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. 

Bell Peppers Rotting on the Vine

What causes bell peppers to rot while still hanging on the plant?

There are a number of fungal diseases that will cause fruit rot. In many cases, however, the rot is secondary to an insect that may either feed on the fruit or lay eggs. The tiny wounds made in this manner create an opening for the fungus or bacteria to enter. 

                                              
                                                                                                       Photo by Paul Choate, Univ. Florida

In the case of peppers, the pest is probably the pepper weevil. They make holes in the young pepper fruit and in the flower buds by the feeding of adult weevils, or egg laying of females. Then the weevils develop inside the fruit and as adults, burrow their way out.

Infested fruit that does not rot will have a blackened core and seeds as a result of larval feeding. Mature peppers are not susceptible to weevil attack, because the skin is usually too hard.  In order to lay her eggs, a female first punctures the fruit or the bud with her mandibles (mouth parts), then inserts an egg into this hole and seals it with a clear yellow fluid which hardens.

Sanitation is very important in controlling this pest. Remove all peppers that have dropped to the ground. If weevil numbers build up and are causing major damage to the crop, there are registered pesticides available. 

Rats

Rats have been a destructive pest to mankind and agriculture since antiquity. They are especially important as disease carriers including plague, murine typhus, leptospirosis, and salmonellosis. The Hamakua coast area was a former plague epidemic area, but the last reported human case occurred in 1949.  Rats and other rodents consume and contaminate the foods in the homes and in the field. Their control is not easy due to their ability to adapt to changes and their capacity to reproduce.

There are four rodents of economic importance in Hawaii: the roof or black rat, the Norway or brown rat, the Polynesian or Hawaiian rat, and the house or field mouse.

Roof rat (Rattus rattus)- is a medium to large rat, 5 to 7 inches long.  Body color varies from grey to jet black; the underside grey, grey-white, or white.  They are expert climbers and can easily walk along electric wires. They frequent cane fields, macadamia nut, coffee, papaya, and banana groves and, in other part of the country, are commonly found damaging citrus groves.  They will gnaw on the bark of limbs and hallow out mature citrus fruit. They also feed on avocado fruit and have been found nibbling on a snail or two. They nest in attics, trees, palms, and dense vines.

Norway rat (Rattus norvegicus)- Largest of the rats in Hawaii, weighs 10 to 18 ounces and measurers 8 to 10 inches long. Body color reddish brown to grey to black; underside whitish color.  It is commonly found around chicken coops, in sewers, rubbish heaps and ships; also found in houses, farm buildings and warehouses, usually within one story of ground level. They are well adapted for swimming and burrowing. They are also aggressive and will drive other rats from the lower floors of buildings and will not hesitate, at times, to attack children.  

 Polynesian rat (Rattus exulans)- is the smallest of the three,  weighs 2 to 3 ounces and measures 4 to 5 inches long.  The body color is cinnamon-brown to grey, stiff black guard hairs on the back and sides, the underside light buff or grey.  This rat is basically a field rat.  Rarely found near buildings in Hawaii, it nests in gulches, rock piles, rock walls, wastelands, fields, and embankments. It can cause great damage to sugarcane, pineapple, macadamia nut, coconut, coffee, and other fruit and vegetable crops.

Rats are nocturnal. They have excellent memories and very repetitious habits.   They are easily frightened of new things placed in their environment. The roof rat is most pronounced in this tendency. Rats have a keen sense of smell and hearing, and only a fair sense of sight with the ability to see in the dark.

Common signs that rats are nearby are droppings, rub marks,  gnawings, nests, and rodent odors.  In controlling these pests, the first step is to clean up the environment by removing accesses to food and shelter.  Physical barriers such as screens may need to be installed.  When only a few rats are involved trapping can be effective.  For field problems, snap traps can be secured to limbs and baited with raisins, citrus or other fruits. Traps should be baited and left unset until bait is readily consumed. 

Paraffin-type bait blocks containing anticoagulants are very effective in controlling rats.  They appeal to the rats gnawing instinct; especially those blocks with numerous ridges.  They should be replaced immediately as they are eaten, since only a single feeding on the first generation anticoagulants (warfarin, diphacinone) will not control rats.  Baits must be eaten over a period of several successive days.  For safety, baits other than in block form, should be placed in bait stations.   Read and follow label instructions.

Some poisons have a secondary effect which will affect animals which consume dead or nearly dead rodents. Thus, it is imperative that strict safety precautions be used in the placement and disposal of poison baits for rodents.

Ohia Rust

Ohia rust is a fungal rust disease similar to plumeria rust, although caused by a different organism. This particular rust disease was first noticed in O`ahu in 2005.  Since then surveys have shown the disease to be widespread throughout the State of Hawaii. It infects a number of different plants in the Myrtle family, like rose apple, paperbark tree and rarely in guava. In wet climates, the disease can be quite devastating on rose apple.

It is sad to see a new disease attack such a valued tree like the ohia.  In some areas ohia trees compose up to 80% of the native forest.  It is one of the first plants to colonize recent lava flows.  Ohia trees are adapted to wet and dry weather conditions and grow from sea level to high elevations.  Ohias also provide habitats for many animals and plants of the forest.

Disease symptoms can easily be seen as bright yellow powdery spots on the leaves, stems, flower buds and growing tips. These are the spores of the fungus erupting from the plant tissue. Often times they are in a circular pattern.  The spots will expand, killing the leaf tissue and causing the leaves to drop.   It is unlikely that this rust disease will actually kill an ohia tree. 

No fungicide has been approved for controlling this disease.  If possible, remove and destroy all infected parts of the tree.