Weeds are weeds from our ‘superior’ human point of view because they grow where we don’t want them. However, in nature they play a diverse and interesting role.
A plant becomes a weed through its relative position—a plant in the wrong spot at the wrong time is a weed.
Not all weeds are bad; there are advantages with some weeds, and disadvantages with others.
The same weed in different crops will have an advantage, or a disadvantage, depending on the crop and consequence of the weed.
An IPM note: Weed suppression does not mean weed-free. Suppression of weeds to prevent competition for moisture and nutrients is usually enough to maximize crop yields.
With irrigation and good nutrient levels, trees can generally tolerate a level of weed competition. It is important to know when they can tolerate the competition, and when this competition will affect growth and yield.
Critical weed-free periods
The critical weed-free period for new orchards is the first three months after planting.
New trees trying to establish themselves are very susceptible to weed competition. Competing with weeds at this time will affect the growth of the tree for the whole season.
Intensive orchards have a small and shallow root system. Weeds growing in this root area are in direct competition with the tree for water and nutrients.
These trees are poor competitors. Weeds at any time in the treeline can affect tree and fruit growth.
Weeds host beneficial & pest insects
Weeds affect orchard productivity and health by acting as hosts for a variety of beneficial and pest insects.
Flowering weeds are a food source for beneficial insects like hover flies, lacewings and Trichogramma—a wasp parasite for light brown apple moth (LBAM).
However, these flowering plants can also be very good hosts for pests like western flower thrip (WFT), mirids and Rutherglen bugs.
White clover is a great source of free nitrogen, but has a huge negative: it is a favoured host for western flower thrips.
Damage to fruit by WFT (especially in nectarines and Granny Smith apples) is greater where white clover grows in the treeline and inter-row areas.
And the damage is more severe in intensive growing systems where fruit is closer to the ground than in large vase shaped trees.
But the potential is always there for much greater damage.
An IPM note: Have the right weed in the right place. If you are growing nectarines and fruit that are susceptible to WFT, then white clover is the wrong weed in your orchard.
Weeds host nematodes
Root lesion and root knot nematodes affect many trees directly by feeding on roots, and indirectly by damaging roots and leaving an entry wound for diseases like verticillium wilt that contribute to premature orchard decline.
There are over 200 weed hosts for nematodes. White Clover is in the spotlight again, and with the common weed, fat hen, these weeds are very susceptible to root knot nematodes.
An IPM note: Susceptible translates to ‘host’—these weeds will affect tree growth, productivity and longevity in high density orchards as they are hosts for plant damaging nematodes.
Weeds and frost
Maintaining a bare soil surface under trees can minimize damage from spring frosts.
Bare soil surfaces, free of weeds or plant residue, absorb more heat during the day. The release of the absorbed heat at night can increase orchard temperatures by a few degrees.
This is commonly referred to as the radiant heat benefit. Although temperature changes are modest, they can be enough to prevent fruit loss during spring freeze events.
An IPM note: No weeds at this time is the greatest benefit.
Weeds indicate soil health
There are weeds that like acid soils, for example sorrel and dock, and weeds such as stinging nettle that prefer very fertile soils.
Weed growth is an indicator to soil types by their preference to grow in certain conditions.
Benefits from weeds
Weeds provide intangible benefits of value to the orchard system.
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Soil protection—weeds protect bare soil and act as mulch (especially when they are dead).
Very important IPM note: Mulch is exceptionally important in protecting soil and tree roots.
Mulch also provides a habitat for soil dwelling predators such as Carabid Beetles and others, and it protects earthworms and other beneficial soil organisms. - Weed suppression—heavy growth of some weeds (e.g. grasses) can suppress the less desirable weeds such as hairy willow herb.
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Nutrient recycling—Marshmallow is very deep rooted and can absorb nutrients from lower in the soil profile and redistribute them to the tree root area when the plants decompose. However, marshmallow is a great host for LBAM over the winter and WFT and Helicoverpa (Heliothis) over summer.
Here is another example of a weed that can help or hinder depending on the fruit crop.
Weed management—an open ended story
Weed management is an open ended story, this article doesn’t finish here.
Weeds can have benefits, can create problems, and are expensive and time consuming to control.
The leading question, “Is it a problem?” leads into, “Does it need controlling” and then, “How much control does this weed in this situation warrant?”
Perhaps look at your orchard floor from a different perspective.
To contact Russell and for other great articles see the May-June 2012 issue of Tree Fruit.
