Weed Barriers for Perennial
Controlling weeds around perennial horticultural crops can be challenging,
regardless of the production system. Perennial weeds can be particularly
difficult to control within the crop row because pulling them out
by the roots is often not possible and hoeing is rarely effective
without damaging the crop. Weed barriers for perennial crops need
to be durable to last the many years the crop is in the ground.
Physical weed barriers increase initial establishment costs, but
these will often be offset by reduced labour costs over time.
Physical weed barriers are most suitable for perennial horticultural
crops that do not spread over time such as lavender, some medicinal
herbs (e.g. Echinacea), perennial culinary herbs, many berry and
fruit crops, tree nuts, hops and nursery crops. Plastic mulches
are less suitable for spreading crops such as asparagus, June-bearing
strawberries, and cane berries, since spread of these crops and/or
emergence of new shoots can be restricted.
There are many different types of weed barriers, each with specific
advantages and disadvantages. There are three main types:
Organic mulches (e.g. straw, wood chips, hulls, bark, leaves,
- The cheapest option available. While they may need to be re-applied
over time, the costs are significantly lower than the use of plastic
- Do not restrict water or air flow to the soil and can improve
aeration of the soil by preventing pounding rains from destroying
- Restrict the growth of annual weeds and some germinating perennial
- Their breakdown over time can increase organic matter content
of the soil, increase moisture holding capacity, and improve microbial
action in the soil
- Soil and air temperatures around the plant are moderated, reducing
the impact of hot and cold periods
- Perennial weeds can push through the mulch if growing in from
between the rows or beside the field. Hoeing or other manual controls
can be difficult without disturbing the mulch and introducing
soil to the surface.
- Usually have a high carbon to nitrogen ratio, and microbial
action to break them down will rob the soil of nitrogen. Nitrogen
must be added to the crop to compensate.
- Large seeded weeds can germinate and grow through the mulch.
The mulches can contain volunteer grain (straw) or weed seeds
that can also grow through the mulch.
- Can increase or decrease soil pH, potentially affecting the
growth of the crop if soil pH is already marginal for the crop.
- Can contain pathogenic organisms that may attack the crop (e.g.
Fusarium in wheat straw). They may also promote a humid environment
around the stem and roots of the crop, which can promote disease
- Break down over time and must be re-applied on a regular basis
Figure 1: Straw mulch
around young hop plants.
Solid Plastic Mulch
- Less expensive than woven plastic mulches
- Provide a solid barrier that is impenetrable by most weeds,
depending on the thickness of the mulch
- Can be applied by machine and the crop can be planted mechanically
into the mulch, reducing establishment time and labour costs.
- Many different colours and types available that are suitable
for a wider range of crops
- Can provide a significant increase in soil and air temperatures
around the crop, which may increase growth of some crops.
- Can restrict aeration of the soil and promote some root diseases
- Mostly impenetrable by rainfall or overhead irrigation, therefore
requiring drip/trickle irrigation
- More difficult to fertilize after establishment with organic
sources (e.g. compost)
- Generally less durable than woven plastic mulches
- Temperatures can be excessive during heat waves and may damage
some crops, especially young plants in contact with the mulch.
Different colours are available that do not cause the same heating
- High costs for removal and disposal. For certified organic
production, plastic mulches must be entirely removed from the
field before they begin to breakdown.
- Re-application if the mulch has broken down is not usually
a viable option
Figure 2: Lavender plants on solid black
Woven Plastic (Fabric) Mulches
- Generally the most durable and can last a decade or more, depending
on the grade.
- Impenetrable by most weeds while still allowing water and air
flow through the mulch
- Drip/trickle irrigation not necessary
- Sturdy enough to allow for foot traffic on top, therefore suitable
for agri-tourism operations (e.g. display arden of lavender)
- Both white and black types available that are suitable for
a wide range of cropsDisadvantages:
- Most expensive
- While mulch can be laid by machine, transplants often have
to be planted by hand.
- Fabric can fray if the edge of the planting hole is not burned
to melt the fibres together
- Similar temperature, organic fertilization, and disposal concerns
as solid plastic mulch.
Figure 3: Lavender plants on black woven plastic
It is also possible to combine organic and plastic mulches.
Cheaper plastic and fabric mulches can break down if exposed
to UV light in the sun. Covering these with an organic mulch
can prevent breakdown and can provide a very effective weed
barrier at a lower price. However, the plastic mulch must be
removed before it begins to break down and this could be very
difficult to achieve once covered with layers of organic mulch
Depending on the crop and the between-row spacing, growers
often have the option to combine a weed barrier in the row with
a living ground cover (e.g. grass, clover) between the rows.
These can be kept under control by mowing. However, it is important
to consider how much these plants will compete with the crop
for nutrients and water. For lavender production, evidence suggests
that plants grown with grass between rows grow slower than the
same cultivar grown with a solid fabric mulch within and between
rows. Irrigation and fertilization may have to be increased
to compensate for the competition. While legumes such as clover
can fix their own nitrogen, this nitrogen is only released when
the plants are killed and competition may still be an issue
when the plants are growing.
Plastic and fabric mulches used for annual crop production
are not usually durable enough to be used for perennial crop
production, unless growers only need to control weeds during
the establishment year. Plastic mulches should be 2 to 4 mil
(1 mil = 0.025 mm) thickness or thicker for perennial crop production.
Woven plastic mulches are often sold as ground covers for nurseries
and should have a lifetime of longer than 5 years. Organic mulches
should be 5 to 10 cm (2 to 4 in.) thick to provide an effective
weed barrier, while not restricting growth of the crop.
Not all organic or plastic mulches are permitted in organic
production systems. Polyvinyl chloride mulches are not permitted.
Organic mulches must come from a source that has not been genetically
engineered and has been managed with only permitted substances
for at least 60 days prior to harvest. Growers should always
check with their certification body to determine if mulch is
permitted. Specific restrictions apply to biodegradable plastic
mulches, but these are not usually durable enough to be used
for perennial crop production unless weeds only need to be controlled
until the crop is established.
For more information:
Toll Free: 1-877-424-1300