How to Control Those Pesky Weeds

What is a weed?  Dandelion is cultivated for salad greens, but in a lawn, it is a weed.  Cantaloupe is a very tasty fruit, but growing in your flower bed, it would be a weed.  A weed is simply a plant growing where it is not wanted or, a plant out of place (p.o.o.p.)!  In some agricultural areas, more money is spent on weed control than any other pest.   

What’s so bad about weeds? 

·        They compete with our crops for space, nutrients, water and sunlight.  As a result, the crop may appear stunted with reduced yields and lower quality.  

·        Weeds can serve as hosts for insect pests and disease organisms.  

·        They provide cover and food for many rodent pests. 

·        Many people are allergic to certain weeds.

·        Some weeds are poisonous to livestock. 

On the other hand, here are some redeeming features of weeds:

§  They reduce dust and soil erosion.

§   Provide cover and food for birds, nectar for honeybees and a habitat and food for beneficial insects.    

§  Some, with an extensive root system, penetrate deep into the subsoil to improve drainage and bring nutrients to the surface.

Strategies for control:

 Prevention – don’t allow weeds to go to seed!  Get rid of existing weeds before they flower and produce seed. The average number of weed seeds in soil is 30,000 – 350,000 seeds/square meter, or 120 million to 1.4 billion per acre. 

Cultural – modify the environment to improve the crops competitive advantage and decrease the weeds advantage. This includes, proper soil preparation, crop rotation, maintaining good soil fertility and moisture, choosing the best varieties for your area and planting at the proper time of the year.  In turf, watering deeply and infrequently discourages many weeds as does maintaining the proper mowing height.

 Mechanical – this method includes hoeing, hand pulling, and mowing. Flaming is also a method of weed control - using a propane tank, hose and appropriate applicator; best when weeds are less than three inches tall.

 Although time consuming, here is another effective method using mechanical weed control: first work up the ground to be planted, water it and allow weeds to grow.  Then either spade them out or spray with a contact herbicide. Repeat this process again, then plant, disturbing the area as little as possible.  This will rid the area of a high percentage of annual weeds.

 Physical – this method includes placing a layer of opaque material over the soil surface.  Examples - inorganic (synthetic) material such as plastic sheeting, or organic material like bark, wood chips, straw and compost. Place 3-6 inches of an organic mulch for best results.  Living mulches, such as grass or a clover cover crop, are used by many growers. Clovers have an extra benefit in that they can supply nitrogen to the soil.

Biological – employs the use of insects and diseases that would normally attack weed plants and their seeds in order to reduce the weed population. Sheep, goats and geese are also used to chew down many weeds.  Geese prefer grass seedlings, but have been known to feed on the crop when the grass is gone. Chinese weeder geese are preferred to other geese because they walk around the crop and not on it. 

Chemical control – products can be categorized as either a pre-emergent or post emergent herbicide. Preemergent herbicides, also known as soil-residual herbicides, are applied to the soil before the weeds appear in order to prevent their germination.  The residual effect of the herbicide will last from several weeks to a number of years, depending on the particular herbicide used, the rate of application, and soil characteristics. The more common preemergent herbicides will persist in the soil from 1- 12 months.

Post emergent herbicides, sprayed on the weed itself, are either contact or systemic. A contact herbicide, such as weed oil, literally burns the plant. The weed however, could recover if it had a significant root system.  On the other hand, a systemic herbicide, such as RoundUp, will kill the weed as it is absorbed and  taken into the root system.

Cinnamon, clove and thyme oils are contact herbicides and have been used as an organic approach in controlling weeds. They are most effective on small annual seedlings and more effective on broadleaf weeds than grasses.  They are strictly contact herbicides.  If the root system of the weed is old enough or strong enough, the plant will regrow, making repeat applications necessary. In general, they do a moderately good job. However, they are not economical for controlling weeds in large areas (acres). 

Beautiful Bamboos

        

 Bamboos are a beautiful addition to any tropical garden.  Not only are they common on the Islands, but also in Southern California and can grow as far north as New York on the East Coast and Seattle on the West.  The bamboos comprise more than 1200 known species, including 6 inch dwarfs and 120 foot giants. The new shoots, or canes, on some species may grow as much as a foot a day. Bamboos can be planted as an ornamental or for economic purposes such as  food and shelter. 

When buying bamboos, there is one important characteristic to consider; all bamboos are not created equal.  They can be divided into two basic groups - the clumping bamboos which are desirable and the running bamboos which aren’t so desirable. The bamboo plant produces underground stems called rhizomes. New canes, also called culms, shoot up from these rhizomes. With clumping bamboos, the rhizomes grow rather slowly.  Thus the outward spread of the original clump is slow, and the plant is more easily contained. 

With running bamboos, the rhizomes spread rapidly, as much as 2-3 feet its first year, increasing from there, to as much as 15 feet in one season. Those who decide to purchase a running bamboo, usually have lots of land.  Barriers can be placed around a running bamboo planting to confine it, but no guarantees.  The barrier will have to be made of some impermeable material like concrete or strong plastic.  It needs to extend down into the ground 24 – 30 inches and completely surround the plant.  If the running bamboo is in a lawn, it is easier to confine since a mower will constantly cut off the unwanted new canes as they emerge.   However, eventually the runners will make their way to the edge of the lawn and begin to emerge – even if it’s in your neighbor’s yard!

The University of Hawaii CTAHR website has an excellent publication entitled, Bamboo for Forest and Garden. It lists over 60 species of bamboos with a brief description of each.

Biological Control of Weeds

Insects attack weeds as well as our food and ornamental crops.  Weeds get diseases, too.  As early as the turn of the century, biologists have traveled the globe looking for bugs and disease that would be effective in destroying weeds.  These agents usually won’t wipe out the pest completely, but hopefully they can severely diminish the population so the weeds are no longer a problem. 

As early as 1930, workers at Parker Rancher were concerned with a cactus that was spreading in their pasture land at an alarming rate, covering some areas almost solidly. Over the years, a number of insects were introduced that biologists thought had the potential to destroy the cactus.  Finally, through evaluations that were conducted over a ten year period, it was found that an introduced moth and scale insect were able to either partially or completely eliminate the cacti.

Another weed, lantana, was a concern to ranchers because it was quickly encroaching onto pasture lands and displacing valuable forage grasses.  In 1902, biologist went to Mexico looking for insects that would feed on this weed. After importing and releasing many different insects, partial to substantial control was attained.   

Obviously, it’s a long process.   First, an efficient pest of the weed must be found. Second, after the potential pest is imported, there is an evaluation of the effectiveness of the agent as well as its risk to other plants.  The most promising agents are brought into a secure quarantine facility where they are carefully tested for potential impacts on native plants as well as economically important ones. They are also screened to eliminate any parasites that may be hitch hiking a ride.  After many years of evaluation, the weed pest may finally be introduced into its new environment.

Today, scientists are still looking for bugs and diseases that would safely keep our weeds in check; eradication is unlikely. In general, the biocontrol agents will only reduce the rate of spread and the abundance of the weed.