Weeds in the Lawn

Controlling weeds should begin long before chemical herbicide applications are considered. 
Cultural measures ought to be employed from the beginning. These include 
a. proper irrigation -  weeds often invade lawns that are either overwatered or underwatered. Perhaps the worst possible irrigation schedule, yet common, is daily watering for 5–10 minutes. Light, frequent irrigation creates a shallow-rooted lawn which is more susceptible to weed invasion, as well as being less tolerant to drought.  In dry areas, where irrigation is employed, water should penetrate to a depth of 6 to 8 inches. The best practice is to allow the soil to partially dry out between irrigation. The top 1 to 2 inches  should be fairly dry before watering again. For more information on irrigating lawns, in Hawaii see UH CTAHR publication, “Watering Lawns”, at http://www.ctahr.hawaii.edu/site/info.aspx;  or your local University Extension office or website.

b. Proper fertilization – apply suitable amounts of fertilizer on a regular basis; nitrogen is the key nutrient for turf.

c. Proper mowing heights – each turfgrass species has an appropriate mowing height. Mowing some grasses too short can weaken the lawn and predispose it to weed invasion.  A typical guide is to remove no more than one-third of the leaf blade at each mowing. If too much is removed, it will take more time for the grass to recover, giving weeds a chance to invade.

d. Thatch is a layer of organic matter - stems, stolons, roots - that develops between the turfgrass blades and the soil surface. Regular thatch removal will help keep turfgrass healthy and competitive with weeds.  See the UH CTAHR publication “Removing Thatch from your Lawn” for more information on this procedure.

e. Any activities that lead to soil compaction will contribute significantly to turfgrass stress. This in turn, will make it easier for weeds to invade. These activities may include heavy foot traffic as well as vehicles traveling over the area.

It is impossible to get 100% weed control in your lawn. Weeds will occur, but the problems can be minimized with a well-managed, vigorously growing lawn. By combining cultural methods with herbicide applications when needed, weeds will  be minimized.

When controlling weeds in your lawn it's good to know the name of that weed to see if it is listed on the particular herbicide you have chosen to use. But what's more important, before identifying the species, is to know the type of weed: broadleaf or grassy weed.  Distinguishing between the two will determine the appropriate herbicide to use.  Controlling a grassy weed in a lawn (grass) is difficult since most chemicals cannot distinguish between good and bad grass species.

Fortunately, certain herbicides can distinguish between grassy plants and broadleaves. It is the chemical composition of the herbicide and the morphology of the plant that will cause certain herbicides to affect broadleaf plants but not affect grasses. Therefore, controlling broadleaf weeds in a grassy environment is relatively easy. Purchase an herbicide with the active ingredient 2,4-D, MCPP (mecoprop), or dicamba (Banvel). These products are often sold in combination, giving better management and a wider spectrum of the broadleaf weeds controlled.

For light weed infestations, spot treating is more appropriate than treating the entire lawn. Apply just enough of the solution to wet the leaf, do not apply to the point that the herbicide is dripping off the leaf. To insure maximum absorption, stop mowing 2 or 3 days before treatment and allow 3 or 4 days before mowing again. This allows sufficient time for the weeds to absorb the herbicide and transport it to their roots.

Caution: these materials are volatile, especially 2,4-D. The drift can damage other valued broadleaf plants; tomatoes and hibiscus are particularly sensitive.

Photo by Forest and Kim Starr - Nene, the Hawaiian goose, feeding on weeds and grass in a lawn setting.
 Weeder geese are used with great success to control and eradicate troublesome grass and certain weeds in a variety of crops such as corn, flowers, strawberries and grapes. Note: Geese prefer grass species and will eat other weeds and crops only after the grasses are gone and they become hungry.

Mango - Big Tree, Little Fruit

 Mango trees may grow well but produce little or no fruit. This condition is likely brought on by a disease called mango anthracnose (Colletotrichum gloeosporiodes).  During the bloom period, when the weather is warm and wet, mango blossoms are attacked by this fungus.  Blooms turn black and are destroyed and little or no fruit is set.  This disease also affects other fruit trees throughout the world including banana, avocado, papaya and coffee. 

The fungus also occurs on the leaves, twigs and fruit.  On the leaves, it is seen as small angular brown-black spots which can grow larger.  Sometimes the dead spots will dry and drop out leaving holes in the leaves.  Under wet conditions lesions on the stems may produce eye-catching pinkish-orange spore masses. Most of the infections on the green fruit remain latent until the fruit is ripe.  Then dark, sunken decay spots will develop and eventually will penetrate deep into the fruit. 

                                                        Anthracnose fruit symptoms

                                        Anthracnose fruit symptom know as tearstaining


What to do.  1.  Plant resistant varieties, especially in wet areas. The following varieties are listed as moderately resistant - Carrie, Earlygold, Edward, Glenn, Julie, Keitt, Florigon, Tommy Atkins and Van Dyke.   The one tree that shows the best resistance to this disease is Florigon.

2. Prune trees yearly and remove fallen plant debris from the ground. 

3. Spray with a copper fungicide when blooms first appear and continue until fruit is 1 ½ to 2 inches long.

It is difficult to grow mangoes in areas with high rainfall.  According to a UH publication, mangoes are best adapted to hot, dry leeward areas receiving less than 60 inches of rainfall annually.

Some mango varieties, especially Haden and Pirie, tend to alternate bear.  That is they produce a large crop one year, followed by an ‘off year’ with low yields.  

Photos: Dr. S. Nelson, University Hawaii

Air Layering

Questions often arise concerning air layering of plants. This is a method of reproducing a plant in order to create another exactly like the original. It has been used successfully as an alternative means of propagating some plants difficult to root, i.e. plants where grafting or cuttings have not been successful or where seeds do not come true to type.  Another advantage to air layering is that these plants have the ability to bear fruit much sooner than a seedling. Some fruit trees may take 10 years to fruit from seed. Plants such as citrus, figs, lychee, longan, mountain apples, camellias, azaleas, croton and dieffenbachia are frequently propagated by air layering and bear fruit (where applicable) in less time than seedlings.

How to Proceed

After selecting the branch you want, come down about a foot from the tip.  Here you will remove a one inch wide ring of bark by making two parallel cuts (1 inch apart) around the circumference of the shoot. This will leave the branch with a “naked” or barkless band.  Don’t cut through the branch, just cut deep enough so the outside bark peels off.  This is known as girdling.

Then wrap the wound with moist, unmilled sphagnum moss. You may need to tie some twine around it to keep it in place. Cover the moss with plastic wrap to form an airtight pouch. Alternatively, you may want to place the moss first, then wrap both moss and plastic around the wound at the same time. Make sure the moss is moist – not wet. Some gardeners will place a final wrapping of aluminum foil to shade the area from the hot sun. Also, some add a little rooting hormone to the wound to speed up the rooting process - a light dusting will do. For many trees, this is not necessary. 

Tie the bundle at each end, secure enough to prevent moisture loss.  Later, when roots are visible, all around the bundle, cut the branch just below the developing roots, remove plastic and plant the newly rooted branch in a pot.  Keep it in the shade until the plant develops more roots. Rooting time will vary depending on the type of plant and the season.  Some plants will begin to show roots in 4-6 weeks, others may take up to 2 years.

In the process of air layering, the removal of the bark (girdling) prevents carbohydrates from flowing down past this point, but still allows water and mineral nutrients to flow upward to the leaves. This keeps the leaves of the shoot from drying out, but the presence of the carbohydrates and the water in the sphagnum moss, will cause dormant buds in the area to grow into roots.

For optimum rooting, make air layers in the spring on shoots produced during the previous season, or in mid-summer on mature shoots from the current season's growth. On woody plants, stems of pencil size or larger are best. 

East Hawaii Residents:  Master Gardeners' Annual Plant Sale is Saturday, April 25, at 8:00am-Noon at the Komohana Agricultural Center. Nick will be there to answer questions.