Nasturtiums are easy to cultivate, thriving even in less-than-ideal soil conditions, and are known for their vibrant colors. However, the plant’s succulent, soft tissues and specific chemical compounds make it uniquely appealing to a wide array of garden pests. Because of this strong attraction, nasturtiums often function as a sacrificial host, drawing insects away from other prized vegetables or ornamental plants.
Identifying the Most Common Nasturtium Pests
The most frequent visitors to nasturtium patches are sap-sucking insects, with aphids leading the list. Aphids tend to cluster densely on the newest growth, stems, and the undersides of leaves. As they feed, these soft-bodied insects excrete honeydew, a sugary waste product that leaves the foliage sticky and may lead to the growth of sooty mold.
Another common issue involves foliage-feeding caterpillars, most notably the larvae of the imported cabbage white butterfly. These pests are characterized by their velvety green bodies and actively chew large, irregular holes in the leaves. Their presence is frequently confirmed by frass, the small, dark droppings left behind near the feeding damage.
Spider mites are persistent pests that thrive in hot, dry conditions and can quickly infest nasturtiums. These minute pests feed by piercing plant cells, causing characteristic stippling—tiny, light-colored dots—on the leaves. As the infestation progresses, fine, silky webbing becomes visible, particularly near the stems and on the undersides of the leaves. Flea beetles are another concern, as these small, dark beetles hop when disturbed and leave behind numerous tiny “shot holes” in the leaves.
Cultural Practices for Pest Prevention
Managing the growing environment is the first line of defense against pest infestations, reducing plant stress. Proper spacing between plants is important, as overcrowding decreases air circulation and creates a humid microclimate that favors certain pests. Allowing adequate space helps the leaves dry quickly after watering, which discourages mites and aphids.
Gardeners should exercise caution with fertilization, as nasturtiums prefer soil that is not overly rich in nutrients. High-nitrogen fertilizers promote a flush of tender, soft, new growth, which is more attractive to sap-sucking pests like aphids. Using minimal or no fertilizer encourages harder, less appealing plant tissue that naturally resists infestation.
Maintaining consistent soil moisture through regular watering helps keep the plants healthy and resilient. Water-stressed plants are often weaker and more vulnerable to attack, though overwatering must be avoided to prevent root issues. Regularly inspecting the plants, especially the undersides of the leaves where pests hide, allows for early detection and intervention before populations can explode. Early removal of a few infested leaves can often prevent the need for more aggressive treatments later.
Direct Mechanical and Biological Controls
Once pests are detected, physical removal methods are used to reduce their numbers. For larger pests like the cabbage white caterpillars, direct hand-picking is highly effective. These larvae can be easily spotted, removed from the foliage before they cause extensive damage, and disposed of away from the garden.
A strong, direct spray of water from a garden hose is an effective mechanical control for dislodging colonies of aphids and spider mites. The high pressure physically knocks the pests off the plant, and many soft-bodied insects cannot easily return. This technique should be performed early in the morning so the foliage has ample time to dry, minimizing the risk of encouraging fungal growth.
Encouraging beneficial insects represents a key biological control strategy. Ladybugs, for example, are voracious predators of aphids, consuming large numbers during both their larval and adult stages. Lacewings are another valuable ally, as their larvae, known as “aphid lions,” actively hunt down and consume soft-bodied pests. By avoiding broad-spectrum chemical sprays, gardeners ensure that these beneficial insects can establish a presence and maintain a natural balance.
Effective Organic and Chemical Treatments
Topical treatments become necessary when mechanical and biological controls are insufficient. Insecticidal soap is one of the safest and most common organic treatments, working by dissolving the protective outer layer of soft-bodied insects like aphids and spider mites. A simple solution involves mixing a small amount of pure liquid soap, such as Castile soap, with water.
The soap solution must be applied thoroughly, completely coating the insects to be effective. Focus heavily on hidden areas like the undersides of leaves.
Neem oil, derived from the seeds of the neem tree, is another organic compound. It acts as a repellent, an anti-feedant, and a growth regulator, disrupting the life cycle of many pests. It is typically mixed with water and a small amount of soap, which acts as an emulsifier.
Neem oil application is most effective when done in the early morning or late evening, avoiding the intense midday sun that can cause leaf burn. Treating the plants outside of peak daylight hours also protects beneficial insects, such as bees, which are not active during these times. Treatments should be repeated every seven to ten days until the pest population is under control, ensuring thorough coverage.
Conventional chemical interventions, such as pyrethrin-based products, are used for managing nasturtium pests. These products are fast-acting but should be used with caution, as they can harm beneficial insects and other non-target organisms. If chemical treatment is necessary, carefully read and follow all label instructions regarding application rates, safety precautions, and pre-harvest intervals.