Urea or soap best ways to control dodder

Aloha, Nick. I need advice on how to get rid of laurel dodder. We have it on a large banyan tree (taller than 65 feet) and on cactus. We previously tried spraying with hydrated lime, which did appear to kill it. But then it returned. We also tried a copper sulfate mixture, which did not appear to have a significant effect. Someone also suggested urea.

We are in the Kapoho area and I’ve been told laurel dodder is more common here than other parts of the island. I would very much appreciate your advice. Mahalo, B.B.

First, what is dodder? Dodder appears as small orange colored strands growing over the host plant. It is a parasitic, leafless, flowering plant that infests a broad range of hosts, especially those on sandy dunes, throughout the Hawaiian Islands and in tropical areas worldwide.

Being entirely dependent on other plants for their nutrition, water and even physical support, dodder will vine around the host and produce adhesive structures called haustoria. These structures penetrate the outer layer of cells of the host plant in order to extract water and nutrients. The host becomes slowly depleted while the parasite garnishes nutrients to grow, flower and produce seed.

Controlling a parasitic plant on large specimen trees such as the banyan is rather difficult. Manually removing the parasite is recommended in many situations but seems unfeasible in this case. Other methods such as fire and herbicides will eliminate the dodder but will damage and possibly kill the tree.

An herbicide such as Roundup has been successful in controlling dodder, although spraying the entire tree with the herbicide could kill the tree. On the other hand, the treated tree might decline by dropping all its leaves, but after a period of time, it could recover, sending forth new growth.

Perhaps the most plausible solution is to spray the dodder with a urea solution, as you mentioned; another recommendation is using a 10 percent soap solution. Urea is a concentrated fertilizer that applied at a strength that would kill the parasitic plant, could burn the leaves of the tree and cause them to drop. This should not, however, kill the tree. The same is true for the soap solution; leaf burn and leaf drop, but the tree should recover.

I would suggest spraying just a small portion of the tree with either of these solutions. Wait two to three weeks and observe any negative effects. Then, you have the option of proceeding by spraying the entire tree or portions and waiting for a recovery before beginning again. It’s no easy task, but would be worthwhile in saving a large banyan.

Read more about dodder at http://aces.nmsu.edu/pubs/_a/A615/.

Current recommendations include the use of a 10 percent soap (mild) solution to burn the stems prior to flowering.

Spread mulch to combat drought

Even though rainfall is usually abundant this time of year, plants might still need protection during periods of drought.

Perhaps the most efficient and practical method to combat the lack of rainfall is to spread mulch. Apply a layer of mulch around each plant — straw, wood chips, grass clippings, bark chips, sawdust and leaves; even crushed rock and plastic can be used as mulch.

During a drought, young trees and shrubs and other shallow rooted plants fair better under a layer of mulch. In addition, not only will mulches help conserve water, they improve soil structure and add nutrients to the soil. With 4-6 inches applied, mulches will prevent weed growth and help suppress root diseases. Established trees also gain from a layer of mulch, but are usually deep rooted enough to withstand a drought.

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 askthegardenguy@earthlink.net.


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