Watching the Waltz: Weed Seeds and Tillage

This article is part of a series if you'd like to read the others here they are: Part I Tillage, Soil Health, and Weeds: WSU Organic Transitions Project and Part III Watching the Waltz: Weed Seeds and Tillage.

Tillage is essential for termination of overwintering foliage (e.g., multifunction crops, weeds) and seedbed preparation, but can degrade biological and physical soil function, or health^1–3. Weed seeds are deposited on the soil surface at various times throughout the year and can be incorporated into the soil through tillage and cultivation activities. By this same action, previously deposited and incorporated weeds into the soil can be redistributed to the soil surface. Weed seeds are part of the weed seedbank that is dynamic and many driving factors, such as tillage, influence them through their impact on seed germination and survival ⁴. The density of seeds and the composition of species are primarily influenced by characteristics (soil and environment) interacting with management practices ⁵.

Role of Different Tillage Implements

Two studies ^6,7 found that tillage is the primary driver of vertical weed seed movement in fine-textured soils and others added that tillage implements differ in their impact on soil health indicators ^8,9 and weed seed distribution within the soil profile ¹⁰. Moldboard plowing more uniformly distributes weed seeds than chisel plowing ^11,12 or reduced tillage ¹³ in finer textured soils. More specifically, chisel plowing resulted in 63% of the weed seeds present in the top 2” of the soil. Another study found that chisel plowing increased the number of annual weeds when compared to moldboard plowing ¹¹.

One important factor is the association of specific tillage or cultivation activities/timings with specific crops and rotations. Crop rotations create an unstable environment for weeds by altering temporal patterns of resource competition, may have allelopathic effects on particular species, fluctuation of nutrient availability and timings, and inconsistent timing of soil disturbance ¹⁴. Additionally, rotations impact weed growth and prevent domination by a few weed species ¹⁵.

Vertical distribution and weed seed size

Weed seeds vary in size and shape and several studies have reported the impact of tillage on the distribution of varying-sized weeds ^6,16–18. Authors of these studies found higher densities of small-seeded species deeper (6-12”) in the soil profile when compared to larger seeded species in soil tilled with a moldboard plow. Other authors ¹⁹ contrasted these findings and found that moldboard plowing resulted in a higher density of small-seeded species in the surface layer and a lower density using a PTO-driven rotary harrow. One paper pointed to the differences between these experiments may have been the result of the initial density and composition of the seedbank ²⁰.

Influence of light on seed germination/dormancy

Soil disturbance introduces light into the soil profile. Even very brief light signals trigger the germination of seeds  ²¹. The mechanism by which seeds can sense light is influenced by soil temperature, soil water, and seed life history ²². Numerous studies have attempted to utilize germination biology by adjusting agricultural practices such as nighttime tillage ^23,24, but the results are inconsistent, and the authors of a review article ²⁵ point to 12 different factors that are complex and difficult for land managers to control. Additionally, having direct experience in evaluating nighttime tillage, it’s just difficult to do! Regardless, awareness of this fact can influence decisions that can be made such as tillage depth and tool selection.

Stale and False Seedbed Techniques

Two pre-plant weed management strategies deserve some attention when we discuss tillage and weeds. These two techniques largely differ in how they approach achieving the same end goal.

Stale Seedbed Technique

The first is the stale seedbed technique as shown in the video below (https://www.youtube.com/watch?v=m4Hd8Fh1X50). This technique is based on three principles: 1) soil disturbing activities such as cultivation promotes weed seed germination, 2) a small percentage of weed seeds in the soil are non-dormant and able to germinate at any given time and those that can, mostly germinate quickly, and 3) the majority of weeds only emerge from seeds in the shallow layer (<3.0 inches) of the soil, and most typically emerge only in significant numbers from the top one inch of the soil ²⁶. A field is prepared for planting with a normal series of events, but then it is left fallow for 7 to 14 days to allow for weed germination, but not too long as to allow for good weed establishment. Then immediately before planting/transplanting weeds are buried and uprooted through very shallow cultivation or flaming (flaming offers the benefit of true no disturbance). It is important to limit soil disturbance to avoid stimulating further germination of weeds ²⁷.

False Seedbed Technique

This technique is similar to the stale seedbed technique in that a field is traditionally prepared in advance of planting, but repeated shallow cultivations occur before planting/transplanting. Ideally, each successive cultivation is shallower than the last. This leads to the depletion of viable seeds in the top layer of soil.

In essence, the stale seedbed technique is based on the assumption that controlled, limited disturbance can lead to lower weed pressure early on in a crop’s life. While the false seedbed technique utilizes frequent shallow cultivations to remove all active weeds in the top layer of soil.

There are some factors to consider when deploying these techniques. First is soil condition, if the soil is too wet or clumpy shallow cultivation may be ineffective (based on the cultivation tool). Second, the timing of these cultivations is critical, especially for a false seedbed. Ideally, shallow cultivations target the white thread stage (Fig 4, https://eorganic.org/node/2596). Third, not all weed species will be effectively managed by these techniques. Some weeds have a very short time from germination to emergence while others have long emergence periods. Knowledge of field histories such as weed species present and density distribution across a field is essential to deploy many of these tactics.

References

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