Weed control is the single largest labour requirement for lavender production. There are no herbicides registered for control of weeds in lavender, and therefore they have to be controlled by hand. However, mulches can be used to greatly reduce the labour requirements, reduce competition with the lavender plants, and increase lavender growth. Black plastic mulches may provide heating during the growing season and increase growth, but may be too hot during hot spells and can occasionally burn low hanging flowers. There are two main options for plastic mulches in lavender production: solid plastic mulch and woven plastic (fabric) mulch.
Solid plastic mulch has the advantage of being cheaper than fabric mulch. The plastic can be laid and the lavender transplanted by machine, reducing labour costs. Because water cannot penetrate the plastic, drip tape must be laid underneath to provide water to the plant until the root system is developed. Grass is often grown between the rows, which can be controlled by mowing. While thick plastic mulch is available that will have an extended life span, solid plastic mulches are generally less durable than fabric mulches and do not handle foot traffic well.
Fabric mulch is a more expensive option. While cheap fabric mulches are available at garden centres, they are not durable enough for lavender production and will break down rapidly when exposed to UV light. Fabric mulch allows water to flow through and does not require drip or trickle irrigation. Since its primary use is as a ground cover in nurseries, it can handle considerable foot traffic, meaning it can be laid both in and between rows if desired. It also provides more aeration of the root system than plastic mulch and will reduce the build-up of humidity under the mulch, which may reduce disease pressures around the crown of the plant. The disadvantage of this mulch is the increased labour for establishment. Transplanting machinery cannot penetrate the fabric mulch. Planting holes can either be burned into the fabric with a blow torch, or cut by hand. However, cutting by hand can result in fraying of the edge and these edges need to be melted with a blow torch after cutting.
A research trial has been established at the Simcoe Research Station, University of Guelph by Cathy Bakker and Mary Ruth McDonald to test solid plastic and fabric mulch in both white and black (Figure 1). These mulches are being compared to weeded, bare-soil plots. There has been debate over whether white mulch can provide the same benefits as black mulch, but without the excess heating that can occur over black plastic in the summer. While it is too early for any data to be collected, the trial is already providing useful information.
Figure 1. Black fabric, white fabric, white plastic and black plastic mulches are being tested for lavender production.
Figure 2 shows the white fabric mulch. Due to light transmission through the mulch, weeds can grow underneath. The weeds have pushed up the mulch, and the small lavender plants are “sinking” below the surface of the mulch. There is no way to control these weeds. The only hope is that the lavender plants will grow fast enough to shade out the fabric and reduce weed growth. The same is not occurring in the white plastic mulch plots because this particular mulch is lined with black underneath, preventing light transmission through the mulch. Unfortunately, a black-lined woven plastic mulch is not currently available.
Figure 2. Weeds growing below this white fabric mulch have pushed up the mulch and some lavender plants are dissappearing below the surface.
It is too early to determine which mulch is best for lavender growth. Trial results will be reported over the next few years.