Considerations for Weed-Free Strip Width

In perennial fruit crops, the orchard or vineyard floor is often divided into two distinct zones.  Within the planted row portion of the field, weeds are managed- typically with herbicides.  The between-row spaces or “row middles” consist of planted grass or native vegetation and facilitate the travel of equipment.  Determining where the two zones meet is intriguing to a weed scientist.

Weeds are most commonly defined as “plants out of place”.  The orchard floor is the perfect example of this definition.  When the same non-crop plant allowed to exist in the row middles encroaches into the weed-free strip, that plant becomes a weed (Figure 1).  As a rule of thumb, weeds closer to the crop compete more for light, water, and nutrient resources than those same weeds farther away.  But how close is too close?

 An Example:
In the interest of time, I’ll focus on results of research into how weed-free strip width influences thornless, floricane-bearing blackberries.
The generally accepted weed-free strip width for blackberry is 4 ft with 2 ft on either side of the row.
But the influence of weed-free strip width may differ between newly planted and established blackberries.
Results in newly planted blackberry:
During my time as a graduate student at North Carolina State University, I evaluated the influence of weed-free strip width on newly planted ‘Navaho’ blackberry.
Plants were started from 50 plug cell trays, and we established and maintained weed-free strips of 0, 1, 2, 4, 6, and 8 ft.
We found no influence of weed-free strip width on primocane or floricane number or size.
Of course, yields in the year following establishment were low, but there was a trend.  Blackberry fruit yield increased as weed-free strip width increased from 0 ft to 3 ft (Figure 2).  However, there was no benefit to increasing the strip greater than 3 ft.
Blackberry fruit weight increased as weed-free strip width increased from 0 to 8 ft (Figure 3).
With the exception of soluble solids content, which was greatest with a 0 ft strip, we observed no effects on blackberry fruit quality.

Results in established blackberry:
A colleague of mine, Nick Basinger, conducted a similar study with established, 5 year-old ‘Navaho’ blackberries, and reported that cane number, blackberry fruit per plant, and total blackberry fruit yield increased from 2 to 6 ft.  His findings suggest that increasing the weed-free strip width from 4 ft to 6 ft could result in more fruit and greater overall yields.
He also found a trend of increased fruit weight with increasing weed-free strip width, but very little influence on fruit quality.

 Is it possible to have a vegetation-free strip that is too wide?
The short answer is “yes”.  Excessively large vegetation-free strips in perennial fruits have been documented to result in increased vegetative growth, which may require additional labor in pruning or cane thinning.  Under some conditions, increasing weed-free strip width can also result in a decrease in fruit soluble solids content.  For fruit marketed for fresh consumption, the difference is minimal.  For wine grapes, a reduction in soluble solids may be more impactful to quality.

There are all kinds of other factors that influence vegetation-free strip width decisions.  Below are some of the common ones I have encounter when speaking with producers:
Risk of erosion.  Orchard sites on steep slopes may utilize smaller weed-free strips to reduce the risk of erosion from increased exposed soil.

Equipment size.  Row middle width can be increased or decreased to accommodate equipment.  The most frequent example I’ve been given regarding weed-free strip width and equipment relates to mower width.  For example, if an operation owns a 6 ft mower, the row middle width will often be 6 ft wide or less to avoid the need to make two passes.

Irrigation and soil type.  The impact of weed competition can be, in part, negated through increased irrigation.  Researchers investigating the impact of weed-free strip width on peach found that the strip width could be decreased if irrigation was increased.  On sites with coarse textured soils prone to leaching, it may also be necessary to increase fertilizer applications to replenish nutrients lost to increased irrigation.

Hand-harvesting crews.  Many fruit crops are harvested in the early morning when field heat is lower and dew is abundant in row middle vegetation.  Larger weed-free strips allow picking crews to keep their feet dry as they pick.