When we cleared the slope for the orchard, we cleared a small area that had been choked with vegetation, including a number of invasive species, and dead, or diseased trees. This resulted in much more sun reaching the slope, and a significant amount of soil disturbance.
Since the deer fence was installed, and the trees were planted, we’ve battled a succession of pocket gophers, and meadow voles, invasive bull thistles, and French broom, but overall, a few years further on, the orchard is now thriving, the trees are maturing, and many of them are starting to set fruit.
This year though, we seem to be battling a new thug at the base of some of our fruit trees. Sheep Sorrel (Rumex acetosella).
Sheep Sorrel, also known as Red Sorrel, is a short-lived perennial herb in the Buckwheat Family (Polygonaceae) that is native to Eurasia, and is thought to have been introduced to the United States around the mid-late 19th Century. When it arrived isn’t really that important, as this common weed is now distributed throughout the United States and Canada, in fact throughout much of the Northern Hemisphere.
In California, Sheep Sorrel is most pervasive in Northern and Central California.
The California Invasive Plant Council (Cal-IPC) classifies Rumex acetosella as moderately invasive, as this species is capable of displacing native grasses by shading, and smothering. Sheep Sorrel is known to compete with important native host plants used by some endangered butterfly species in the Bay Area.  It is also suggested that Sheep Sorrel is non-mycorrhizal, and may potentially eliminate soil mycorrhizae in areas where large dense stands of plants occur.
Sheep Sorrel favors growing in moist, low-fertility, acidic, fast-draining soils, and is commonly found in open, sunny annual grasslands, and meadows. Sandy or gravelly soils are especially favored. It is frequently found in recently disturbed soils, and in areas that have recently been burned.
Some farmers cultivating Blueberries struggle with Sheep Sorrel as Rumex acetosella favors similar growing conditions. For some crops, the presence of Sheep Sorrel in soils may be used to suggest when soils need liming to increase soil pH, although liming alone is not sufficient to control Sheep Sorrel growth.
Needless to say, installing our irrigated orchard in the midst of a coastal mixed-evergreen woodland, on a recently cleared sandy slope, with acidic surface soils, has set up the perfect conditions for Sheep Sorrel growth here.
Sheep Sorrel is a relatively unassuming plant, that grows from 4 to 18 inches in height. Its distinctive arrow shaped leaves somewhat resemble a fleur-de-lis, and occur as a basal whorl beneath the flowering stems, although stem leaves are occasionally also observed.
Above the leaves tiny flowers appear on erect stems that branch at the tips.
Stems are ridged, and may be green or maroon-tinted, depending on stage of growth. Flowers may also be green or maroon-red in color.
The flowers darken, and become more red, with age.
Rumex acetosella reproduces both by seed, and from creeping rhizomes. A single plant can reportedly produce up to 1,600 seeds, which are rust-brown in color, and may remain viable in undisturbed soils for decades.  Needless to say, this is NOT a plant to add to the compost pile!
Seeds may be consumed, although not digested, and subsequently distributed, by birds and small mammals.
One recommended method of Sheep Sorrel control is hand pulling, and up until now, this is the method we’ve employed. It’s laborious, and requires frequent revisiting to an area to keep growth in check. The challenge with hand pulling, other than being labor intensive, especially for large stands, is that any root and rhizome fragments left behind have the potential to resprout. Pulling also disturbs surface soils, and may encourage germination of dormant seeds. As such, hand-pulling is usually only recommended for small stands, and young plants. Mature plants can develop extensive fibrous root systems making complete and effective hand removal difficult.
It’s fair to say that mechanical removal has not worked for us. In fact, the problem is significantly worse than it was a couple of years ago. When I first found Sheep Sorrel in the orchard 2 years ago, the patch of growth was less than a square foot, immediately adjacent to a recently planted apple tree. I set to work to remove it, and repeatedly pulled new growth as it appeared. Despite attempting to hand pull it numerous times since then, it has continued to resprout, and spread, with a vengence, and the patch is now approximately 150 square feet in total, over two areas, and continuing to spread. Clearly we need an alternate strategy for control.
Although we have goats, grazing is not recommended due to the high oxalate concentrations in Sheep Sorrel, which can be toxic to some species of livestock, and wildlife, when consumed in large quantities. Our goats are well fed, so it’s unlikely a little grazing would bother them, but Sheep Sorrel isn’t particularly palatable to livestock. As our goats are spoiled, they’d have absolutely no incentive to give these weeds so much as a sideways glance. That’s not a bad thing though, as much like hand-pulling, grazing can also result in soil disturbance, encouraging dormant seeds to sprout, and result in seed, and rhizome, redistribution.
Deer, however, are known to browse this species, and it’s possible our robust deer populations have been helping to keep this weed in check outside of the orchard, as thus far we’ve only found it growing within the perimeter of the deer fence.
Repeated cultivation can also reportedly control Sheep Sorrel as continued removal of top-growth will eventually starve the roots.
Herbicides are frequently, and successfully used to control Sheep Sorrel, but as we don’t use herbicides in our orchard, we need to find alternate means of effective organic control.
A study regarding suppression of weeds around organic highbush blueberry plantings suggests that some mulches may successfully suppress the growth of Sheep Sorrel, including manure-sawdust mulches, both through mechanical suppression due to mulch thickness, and also through alteration of surface soil pH. 
As we frequently source large quantities of such material from a nearby stable to augment our own compost with, this is a method we’re interested in trying, providing it doesn’t provide too much cover for voles of course. We don’t mulch extensively here, primarily to reduce cover for rodents, but as the Sheep Sorrel is relatively contained at the moment it might be worth trying to smother it. Hand pulling around the trunks of the trees though will still be necessary, especially around this Honey Crisp apple, which is where this problem originated.
Our strategy will be to remove the existing plants as best we can now the weather is warm and dry, and the rains have ceased for the season. After cultivation, we’ll leave the area exposed, long enough for the surface soil to dry out thoroughly, so any disturbed roots left behind will dessicate. Cultivation during wet weather, or in damp soils, can encourage rapid regrowth.
We’ll then apply the mulch generously to the areas in the orchard where the Sheep Sorrel is currently gaining a stronghold. We won’t eliminate growth, but even if the rate of spread is slowed, and the mulch eases the removal of the plants when young, it may help us to finally get on top of this problem before it really spreads out of control.
Maybe then we’ll have time to develop a strategy for outsmarting our exploding squirrel population, before they abscond with the remaining crop of Apriums this spring!
 Plant Assessment Form for Rumex acetosella. California Invasive Plant Council.
 Weed of the Week ‘Sheep Sorrel’ (Rumex acetosella L.)
 Burkhard, N. and Lynch, D. 2009. Organic Mulch Impact on Vegetation Dynamics and Productivity of Highbush Blueberry Under Organic Production. HortScience vol. 44 no. 3 688-696.
 Fitzsimmons, J.P., and Burrill, L. C. 1993. Red Sorrel (Rumex acetosella L.) Pacific Northwest Extension Publication. PNW 446. March 1993.