Gladiolas should be planted deeply

: Why are my gladiolas falling over? They look healthy and have beautiful blooms, but they don’t stay upright. Is there something wrong with the stem or root? Are there insects that cause this?

A: The insects that primarily infest gladiolas are called thrips, but they feed on the blossoms and leaves. The stems are simply falling over because of the weight of the blooms. The remedy is to plant the bulbs deeper for better support. If the soil is rocky, and if it is difficult to dig a deep enough hole, the plants will require staking. The same is true if they are grown in windy, exposed areas. Stake individual plants if needed.

Gladiolas are relatively easy to grow. They require well-drained soil in a sunny location. While often classified as bulbs, gladiolas are actually corms, enlarged stems. Corms which have a plump shape with high centers are better than large, flat ones; the thicker the corm is, the higher the quality of bloom will be.

It is best to select corms 1 ½ to 2 inches in diameter. They will produce full, attractive flower spikes. Small corms produce foliage but may not bloom. Planting depth varies with the size of the corms. Large corms should be planted 4 to 6 inches deep and 6 inches apart. Small corms should be planted at a depth of 3 inches and about 2 inches apart. A general rule of thumb is plant deeper in sandy soils and shallower in heavy soils. Successive plantings every two weeks will provide continuous bloom.

For a lovely flower arrangement, cut gladiolas as soon as the bottom flower on the spike has opened; the remaining blooms will open as the spike is brought indoors and placed in water.

Q: My problem is that the blooms of gladiolas have white specks in them, and sometimes they don’t even open. Can you shed light on this? — C.N.

A: These gladiolas are infested with an insect called thrips; they are the major insect problem for the blooms. Thrips are tiny, slender black insects, about 1/25th of an inch long, and they are difficult to see with the naked eye. Thrips cause white streaking on the leaves and blooms.

Flowers can also be misshapen, discolored and occasionally will fail to open. Thrips feed by puncturing the plant tissue and sucking out the cell contents. Preferring to feed on rapidly growing tissue, thrips often cause stunting. These insects are the culprits; they also cause silvery to brownish scarring on avocado and citrus fruit surfaces, but this cosmetic damage does not harm the internal fruit quality.

Horticultural oils, azadirachtin (Safer BioNeem), neem oil and insecticidal soaps can be somewhat effective for the temporary reduction of the number of thrips — if applied when damage first appears. Sprays must be applied to thoroughly cover susceptible areas of the plant, including new leaf growth and buds.

Note: these insecticides only kill newly hatched thrips and recently emerged adults, thus repeat applications are required, usually five to 10 days apart, depending on the temperature. In the future, moving the location of the gladiola bed each year will discourage the spread of thrips.

Avoid the use of insecticides such as malathion and carbaryl, because they are highly toxic to natural enemies and can cause dramatic increases in spider mite populations and are generally not effective against most thrips.

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.