Sticky plant leaves can be unsightly with houseplants, but it is not uncommon. The causal factors may be natural or from the excretion of unwanted pests on your plants.
The sticky substance on plant leaves can be naturally caused by plants or by excretion from pests such as aphids and scales on the leaves. Plants secret nectar for protection and to attract food, whereas pests suck nectar from plants and produce Honeydew which also leaves a sticky residue on plant leaves.
Solving this sticky situation requires knowing where the sticky sap is coming from. By knowing its origin, you can have a better idea of what you are dealing with. Sticky sap can come both from plants themselves and pests.
With closer inspection, you can determine if it’s just your plant or if there is an underlying pest infestation going on. We will discuss them both and how to clean up this sticky plant situation.
What Can Cause Sticky Substances on Plants?
1. Secretion from Insects (Honeydew)
Insects such as mealybugs, aphids, and scale are the usual suspects when it comes to sticky secretion on plant leaves.
These sap-sucking insects feed on the nectar of the plants by tapping into the phloem of the plant.
The Phloem is the living tissue in vascular plants that transports sugars and other metabolic compounds to other parts of the plants (source) .
These insects rob the plant of the food it produces in the leaves and transport to other parts of the plant by sucking it directly from the leaves.
The byproduct of this is honeydew which is poop produced by the insects after they digest the sugars from the plants.
The honeydew remains on the plants and sometimes drops onto lower leaves which in turn attracts ants and other unwanted insects to the plant.
Insects attracted to honeydew can cause mold and other fungi to grow on the surface of leaves, sometimes even coating it totally. It is important to clean the stickiness off the leaves as soon as you spot it.
The root of the problem, in this case, is the pest, which has to be addressed quickly.
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2. Secretions from Plant Nectaries –
Plants produce sticky nectars to attract certain insects or sometimes to ward them off.
Nectar Secretions for Plant Protection –
Plants have secretory structures called extrafloral nectaries. Thousands of plants have these glands that are located all over the non-flowering parts of the plants.
These nectaries are used to produce nectars that are aimed towards attracting ants, which in turn will protect the plant from herbivores. This forms a symbiotic relationship where the sticky substance is necessary for the plant’s survival in the wild.
In the case where plants secrete sap to ward off insects is actually a survival technique.
When the invading insects get too close, the plant gets into defense mode and starts secreting nectar that is unattractive to that particular insect, and as a result, the insect leaves.
Nectar Secretions for Plant Reproduction –
In the case where nectar is secreted in floral nectaries (nectar secreted at the flowering parts) to attract insects such as bees and butterflies; it is done so that the insects may help to carry on the plant species through pollination, which is called entomophily
Nectar Secretions to Attract Food –
The attraction may not be all that good for insects when it comes to plants like sundews or other carnivorous plants.
The aim is to attract insects for food and survival. Secretions are made and can linger on the leaves and stem causing it to become oxidized which gives it a brownish appearance. Insects come to eat the nectar and are captured and eaten by the plants.
3. Bacterial Infections
In some cases, the sticky substance found on plant leaves may be the cause of a bacterial or fungal infection.
Bacterial infections are often caused by cuts or broken parts of the leaves which are exposed to open air.
Plants that are subjected to bacterial infections as such are –
- Snake Plants
- Monstera Deliciosa.
A common type of bacterial infection is Erwinia infection.
The symptoms of its onset are a sudden rotting of the tuber, which gives off an unpleasant smell, and a drooping of the leaves.
The bacterium is present in most soils, but it is fostered by too much warmth, too much fertilizer, and the use of too heavy a growing medium which makes for too much humidity.
Erwinia infection can be often mistaken for fungal infection and therefore is treated the wrong way. There is no chemical or medicinal way of treating the disease and the best course of action, in this case, is to remove the affected parts by cutting it off.
Now cutting off the affected area may also lead to additional exposure if there is nothing done after the cut is made.
The best antifungal treatment for a severed leaf or stem is cinnamon added directly to the affected area. Cinnamon oil also works in the same way when used to treat open wounds in plants.
How to Prevent Bacterial Infections in Plants
- Ensure strong vigorous plants by arranging for plenty of light and ventilation eliminates infected or suspect plants
- Avoid sudden variations in pH
- Do not mist-spray as a means of lowering temperature avoid irregular watering (over-watering, drought)
- Avoid over-acid soil (pH < 5.5)
- Avoid too peaty a growing medium (ensure a well-aerated soil) as well as excessive nitrogen content or large doses of fertilizer all at once
- Disinfect materials and tools used, with alcohol or dilute bleach
- Avoid excessive humidity, and temperatures either too high (over 25ºC – 77ºF) or too low (below 10ºC – 50ºF)
- Do not reuse either the earthenware pots or the compost used by infected plants; disinfection is not easy to achieve.
- Do not cover the tuber with compost at re-potting time.
- Watch out for infected drainage water carrying infection from pot to pot (take care with sub-irrigation and gutter irrigation)
Why does the Nectar and Honeydew Look Brown and Sticky?
After the nectar is produced it can drop unto other leaves beneath it or unto the floor.
It may not be a good idea to leave the sticky nectar there (if you notice it right away) as they may attract more insects.
The sticky stuff may also cause dust and other pathogens to become caught and build up making it even more unattractive to see.
The brownish appearance as mentioned above is because of oxidation of the organic sugars contained in the nectar. Additionally, honeydew may cause fungus to colonize and build upon the honeydew giving it a dark appearance.
Unwanted Attraction of Insects-
Nectar and other secretions may attract unwanted visitors to your home. This is naturally what the plants do in the wild. No need to start bashing the plants!
You, as a plant owner, may not want to have ants, lizards, and other little insects venturing inside your home since they will not only go after nectar but other seemingly edible goodies within your home.
For this reason, you should clean the sugary stickiness off the floor or leaves of the plants before the invasion begins.
How to Control Scale and Other Sap-Producing Insects?
Some of the more common insects that produce honeydew are aphids, Mealybugs, and scale.
The method of control may depend on the level of infestation of these insects on the plants.
Precaution has to be taken when applying control methods for these insects within the home.
It is never recommended to use chemical insecticides within the home as it may not only kill pests but also affect you or anyone else living in the same space.
1. Scale –
Scale is small, oval, and flat, with protective brown shell-like covering insects.
They may be difficult to control since their shell-like coating protects them from insecticides.
The scale can be removed by manually wiping them off the plant by using a cloth soaked in warm water or by soaking the scales with a cotton swab soaked with rubbing alcohol.
House plants that are too far gone being infested with scale have to be discarded and hopefully, after reading this article you will not have to reach this point before action is taken to eradicate these pests.
2. Aphids –
Early detection is the key to stopping aphid infestations. Aphids are soft-bodied insects that use their piercing mouths to feed on plant sap from the phloem.
They are often found on the underside of the leaves and start off white in smaller batches and then turn brown as the sugars they suck start to oxidize.
Once an aphid problem is detected you can use insecticidal soap or a mixture of dawn and warm water and apply directly to the aphids by spraying using a spray bottle.
Horticultural and neem oil also does a good job in dealing with aphids.
Again, mix the oil with water and a little Dawn dish soap into a sprayer or a spray bottle and apply on plants struggling with insect issues.
These applications are effective because they disrupt the aphid’s cell membrane causing them to die.
3. Mealybugs –
Mealybugs are soft-bodied insects that often appear as white cottony masses on the leaves and stems. They are also controlled using a mixture or neem oil and or dish soap.
Additionally, a solution of isopropyl (rubbing) alcohol in water may be dabbed directly on mealybugs with a cotton swab to kill them or remove them manually.
Straying away from chemical warfare other biological methods can also be used if you are willing to have them in your home.
Ladybugs when introduced to plants with aphids and mealybugs can eat and destroy the entire pest population within a couple of days. Again, only if you are willing to have them around.
How to Clean Sticky Plant Leaves
The sticky sugars produced from Nectar or Honeydew can be removed by-
- Mix 1 teaspoon of DAWN dish detergent into 1 quart of warm water
- Dip a cotton swab into the dish detergent / warm water mixture
- Wipe the area down completely
Additionally, if you don’t have dawn available you can use a cotton ball soaked in rubbing alcohol and gently wipe the sticky sap off from the plant.
The Dawn and alcohol break down the sugars causing them to be released from the surface of the leaves which is then easily removed.