Plant problems: prevention and diagnosis


Let’s be honest: gardening in Hawaii can be challenging. We have a year-round growing season, which is great for growing all kinds of wonderful plants. The warm weather also means that insects, diseases, weeds and other problems don’t take a rest. Kona soil is often rocky and nutritionally lacking, and some areas get salt spray or lots of wind. We often have long, dry winters and the rains of summer can drown plants. What to do?

Preventing garden problems is the best cure, and it is not hard. Start with healthy soil that will adequately support your plants. A soil test can help determine its nutritional content, but if you are adding compost and can mulch your plants once they are installed, you are on the right track to soil health. If your soil lacks the texture and nutrients it needs and you want to plant now, buy some soil to get started. Be sure the soil you’re planting in drains well. Most plants prefer to grow in soil that is not constantly wet.

Once you have the soil ready, then you can select your plants. Careful plant selection goes a long way toward preventing problems later. Choose plants known to grow well in your area. Not all plants can tolerate the heat and salty spray found at lower elevations or the windy areas of Waikoloa or North Kohala.

At lower elevations, or where drying winds prevail, you need to choose plants that are drought-, wind- and salt-tolerant. Plants grown on the mainland may not do well here. Lilacs, forsythia and some apples need a chill, even a freeze to thrive. Some rose and gardenia varieties, as well as some cole crops, will be challenged here at lower elevations.

Properly chosen plants in healthy soil, with adequate water and fertilizer, are less likely to develop problems. If problems do arise, start with an accurate diagnosis. Do some research before treating. It’s best not to guess.

Identify the problem plant and review its normal appearance and its needs. If this information does not offer clues, dig deeper.

Look for patterns. Damage occurring throughout the plant indicates a systemic problem. If it is localized, look closely for a cause in that area. A systemic problem, or one at the growing tips, may indicate root damage. Dig gently into the root zone to see if it is too wet or if you can see nematode nodules on the roots.

Examine the spread of the problem. Damage caused by plant pathogens, insects or foraging animals often affects only one plant species or family. Most disease-causing fungi, bacteria, viruses or nematodes are host-specific. Early diagnosis and treatment of infectious diseases is important to prevent spreading.

If symptoms are found on several plant species, the problem is more likely not infectious, though the causes may still be living. Gradual symptom development implies a living causal agent. Though damage caused by insects or animals is usually not infectious it can spread if not diagnosed and treated. Try to identify the cause by looking carefully for evidence of pests, even if the pest is not present.

Wilting, dying or dead leaves, beyond what is normal for the plant and the season, are strong clues. First look for insects or evidence of their presence. Observe damage patterns on the leaves or the stem that might help identify the causal critters. Misshapen or chewed leaves can indicate mites or caterpillars. Yellowing leaves may be caused by sucking insects or by nutritional or chemical problems. Tiny holes in the stems can indicate the presence of borers.

If the problem appears suddenly on several plants in an area, it may have a nonliving cause, such as storm damage, machinery error and environmental changes. Slight variations in temperature, light, moisture, vog or pollution levels can adversely affect plants.

Frequent and careful observation is key to trouble-free gardening. Check your plants regularly. If you find problems, research, diagnose and treat them quickly. Hopefully, this will put you on the road to happy gardening in 2013.

Tropical gardening helpline

Judy asks: I love the fragrance of tuberose and have tried growing the plant in my garden several times. It never seems to flower. What’s the problem?

Answer: One of the best resources on tuberose cultivation is Donna Mah from J&D Farm. She sells her flowers at the Keauhou Saturday Farmers Market and also has a business as a horticultural consultant.

She says the best time to plant tuberose bulbs in Hawaii is early spring. This gives them time to build up the energy they need to flower by fall. It usually takes six to eight months for tuberose to flower.

They grow best in full sun, in deep, rich, moist soil with good drainage. Do not let them dry out and be careful that the soil is not too wet. You can keep your tuberoses moist with the help of a 3-inch mulch layer.

Tuberoses are moderate to heavy feeders. Applying a slow-release fertilizer at the beginning of the season can help them produce flowers. A 5-10-5 fertilizer can offer extra phosphorus, which will encourage flowering. According to Mah, tuberoses require lots of food and water to flower well.

Each stem can bear a dozen or more white blooms, which may remain closed if the heat is particularly stifling. Also, the blooms can shrivel in direct sun when temperatures are 95 degrees or greater. If your garden gets this hot, plant the bulbs where they will receive some afternoon shade.

Email plant questions to konamg@ctahr.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 with an organic farm in Captain Cook.