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Why are my plant leaves sticky?

Sticky leaves have one cause. Something is drinking sap above them and excreting the surplus as honeydew, which falls onto whatever is beneath. Finding out which insect is a short job once you know to look upwards.

Look above the sticky leaves, not at them

Honeydew falls, so the insect producing it is on the growth overhead rather than on the leaf you are holding. Follow the stickiness up the plant to the shoot tips and the young stems, and check the undersides. On a sticky patio or a sticky car, look at what is growing over it.

Work down the list

  1. Aphids

    What you would see
    Dense colonies of green, black or grey insects packed onto shoot tips, flower buds and the undersides of young leaves, from May onwards.
    The check
    Look at the newest growth first. Aphids go for the softest tissue on the plant, and a column of ants running up the stem will take you straight to them.
    Aphids

  2. Scale insects

    What you would see
    Immobile brown, yellow or white bumps along stems and the main veins, looking like part of the bark, on bay, citrus, camellia, holly and many houseplants.
    The check
    Run a thumbnail along a stem. Scales pop off, and the young stages are flat translucent scurf that most people never notice.
    Scale insects

  3. Mealybugs

    What you would see
    White waxy tufts in leaf joints and under leaves on a houseplant, with the stickiness worst directly below them.
    The check
    Squash one and look for an orange or pink smear, which confirms an insect rather than a fungus.
    Mealybug

  4. Whitefly

    What you would see
    Tiny white insects rising in a cloud from the undersides when the plant is knocked, in a greenhouse or on indoor plants.
    The check
    Turn a lower leaf over. The scale-like immobile young stages are what produce most of the honeydew, and they are easier to miss than the adults.
    Whitefly

  5. A tree overhead

    What you would see
    Sticky spotting on a car, a patio, garden furniture or the plants under a lime, sycamore, cherry or willow, from June to September.
    The check
    Look up. Lime aphid on a mature lime produces enough honeydew to make the pavement tacky, and nothing can or needs to be done to a tree that size. Move the car and the furniture instead.

  6. The black film that follows

    What you would see
    Leaves that were sticky a fortnight ago now covered in a black or dark grey film that wipes off to leave sound green leaf.
    The check
    Wipe a leaf. Sooty mould grows on the honeydew rather than on the plant, so removing the insect stops the black returning.
    Sooty mould and honeydew

  7. The plant making it itself

    What you would see
    Sticky glandular hairs over the stems and leaves of a plant that has always had them, with no insects anywhere.
    The check
    Feel a new leaf on an undamaged part of the plant. Salvias, petunias, tomatoes, tobacco plants and sundews are sticky by design, and a plant that is sticky all over evenly is doing it on purpose.

Still not sure which one it is? Photograph the leaf and compare it against the full library.

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