Agastache Pests

Pests are not a common problem for Agastache plants in the garden. The larger wildlife in your garden such as deer or rabbits tend to leave the fragrant and leathery foliage alone, allowing the plants to grow large flower spikes for attracting more desirable pollinators and beneficial insects. While young, Agastache is susceptible to a few common pests. Slugs are attracted to the new growth of just about any herbaceous perennial. Whiteflies and aphids are common pests during periods of rapid foliage growth, and spider mites are typically only an issue for plants growing in the hottest and driest of the USDA growing zones. 

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Common Agastache Pests

Slugs

Slugs are skilled in finding all of the tasty new growth in a garden. Often early in the spring, the new emerging shoots of Agastache will appear ragged or uneven. The telltale shiny slime trails left by slugs will be visible during the day as well as other damage to tender shoots. The good news is that once the plants have grown to 6 inches or more, slugs rarely present a problem. Mature Agastache in a healthy growing environment will quickly overcome any damage when measures are taken to combat the slugs. 

Treating Slugs on Agastache

Slugs feed and move about the garden mainly during the night when temperatures are cooler and the risk of drying out is lower. Placing baited traps around the garden is a good way to collect and kill adult slugs. Shallow containers (cat food or tuna cans) of cheap beer or soapy water placed at soil level will attract and drown the slugs when they go in for a drink. To be effective, the cans must be emptied daily and refilled. The more slugs that are trapped means fewer eggs that are laid around your garden. 

If control by beer traps is not successful, there are wildlife-friendly commercial bait pellets that work well to control slug populations. Sluggo is one brand name to look for, although other brands use the same formulation. The active ingredient, iron phosphate, attracts and kills slugs but does not affect pets or wildlife and is safe to use around waterways and fish. Reapplication will be needed in climates that receive heavy summer rains or after a period of heavy irrigation. 

Preventing Slugs on Agastache

Removing all of the spots that slugs like to hide during the day is one way to keep them out of your garden. Any dark, damp location is prime real estate for a daytime hiding spot. Keep unused pots, bags of potting soil, hoses, and other containers cleaned, dried, and in a covered location such as a garage or carport. Encourage slug predators such as birds, reptiles, and snakes into your garden. 

Spider Mites

Spider Mites are most active during long periods of hot and dry weather. Plants that are dehydrated or struggling to grow are the first to be attacked. Plants that have been overfertilized are also prime targets for spider mites. Even though spider mites are tiny, they feed on lush growth and can produce many generations during a growing season. Mature plants may be able to sustain large populations while continuing to grow normally. 

Treating Spider Mites on Agastache

The mites may be hosed off the plant with a sharp stream of water. This not only will dislodge the mites, but it will also destroy the webs and eggs. For heavy infestations, a horticultural soap spray may be needed. Soap sprays will destroy the mItes by dissolving their soft bodies. Repeated applications may be required. Always follow the manufacturer's instructions.

Affected parts of the Agastache can also be pruned out to remove webs and insects. For late infestations, the plant can be cut back to within 6 inches of the soil. Agastache goes dormant in the winter and will resume new growth the following spring. Removing the top growth will not permanently damage the plant. 

Preventing Spider Mites on Agastache

Only use slow-release fertilizers when feeding. Agastache often grows best with minimal feeding. Mulching with 1-2 inches of organic compost in the spring is often enough to support healthy and vigorous growth. Do not allow plants to become heat stressed or dehydrated during extended periods of dry weather. Encourage beneficial predators to inhabit your garden. Lady beetles, lacewings, and predatory mites are natural enemies of spider mites. Using broad-spectrum insecticides affects not only pests but beneficial insects.

Aphids

Aphids are sap-suckers that mostly damage new tender growth, although even small outbreaks can cause twisted and deformed foliage or stems. Aphids may be white, green, or black, and most are host specific. Large outbreaks can lead to secondary infestations by ants and other insects that harvest the sticky honeydew aphids excrete as they feed. The honeydew can also be an annoyance when it drips on decks, driveways, or cars. Diseases such as sooty mold may settle on the honeydew, causing even more problems for the affected plants. 

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Treating Aphids on Agastache

Aphids are easily knocked off plants with a sharp stream of water from a hose or spray bottle. Be careful not to injure any new growth or flower spikes. Pruning out affected branches works well if an infestation is caught early. Make sure to remove all of the affected stem down to fresh growth. Use caution when applying insecticides to manage aphid colonies. Broad-spectrum insecticides will kill all insects, beneficial or pest. Using a less potent insecticidal soap spray that specifically lists aphids on the label is a safer and more environmentally sustainable approach.

Preventing Aphids on Agastache

Preventing aphid outbreaks is the best way to protect most of your plants in the garden. Overfertilizing is the number one reason that Aphids attack plants. High nitrogen fertilizers boost vegetative growth without supporting fast root growth. The imbalance between heavy top growth and limited root growth makes the foliage weak and less likely to fend off attacks from pests. Aphids also take advantage of plants weakened from growing in too little direct sunlight. Although Agastache grows fairly well in partial shade in a hot climate, it prefers at least 6 hours of full sun exposure and will not grow well in total shade. Encouraging natural predators of aphids (e.g. lady beetles, lacewings, syrphid fly maggots, and soldier beetles) to inhabit your garden will help control aphids and prevent widespread damage. 

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Author Robbin Small - Published 7-17-2023