient system. One farm couple who operates a 950 share CSA calculated that they save $3,000 an acre by taking land out of production. Their estimate represents savings in labor for cultivating and weeding as well as a substantial reduction in disease and insect management inputs. It also includes the dollar value of the nitrogen and organic matter provided by the fallow year cover crops.

Along with the necessary discipline and skills, these experienced farmers had developed the insights and wisdom to adapt the details of our system to the site-specific requirements of their own operations.

  • Soil type, condition and fertility levels.
  • Climate, rainfall patterns, and the length of the growing season.
  • Land configuration and access.
  • Irrigation.
  • The life cycle and nature of the local weeds.
  • The planting dates and cultural requirements of the crop mix.

All of these factors will influence the design of the rotational cover cropping plan and the orchestration of cultural practices for weeding the soil.

If implementing the system sounds daunting at first, keep in mind that fallowing half of the market garden each year provides a lot more time and space for trying out new ideas and making mistakes than growing vegetables intensively year-in and year-out. Psychologically, we found everything became so much easier when we set aside a whole growing season just for building the soil and reducing weed pressure.

Cultivating Questions Weed the Soil Not the Crop
EARLY PLANTED ONIONS the end of June, 1991, the second time through the four year cover crop rotation. We had cultivated the onions a few times but had not done any hand weeding in the rows at this point. Low weed pressure was due to “weeding the soil” during the previous fallow years by using mature cover crops to build soil structure so the weeds do not have to perform this important job, and by using a bare fallow period midsummer to reduce the weed seed bank in the soil. The fallowlands on either side of this onion field are preparing weedfree conditions for the next year’s vegetables. We have mowed the rye cover crop in fallow fields several times in preparation for plowing the cover down and initiating the bare fallow as depicted in the photo of “Deep Plowing” below.
Cultivating Questions Weed the Soil Not the Crop
SHALLOW TILLAGE is critical to prevent bringing up new weed seeds to grow with the vegetables. For EARLY planted crops, like the onions to the left, or these alternating rows of spring spinach and peas, we use a winter-killed cover crop of oats and Canadian field peas to facilitate shallow tillage. Working the dead residues into the surface of the soil creates a mulch effect which minimizes erosion, improves infiltration, and slows down evaporation.
Cultivating Questions Weed the Soil Not the Crop
We also use SHALLOW TILLAGE for killing an overwintering cover crop of rye and hairy vetch the third week of May, six weeks ahead of planting LATE vegetables in July and August. At this stage in the cover crop’s life cycle, it is easy to kill by knocking it down and crimping it with a small disc. Restricting tillage to the top 2-3” of the soil prevents bringing up new weed seeds to the surface, preserves the soil structure created by the cover crop’s root system, and enhances tilth by concentrating the coarse organic matter into the soil surface.
Cultivating Questions Weed the Soil Not the Crop
LATE PLANTED VEGETABLES following the disced down rye and vetch shown to the left. The six week decomposition period between killing the overwintering cover crop and planting the fall broccoli, carrots and kale in July provides enough time for all the coarse organic matter to break down and to restore the soil moisture the live cover crop removed from the ground. Weed pressure was so low we seeded a single row of rye in the pathways for erosion control and never did any hand-weeding in this half-acre field. In the fallow field to the right, we seeded a cover crop of rye and vetch the second week of August following a six week bare fallow period in preparation for the next year’s LATE planted vegetables.
Cultivating Questions Weed the Soil Not the Crop
SKIM PLOWING is the ticket for killing an overwintering cover crop of rye and vetch the beginning of April in preparation for June planted LATE vegetables. The six week delay allows adequate time for the shallowly inverted sod to decompose and provides another opportunity to weed the soil by intentionally germinating and killing a batch or two of annual weeds before planting.
Cultivating Questions Weed the Soil Not the Crop
We reserve DEEP PLOWING for setting back deep rooted perennial weeds during the fallow year like the dandelions, plantain and volunteer clover in this mowed cover crop of rye. Moldboard plowing severs the tap roots deep in the ground while completely burying the weed’s crown. After plowing down the rye around the end of June we settle the soil with a harrow and roller to produce a firm, moist seedbed to intentionally germinate and kill several generations of warm season broadleaf weeds during the six week bare fallow period. The bare fallow ends with the planting of the second fallow year cover crop the second week of August. NOTE: Mowing the first fallow year cover crop of rye several times over the course of the spring pretty much kills the cover crop and creates a dead mulch of 18-24” long residues which is a lot easier to incorporate with a plow or disc than tackling 6’ tall standing rye.