Application of Oyster Shell Mulch for Lavender Production

Final Report for FW01-052

Project Type: Farmer/Rancher
Funds awarded in 2001: $6,000.00
Projected End Date: 12/31/2004
Region: Western
State: Washington
Principal Investigator:
Mike Reichner
WSU Coop Ext.
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Project Information


Purple Haze Lavender Farm applied oyster shell mulch to commercial lavender with the goal of increasing yields of flowers and obtaining a higher grade of oil produced from those flowers. The study showed no significant difference in the quality of essential oil produced on mulched vs. non-mulched plants. The mulch did, however, reduce weeding between plants, which cut labor costs, and appears to be an improved medium to propagate seedlings resulting from cross fertilization.

Lavender, a perennial flowering herb, is marketed both fresh and dried to the wholesale florist and gift shop industry. Flowers are distilled using steam extraction, and the resulting oil is used in perfume, cosmetics and soaps, and has medicinal applications in aromatherapy. In the Sequim Dungeness Valley, lavender is grown on more than 35 small farms, generating income from agri-tourism and the sale of bundles, sachet and value-added products. The floral industry demands a high quality product, graded on color, size and fullness of blossoms, and the aromatherapy industry grades the essential oil on its constituent profile as shown in Mass Spectra Analysis.

The hypothesis was based on a study that used white sand as mulch for lavender flowers, which resulted in increased flower production. Anecdotal observation showed that crushed oyster shells applied to Purple Haze Lavender Farm’s demonstration beds resulted in more blooms per plant. Under the hypothesis that increased light would increase flower production and floral esters in the distilled oil, the farm sought a local, inexpensive white mulch that would increase light reflection to the plant. Oyster shells, plentiful as a byproduct of the aquaculture business on the Olympic Peninsula emulated the limestone soils of the regions of France where lavender is indigenous.

In the spring of 2001, a 4-inch mulch of washed oyster shells was applied, using a front-loader tractor, to the base of plants to the drip line on two field test plots on the commercial farm. In one plot, the lavender variety Lavendula angustifolia, or Royal Velvet, was selected to measure the effects of mulch on the quality of essential oil extracted from the flowers. The second plot of Lavendula intermedia, or Hidcote Giant, was selected to measure the effect on the quantity of flower production.

The first year’s test resulted in no significant change in oil quality in Red Velvet and a slight increase (0.8 bundles per plant in mulched rows vs. unmulched) in flower quantity in Hidcote Giant. In the second year, the mulch was extended from the drip line of the plant to cover the entire row area between plants, the idea being that a greater increase in reflective surface around the plant would result in an even greater increase in flower production.

The results of the second year were unexpected. Not only did the mulching not increase the number of bundles, it appeared to have had reverse effect, actually lowering the number of bundles per row. In searching for an explanation, it was postulated that the effect could have resulted from an unusually hot, dry summer in 2003 or from aging and the attendant decline of flower production.

The decreased in flower production was tempered however, by secondary results from the study:

1. The oyster shell mulch proved to be excellent mulch for weeds, reducing labor costs because hand weeding and tilling were eliminated. The integrity of the soil profile was also maintained.
2. In the demonstration beds where plants are mulched with oyster shell and watered with overhead sprinklers, a proliferation of young seedlings germinated through the mulch. Transplanted into 4-inch pots, the seedlings generated an unexpected profit from the sale of around 200 plants that germinated on an area less than 100 square feet.
3. The rows of white shells provided a striking contrast to the lavender colors of the bloom, attracting the attention of thousands of visitors to Purple Haze Lavender Farm. This not only reinforced the importance of educating potential lavender growers on the alkaline soil preference of lavender, it exposed visitors to the SARE grant process.

1. Labor costs for weeding and cultivation were reduced over two years.
2. Unlike sand mulches, which required yearly application, oyster shell mulch remained on the surface.
3. No lime or bone meal was required to amend the soil.
4. The local elk herd of 70 or more head, which previously traveled through the field damaging plants and irrigation lines, avoided fields with oyster shells.
5. With the use of a renewable resource, oyster shells, the project established a positive relationship between aquaculture and agriculture.

On the negative side, because of the expense of transport, oyster shell as a mulch is limited to coastal farms. Further, as the grant progressed, oyster farmers discovered that what was once a low value byproduct now had market value, and the cost and availability of shells has changed.

Robin Hill Farm, a demonstration farm in Clallam County maintained by Cooperative Extension, applied oyster shell to its demonstration beds, and farmers in the Dungeness Valley are awaiting results to determine whether they will proceed with shell application. Purple Haze will continue to collect data on bundle production on Hidcote Giant to assess whether the dry, hot conditions caused the reduction in flower production. In addition, Purple Haze will continue to apply shells as much as they are available and affordable, the savings in labor costs and improved aesthetics warranting the investment.

Producers in the valley are experimenting with a variety of mulches. Purple Haze will present its results to the Sequim Lavender Growers Association (more than 35 member) to compare results with other approaches.

The field test resulted in many questions: How does the application of oyster shells affect soil pH? Does the mulching, which increases light intensity, also increase heat units? Does mulching change the soil temperature? Is there an optimum depth of oyster shells that would achieve desired results without restricting the irrigation lines? And how would mulch affect the growth and harvest of Mediterranean herbs like oregano, thyme and rosemary?

Purple Haze has received local and national press about the farm and SARE grant, and thousands of visitors have seen the signed test plots and heard about the grant during walking tours and festivals. The Reichner’s have spoken to dozens of garden clubs in Washington, discussing the grant objectives in each presentation. The results of the SARE grant will be on the Purple Haze Web site,, with results in forthcoming years updated annually.


Participation Summary

Research Outcomes

No research outcomes
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.