Weeds! Is there another word that causes such a groan for organic growers? I know as a first-year grower I am particularly interested in organic weed management methods. Especially since our market garden plot is currently a hay field!
The reality is that we will be dealing with hay popping up for years to come most likely, but we would like to cut down on the amounts coming through as much as we can.
I am learning as much about organic weed management as possible so that we can start practicing what we learn as soon as we can, not to mention keeping on top of other weeds that are bound to be there.
In this article, we will be learning from our friend Pam Dawling, author of Sustainable Market Farming for her expertise on sustainable weed management.
Sustainable weed management is about effectiveness, including removing weeds at their most vulnerable stage and ignoring weeds while they are doing little damage.
The idea is to work smarter, not harder.
Organic Weed Management
As market gardeners, we always strive to restore and maintain balance in the ecosystem. Developing strategies for preventing weeds and controlling the ones that pop up is part of maintaing balance.
One factor to consider is how vulnerable the crop is to damage from that weed at that time.
Weeds that germinate at the same time as veggie crops usually do not really affect the crop’s growth until they become large enough to compete for moisture and nutrients.
However, if allowed to go unchecked, these early weeds have the greatest potential for reducing crop yields.
You need to cultivate or otherwise control weeds before this 2-3 week grace period is over.
The most important time to weed a crop is from 2 weeks after sowing until the crop is halfway to being finished.
For vigorous crops like tomatoes, squash, and transplanted brassicas, this is 4-6 weeks; less vigorous crops like onions or carrots need weed-free conditions for 8+weeks.
During that time it is essential to control weeds to prevent loss of yield.
Weeds that emerge later have less effect, and ones that emerge quite late in the crop cycle no longer affect the yield of that crop, although their seeds give a reason for removing them to improve future crops.
Know Your Weeds
Learn to identify the major weeds on your farm, and any minor ones that may be trouble later.
Start a Weed Log with a page for each weed.
Add information about your foe’s likes and dislikes, habits, and possible weak spots.
Find out how long the seeds can remain viable under various conditions, and whether there are any dormancy requirements.
- it emerges
- how soon it forms viable seed (if an annual)
- when the roots are easiest and hardest to remove from the soil (if a perennial)
- what time of year it predominates
- which plots and crops have the worst trouble with this weed
Monitor regularly throughout the year, every year.
Look back over your records and see if anything you did or didn’t do seems to have made the problem worse or better.
Next, think about any vulnerable points in the weed’s growth habit or life cycle, or responses to crops or weather that could provide opportunities for prevention or control.
List some promising management options.
Try them, record your results, then decide what to continue or what to try next.
Most weeds respond well to nutrients, especially nitrogen. For example, if you give corn too much nitrogen, its productivity will max out and the weeds will use the remaining nutrients.
Sun-loving weeds like purslane are more likely to thrive among crops (like carrots and onions) that never cast much shade at any point of their growth. They won’t be a problem for crops that rapidly form canopies that shade the ground.
Most of the weeds in cultivated soils are annuals, but some of the worst ones are perennials, whether stationary (taproot) perennials like docks and dandelions, or wandering/invasive perennials with tubers, rhizomes, or bulbs (Bermuda grass, quackgrass).
Stationary perennials in their first-year act like biennials- leaves, roots, but no flowers or seeds.
In annually tilled areas, they get killed in one year and don’t often establish.
Wandering perennials are a more difficult problem, and understanding apical dominance is important in tackling them. More on this below.
Preventing Weeds From Germinating
Any technique that helps fill the growing space and the growing season continuously with thriving crops will leave less opportunity for weeds to germinate.
- Keeping soil fertility high, helping the plant canopy to close quickly, out-shading and out-competing weeds.
- Growing vigorous crops, providing the right soil conditions, row cover, or shade cloth if needed.
- Choosing locally adapted varieties with good disease and pest tolerance.
- Using crop rotations that switch between spring and summer crops, so that cultivation takes place at different times of the year.
- Utilizing drip irrigation (rather than overhead watering), leaving aisles relatively dry and inhospitable.
- Using mulches, especially for slow-growing or vertical crops such as garlic that has a poorer chance against broad-leaved weeds.
- Planting promptly after cultivation to give the crop the most advantage over sprouting weeds.
- Using stick seeders or easy-plant jab planters to sow large crop seeds into a seedbed that has already had the weeds removed (by flaming or stale seedbed technique), avoiding bringing new weeds to the surface.
- Transplanting, rather than direct sowing, to give the crop a head start.
- Choosing close crop spacing, leaving less space for weeds.
- Practicing rapid multiple cropping (where one crop is planted immediately after the previous one).
- Interplanting/relay cropping (where one crop or cover crop is planted in the spaces between the standing crop before it is finished).
- Planting cover crops wherever there are no food crops.
- Using no-till cover crops grown to maturity then rolled or mowed to create an in-situ mulch.
- Practicing reduced tillage/strip tillage, thus reducing the new weed seeds brought to the surface.
Reducing Weed Seeding
It’s not always possible to prevent weeds from germinating, so the next step with annual weeds is to remove them before they seed.
It is possible to reduce weed seed banks to 5% of their original levels when weeds are not allowed to produce seeds for five consecutive years.
This will require some energy and commitment, but may just be well worth it.
Techniques for controlling weeds before they drop seed include:
- Timely cultivation, when the soil is dry enough and the weather warm and/or breezy.
- Mowing, cutting the flower stems, prevents weeds from seeding.
- Grazing by cattle, chicken tractors, or goslings.
- Precision cultivation, whether its tractor-mounted or a good hoe.
- Close in-row spacing, and wider row spacing to provide the same planting density, means that most of the weeds will be between rows, easily wiped out by cultivation, rather than between plants in the row, where they are harder to remove.
- Using post-emergence organic weed killers such as corn gluten, or vinegar.
Reducing Seed Viability
Most weed emergence happens within 2 years of the seeds being shed. Not all seeds that are produced will ever get to germinate.
You can help reduce their chances by:
- Farmscaping: plant habitat areas to encourage seed-eating birds, insects, earthworms, mice, etc.
- “Grow” a healthy soil with abundant soil microorganisms to keep the biological activity high. This gives veggie crops, which benefit from mycorrhizal fungi, an advantage over some weeds.
- Mow the crop immediately after harvest (to prevent more weed seed formation), then wait before tilling to allow time for seed predators to eat weed seeds already produced.
If they do not get eaten, dry out or rot, seeds on top of the soil are more likely to germinate than most buried seeds, but long-lived seeds, if buried, may remain viable and dormant in cold dark storage for years, and any tillage during that time can bring them back to the surface, where they rapidly germinate and grow.
Avoid deep tillage if you struggle with these long-lived seed types of weeds.
On the other hand, small, short-lived seeds of weeds with no dormancy period will almost all die within a year or two if they are buried a few inches.
Till and mulch to bury short-lived weed seeds.
Stale seedbed techniques draw down the ‘wealth’ of the seed bank in the soil.
Prepare beds ahead of time, water them, maybe throw a row cover on them, to germinate a flush of seeds. Then remove the weeds by shallow cultivation or flaming.
Soil solarization can be used in hot weather for particularly difficult weeds. This kills anything in the top layer of soil that is unable to move out of the way.
Reducing The Strength of Perennial Weed Roots and Rhizomes
Perennial weeds can regrow from pieces of roots or rhizomes left in the soil after tillage.
Rhizomes are modified stems, usually growing underground, that are capable of growing new shoots and roots from each node.
When a rhizome grows a shoot, chemicals from that shoot prevent other nearby nodes from sending up shoots. This is called apical dominance – the node at the apex dominates other nodes into not sprouting.
These chemical messages get weaker over distance so that on long rhizomes, after a certain length, the dominance effect is too weak and another node can grow a shoot.
When rhizomes are cut into little pieces during tillage, the apical dominance is lost and each piece can grow a shoot.
This is not necessarily as disastrous as it sounds, because these are small, weak plants with only a small source of nutrients.
The danger is in walking away at his point and leaving them to grow.
If you cultivate again before the new shoots have grown enough to be sending energy back to the roots, or manually pull out the pieces to dry on the surface, the depleted pieces of root or rhizome may die.
Simply removing top growth whenever the weeds reach the 3-4 leaf stage can be quite effective in further weakening invasive perennial weeds.
This may need to be done several times at 3-4 week intervals to knock out a bad infestation.
It’s more effective to wait each time until the new top growth has drawn down the plant’s reserves (in the roots) before hoeing or pulling, than to go almost daily after every sprig.
Thickly planting buckwheat or other smothering cover crops immediately after tilling helps put extra pressure on the weed, and can reduce tilling passes.
Examples Of Sustainable Weed Management In Action
Galingsoga can be a troublesome weed for veggie growers, as it can produce seeds in as few as 30-40 days after emergence and its seed has no dormancy period. It sprouts after the nest cultivation.
Fortunately, the seeds are short-lived and have to be in the top ¼” of soil to germinate.
Strategies for annual weed management include:
- Inversion tillage such as moldboard plowing (seeds will die off in the soil within a year or so).
- Mulching – the seeds will not germinate or be able to grow through the mulch and will be dead by next year.
- No-till cover crops, with summer crops transplanted into the dying mulch.
- Stale seedbed technique, including flaming.
Nutsedge is a ‘wandering perennial.’ Their seeds can persist for 3-4 years deep in the soil, but if they are near the surface they germinate or die within three years.
The main sources of new plants are the small tubers that form on the roots and the rhizomes that grow in late summer.
Tubers need a month of cold dormancy before they start spring growth.
Strategies for perennial weed management include:
- Tilling in late spring and early summer after tubers have sprouted, but before new tubers or daughter plants from rhizomes have had a chance to form.
- Repeated cultivations are needed to kill all the young plants, and this can work with a late spring/early summer crop that needs frequent cultivation, such as sweet corn.
- Growing food crops that finish in early summer and follow by deep tillage to disrupt tuber formation, which mostly occurs in late summer.
- Maximizing soil fertility and making dense plantings of crops that can overshadow and outcompete the nutsedge may work.
- Sweet potato can suppress the growth of nutsedge, by releasing growth-inhibiting substances (allelopathy).
- Allelopathic cover crops such as rye (over-winter) and sorghum-sudangrass (through the frost-free growing season) could be used if the area can be taken out of crop production.
- Ducks or pigs are the actions of last resort for organic growers. The land will need to be taken out of veggie production while the livestock roots out the nutsedge.
Knowing and understanding the particular weeds that are giving you the worst problems enables you to design an approach that includes removing weeds at the most important point in their life cycle before they do their worst damage.
While focusing on your primary problem weeds, you can relax and ignore weeds that are not doing much harm.
I know some of this advice sounds counterintuitive, like just leaving some weeds be if they are not taking away nutrients from your veggies.
When I see a weed in my garden, I want it out right away, but on a market garden scale, you could be weeding all day every day if you want every single weed out of your garden.
Take the time and get to know your weeds so you can make the best choices as to the time to get rid of them and how to keep them away for good.
How do you practice weed management in your market garden? Let us know in the comments below.
Kathy & Jon
your friendly neighbourhood growers