Monday, April 21, 2014

Different Variations of Ohia Trees (Metrosideros sp)

To begin with, there are four distinct species of ohia tree that are endemic to Hawaii; there are also eight different varieties. But in addition to genetics, there are environmental factors which will cause physical variations in the trees. Elevation, rain, and temperature all have an effect upon these trees. 

With trees, in general, the shapes of leaves from trees growing in hot, dry deserts are often small in order to conserve water loss through the leaf. Contrast this to the large-leafed plants of the tropics. Ohias occur in a wide range of habitats, from just above sea level to 8,200 feet; from dry forests with less than 16 in annual rainfall to wet forests with more than 33 feet of annual rainfall.
This along with its genetic propensity for variation will produce trees of diverse appearance. The species name, polymorpha actually means many forms. Thus tree height and form, leaf shape and flower color are highly variable.

Concerning their care, ohia trees can be damaged and even killed from a lack of water. During periods of drought especially, supplemental watering will be needed. Trees growing in the pahoehoe lava (exhibiting a smooth, billowy, ropy surface)  may do well when young and water requirements are low.  Nevertheless, as trees increase in size, if adequate water is not supplied, they can dry out and even die.  Mulch will help, but of course, not piled against the trunk.

Since ohias are adapted to grow in low nutrient soils, beware of over-fertilizing. Slow-release fertilizers or natural compost is best. 

Monday, April 14, 2014

Trouble with Palms

The bottom fronds of our triangle palm started to die, and then suddenly the whole palm fell over.

A significant pest of Pritchardia palms is the banana moth, Opogona sacchari. The triangle palm is also attacked by this moth. It seems that the female moth targets wounded or stressed palm tissue to lay her eggs. Stresses can include drought, flooding, mechanical wounding, poor nutrition, and herbicide injury. The larvae generally feed on decaying and dead plant tissue but will feed on living tissue, too, causing extensive damage. In affected palms, larval tunneling, along with the characteristic frass (insect droppings), can often be seen. Fully developed caterpillars measure just over an inch. The adults have greyish brown wings are 3/8 to 5/8 inches long.

The main treatment is prevention: keep palms growing well; give them adequate fertilizer, and supplemental water during a drought. Gardeners also need to be careful with the weed wacker! Female moths are looking for wounds to lay their eggs. Bt (Bacillus thuringiensis), an organic insecticide, can be applied to these wounds, as well as those caused during pruning.

For more information about this pest see the CTAHR Cooperative Extension Service publication, “Banana Moth – A Potentially Fatal Pest of Pritchardia and Other Palms”, by Scot Nelson and Mark Wright.  CTAHR stands for the College of Tropical Agriculture and Human Resources at the University of Hawaii. 

Monday, April 7, 2014

Stink Bug

 A wide range of fruit and vegetables, as well as leaves and flowers, are susceptible to the Southern Green Stink bug, nezara viridula. In some countries it is called the green vegetable bug.  

Damage is achieved by inserting their needle-like mouth part into the plant tissue and sucking out the plant juices. Some stink bugs can cause wart-like growths as the plant reacts to the piercing. Although the green color of the pests blends with the foliage making them difficult to find, bugs can be visible feeding in the garden on beans, cabbage, peppers, tomatoes, okra or in the trees on such fruit as mangoes and macadamia nuts. The adults are active fliers, and when disturbed, will fly away or fall towards the ground.

Control is sometimes difficult. Horticultural oils, including Neem oil, work well, but the solution needs to contact the insect. (Read more about Neem on this website.) To spray when the insect is not there will do no good. Oil, as well as soap, is a contact insecticide. Harsher chemicals like Sevin and Orthene will also control this pest. Yet in many areas biological control is effective, and chemical control is seldom necessary. 

In Hawaii, introduced parasites (good guys) are well established and are generally effective. Predators include spiders and the big headed ant. The ants prey upon the eggs and immature stages of the stink bug, taking them back to their nests. The stink bug is so named because of the strong odor emitted from scent glands when disturbed.