Wilting Sunflowers

Annual sunflowers (Helianthus annuus) have bright, happy faces that faithfully follow the path of the sun across the sky. Sunflowers range from 3 to 10 feet tall and have yellow, mahogany or red petals. These tough, vigorous plants are generally healthy and trouble-free, but several factors can make them start wilting.

Although sunflowers can tolerate some drought, getting too little water over a long stretch of time can make the plants wilt. Water the sunflower plants regularly and deeply during times of active growth, or at least 20 days before and after they flower. University of Minnesota Extension notes that sunflowers need 1 inch of water every week, so water plants if your area doesn't receive that amount in rainfall. Don't overwater sunflowers, or you increase the risk of root rot, a disease that makes plants wilt and die.

Sunflowers attract various pests, including foliage-feeding caterpillars, aphids and cutworms, but carrot beetles and sunflower stem weevils most commonly cause wilting.

Carrot beetles (Ligyrus gibbosus) have dark red-brown, 1/2-inch-long bodies. The pests feed on both above-ground and underground plant parts, causing affected sunflowers to wilt, turn yellow, fall over or die.

Because pesticides have little impact on carrot beetles, make your yard unattractive to the pests by regularly removing weeds and decaying plant materials that serve as hiding spots. The beetles hang around outside lights at night, so discourage attacks by keeping your garden and yard dark at night.

The pale, maggotlike larvae of sunflower stem weevils (Cylindrocopturus adspersus) bore into sunflower stalks and crowns to feed and overwinter. This boring activity causes plants to wilt, turn yellow and collapse. The larval pests reach about 1/4 inch long and have whitish, legless bodies with darker heads, while the adult insects have gray-brown, 1/4-inch-long bodies covered with white spots. The adults don't damage plants by feeding, but the females lay eggs inside the sunflower stems, giving the emerging larvae easy access to food sources.

The University of California's Statewide Integrated Pest Management Program website suggests controlling sunflower stem weevil populations by cleaning up and discarding plant debris. North Dakota State University Extension service recommends delaying planting until late spring through early summer so adult females have no place to lay their eggs.

A carbaryl-based pesticide can control the weevil pests, but don't spray plants within 60 days of when you plan to harvest. Select a ready-to-spray formulation that you just hook up to your garden hose. Spray sunflowers using a slow, sweeping motion until the foliage glistens with moisture. Repeat applications when needed, but not more often than once a week.

Only apply carbaryl sprays during calm weather when no rain is expected for the following 24 hours.

Carbaryl is toxic to bees, so spray between 7 p.m. and 6 a.m. when the pollinators aren't active. 

Pesticides can irritate your skin and eyes, so follow the safety precautions on the product's label and cover up as instructed. 

Keep people and pets away from treated plants until the spray dries, typically at least four hours.

Some plant diseases can affect sunflowers and cause the plants to wilt.

Root rot fungi (Pythium spp., Fusarium spp., Phytophthora spp., Rhizoctonia spp.) can attack sunflowers, causing the plants to wilt and turn yellow. As the fungus attacks the roots, the plants can't absorb enough water and minerals, which also stunts their growth and eventually kills them.

Always plant sunflowers in soils with good drainage and don't overwater them, because waterlogged soils promotes the growth of root rot fungi. The only thing you can really do once root rot infection occurs is to dig up and discard the infected sunflower plant.

Wilt#/SNAME 2#

The Verticillium wilt fungus (Verticillium dahliae) lives in the soil and infects the roots of sunflower plants. It spreads throughout the xylem, or the tissue that carries water, plugging it up so the plant doesn't receive enough moisture. The disease first appears on the lower leaves, causing them to wilt before spreading to the leaves further up on the stem. In addition, the leaf tissue can turn yellow or brown and the stem might turn black close to the ground. Severely affected sunflowers can die before they flower.

Prevent Verticillium wilt by planting disease-resistant sunflower cultivars. Soil solarization can kill the fungi dwelling in the top few inches of soil. To solarize your garden soils, the University of California IPM recommends treating the soil during the hottest part of the year.

Rake

Hose

Plastic sheeting, 1 to 4 mils

Clean up, smooth out and level the surface of the treatment area with a rake. Remove any debris, rocks or dirt clods.

Wet down at least the top 12 inches of soil.

Roll clear or transparent plastic tarps with 1 to 4 mils thicknesses over the soil's surface. Smooth out any air pockets that appear.

Bury the tarp's edges beneath enough soil to keep them down on windy days.

Allow the plastic to sit for four to six weeks during hot, sunny weather.

Remove the tarp when it's time to plant sunflower seeds.

In general, thicker plastic works better in small areas or windy climates, while the thinner plastic allows more heat to get through to the soil.

Black plastic doesn't work well for soil solarization because it deflects heat instead of trapping it. 

Don't leave the plastic tarp down for more than six weeks or the material can become brittle and hard to remove.