Tropical Gardening Helpline: Protect your plants from rot and infestation

  • Mealybugs often group together on stems or leaf undersides. (Photo courtesy / Susan Mahr wimastergardener.org)

  • Scale often appears along the stems or leaf ribs of host plants. (Photo courtesy / ctahr.hawaii.edu)
  • Chinese rose beetles attack and cause shot-hole leaf damage to many plants that grow in Kona. (Photo courtesy / www.ctahr.hawaii.edu)

Loren asks: I recently lost my partner, the family gardener. I need some help taking over her job. Something is eating the edges of the leaves on my lanai orchid plants and a bunch of white bugs are swarming around my gardenia. Can you offer advice on these issues as well as some general tips on plant care?

Tropical Gardener Answer: Sorry about your loss. Good gardeners are a treasure. They usually are well versed in the basics of plant care: adequate water and food as well as pest prevention and control.

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My best general advice is to avoid overwatering or over-feeding your plants and learn ways to prevent pest problems or control them before they become an infestation.

It sounds like you might have slugs chewing on your orchid leaves. These critters come out to feed mostly at night. If you want to be sure you have them, put out tuna cans of cheap beer and see if they come to the “bar.” If you find drunken and drowned slugs in the cans in the morning, you can either refill your bar or put out some slug bait. Be sure to look for bird, pet and wildlife safe products that are only toxic to slugs and snails.

Your gardenia may have white flies. These insects often swarm when the plant they are feeding on is disturbed. They are sucking insects which means they attach, usually to the underside of a leaf and suck the juices from the plant. Once their numbers increase they can cause serious damage to your plants.

Assuming you want to start with a low toxic control, you can spray them with a combination of Safer soap and neem oil mixed in water. Check the directions or go online for the proper mix. This combo will smother the insects on contact, so contact is required.

The best way to prevent pest problems is to keep your plants healthy. Too little water can dehydrate your plants, too much can drown them. On hot sunny days, you may need to water several times a week. Never water, however, when the top of the soil is wet or moist. Overwatering is the main cause for deadly root rot and other pathogenic diseases. Infrequent deep watering is always best.

Plants, like people, need food. Lightly fertilizing potted plants every few months will usually provide adequate nutrition. Always read the label and follow it. Too much fertilizer is as bad as too little. Over-fertilizing, especially with nitrogen, can cause plants to put out flushes of new growth which are very attractive to insects. In potted plants, too much of some nutrients can result in toxicity.

An early sign of poor plant health is the appearance of insects or a disease. Checking your plants often and thoroughly will give you a head start on pest control. Look for scale and mealy bugs along stems or leaf ribs. Especially check the undersides of leaves where critters like aphids and mealy bugs often hide. If you catch their presence early and treat with soap and oil weekly until they are gone, their damage will be minimized.

Another common pest here in Kona is the Chinese rose beetle. They usually feed on plants right after sunset, leaving shot holes in the leaves.

You can discourage them by installing lights that will shine on your plants after dark. The best way to control these beetles is to collect and kill them. Go out after sunset with gloves, a head lamp and a jar of rubbing alcohol or soapy water. Pluck them from your plants and put them in the jar to kill them. Do this for a few nights until you no longer find them. You may need to do this again every few weeks. If you have lots of rose beetle damage or want to build a trap to prevent their damage, go to the UH website https://www.ctahr.hawaii.edu/uhmg/FAQ/faq-chinese-rosebeetle.asp for more information.

Contact the Master Gardener website konamg@cthar.hawaii.edu if you have more questions. You can also reach them by phone at 322-4892 on Tuesday and Thursday mornings between 9 a.m. and noon.

Good luck with your plant care.

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Email plant questions to konamg@cthar.hawaii.edu for answers by Certified Master Gardeners. Some questions will be chosen for inclusion in this column.

Diana Duff is a plant adviser, educator and consultant living part time in Kailua-Kona.

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