Dear Garden Guy, I am having trouble with unexplained slow death of the few ohia trees we have growing on our property in Hawaii Paradise Park. Over the last five years, approximately half of the Ohia have died (10 of 20 total on the acre). We have not used any heavy machinery around them, as I know they are sensitive to disturbance in the shallow root area. They appear to begin dying at the top of the tree, with leaves browning and falling off from the top toward the bottom. This process usually takes over a year. I cannot find any explanation for this (ex. disease) in my online searches. Do you have any explanation for this, and hopefully some suggestions for how to stop the die-off? Thank you for your column and your regular helpful response to my inquiries. Mahalo. — Mary
This is a phenomenon that has been observed as far back as 1906. In the late 1960s, a major decline of ohias took place. In 1986, the U.S. Forest Service published a lengthy article on the subject http://www.fs.fed.us/psw/ publications/documents/psw_gtr086/psw_gtr086.pdf.
The decline is complex and not totally understood. Older trees are not as vigorous as they once were and are more susceptible to stress.
The stress may come as a prolonged period of rain or the opposite, drought. Poor draining soils will compound the problem. Other factors may include vog, low soil nutrients, bulldozers and perhaps dense stands of invasive species. These stresses alone may cause some type of decline.But what usually happens in the next phase is the invasion of a root rotting fungus or perhaps a tree boring beetle.
Fungal organisms are often found infecting the roots of declining trees but are not thought to be the primary cause.
Young ohia trees can grow well for many years on shallow soils. Eventually, decline may set in when they become large trees, and the shallow soil simply cannot support their growth, particularly under drier conditions.
What can be done? Not much.
If the property contains several older ohia trees, plant some young ones to diversify the age group. Keeping the trees healthy is important; fertilize them, if needed. It is impractical to water a stand of ohia trees during a drought, but watering a few trees around the house may be feasible.
We live at the 4,300-foot elevation in Ocean View in the ohia forest, except the forest is dying. We have had only 2 inches of rainfall for each of the last two years. To save our large ohia trees, how much water would we need to supply and how should this be delivered? Or is this a losing process. It pains us to see all the dead/dying trees. Thank you. — Jay C.
Giving the trees supplemental water can help them to survive; it’s worth a try.
However, I cannot give specific numerical value of how much water to supply.
With major crops like corn, citrus, and soybean, researchers have developed proper irrigation values for the various crops. I’m afraid there is no such data for ohia trees.
I would suggest irrigating the trees by the calendar; that is something I would never recommend for backyard or commercial crops. If there is no rain, and the weather has been fairly sunny, irrigate the trees approximately once every four to six weeks for roughly 24 hours, being cautious to watch for runoff. Use either sprinklers for larger areas or allow a hose to trickle at the dripline of an individual tree. This is by no means an exact recommendation; the schedule can certainly be varied.
Another suggestion is the use of tensiometers, a soil moisture measuring device. They are designed to measure the tension or suction that the roots of plants must exert to extract water from the soil. When placed in the soil at various depths, they can measure how dry the soil actually is. Again, major crops have values for when to irrigate by tensiometers, but no such figures for ohia. You will need to purchase one or more tensiometers. When the dial reads around 70, it’s time to irrigate.
Hilo resident Nick Sakovich is a professor emeritus of the University of California. He has worked in the field of agriculture for 30 years. Email your questions to Sakovich at firstname.lastname@example.org.