Some entirely natural plant processes are sent to scare us houseplant hobbyists. Share a photo of white deposits in a plant group and others may have you panicking about pests, leaf spot, powdery mildew or imminent plant death.
Especially if they are outdoor gardeners.
That's because outdoors, the combo of weather like rain and wind, and beneficial bugs like ants, often gets rid of the evidence of these natural plant processes before gardeners see them.
But indoors, we get to see a lot more of what plants get up to (and get to worry a lot more!).
There are 3 common natural processes that can all appear as white deposits, mainly on leaves, sometimes on stems and petioles (the fancy name for the stalk that attaches a leaf to the main stem or branch).
In the case of guttation or phenolic gland deposits, the deposits might appear chalky, salty or waxy, so it's easy to see why others jump to the conclusion it's a pest, like mealybugs, or a disease like powdery mildew or leaf spot.
In the case of extrafloral nectaries, more often mistaken for pest damage or leaf spot, the spots left behind may be papery white, but can be from cream to rusty brown.
Guttation is the most common of the 3 causes of those deposits, with extrafloral nectaries next down the list, and the waxy deposits from phenolic glands the rarest to see (based on my 30+ years experience in the hobby anyway!).
Although, to be fair, I get to see all 3 more often than a typical hobbyist might because I'm also the owner of a plant store (you're probably reading this on my store website right now).
I get all sorts of 'What is this on my plant?' photos sent in daily by my customers (I offer free plant support and advice to local hobbyists if you're based in New Zealand).
But if you came here for help with leaves turning transparent, rusty-looking spots on new leaves, or bumps or pitted damage on foliage, you might be looking at a case of edema (which is related to guttation).
Head over to What is Edema and what to do about it? > for more about that plant process.
Coming up below we're going to take a quick look at guttation, extrafloral nectaries and waxy deposits produced by phenolic glands (maybe one day I'll expand each into their own deep dive article like I have with edema).
1. Guttation
Let's look at the most common one first. When plants 'breath' it's called transpiration, and when they 'cry' it's called guttation. Basic analogy there but but a good place to start.
Guttation is a natural process that appears as droplets of water. Usually on the tips of leaves, sometimes along the edges.
The liquid droplets are xylem sap. A mixture of water, sugars and minerals.
When they dry, the water evaporates, leaving behind the mineral salts and sugars. Depending on how hard your water source is, the minerals in your soil and in your fertiliser, what's left behind can vary.
It may look like faint white chalky marks on the tips, leaf edges or undersides of leaves, a bit like limescale you might see on your shower glass; or can appear as a crusty build-up, usually of white crystals from the dried mineral salts.
You might spot those same white salt deposits on the surface of your soil, around drainage holes and on the outside of ceramic pots.
Pressure inside plants keeps water, nutrients and sugars moving around, keeping plants alive, fueling growth as well as other plant processes like photosynthesis.
During transpiration (that's a fancy name for water loss via evaporation as a gas), water is 'breathed' out of tiny holes or pores in the leaves, called stomata.
When that pressure builds up, usually at night or in high humidity when stomata are closed, you might get guttation happening.
If roots continue absorbing water from the soil even when stomata are closed, it causes a build up of root pressure.
With nowhere to exit through stomata, microscopic pores called hydathodes do the job instead. Hydathodes are located at the ends of leaf veins, which is why you see those droplets on the tips and edges of leaves.
The hydathodes release the extra pressure as droplets of water mixed with sugars and minerals. If they didn't, next step could be edema, where cells in the leaves swell and burst, so guttation is good!
Why guttation might also be bad is because it can be an early sign of over-watering, of the wrong potting mix that's holding too much water and not enough air, or of general conditions not being ideal, such as airflow being too low or humidity too high, or of a sudden swing in conditions.
I see it more often in spring and autumn, but it can happen any time of year. My pothos, philodendrondrons and monsters are particularly prone to it after I water.
2. Extrafloral Nectaries
Less common than guttation, but another form of deposits and marks on stems and leaves that often gets mistaken for pests, or for fungal or bacterial leaf spot, is extrafloral nectaries, also nicknamed EFNs.
These usually first appear as small sticky droplets underneath leaves, but can appear on top, and along petioles and stems also.
That sweetness and stickiness is one reason EFNs get mistaken for pests like scale, that produce sticky, sweet honeydew (their waste). EFN's feel a little tackier than water and are a combo of water, sugars and nutrients.
When EFNs dry, they tend to leave a blemish behind, usually rust coloured but the spots or dots can vary from cream to brown, sometimes red or purple, depending on the pigments in the leaf.
The dried EFNs are more likely to damage velvet or softer leafed varieties, looking almost like a burn, especially varieteies like Philodendron micans.
EFN's are most often reported on philodendrons, but I've heard of alocasia, hoya, calathea, maranta and aglaonema all producing EFN's also.
A plant producing EFN's is not a sign of pests, although plants have been observed producing EFNs when pests are about, as that sweet, sugary liquid helps attract ants to feed on the pests and help protect plants.
Plants can also produce EFN's when happy and healthy, when nothing is wrong at all. Some hobbyists just consider it an unavoidable part of owning philodendrons.
I've found certain varieties more likely to produce EFNs, like Ring of Fire, Spiritus Sancti, Rojo and Micans, and EFNs more likely to leave behind blemishes on certain varieties more than others.
If you're dealing with EFN's and not a fan of the damage they leave behind, a regular mist and/or wipe with water is often enough to get rid of them before the dry and cause damage.
3. Phenolic Glands (Dorsal Wax)
The rarest reason for white deposits, although only rare because it appears on one main houseplant family, Ficus, and because it often gets missed due to where it shows up. That includes Fiddle Leaf Figs (Ficus lyrata). It's actually quite common if you go looking for it.
Here's one example from a Ficus benjamina:
If you suspect phenolic glands are to blame, where the deposit appears is an important clue.
The waxy or salty deposits tend to appear under the petiole, in the joint where the leaf meets the petiole (that's the leaf stalk that connects the leaf to the main stem); or on the underside of leaves usually along the middle rib, which is where the phenolic glands can be found.
These are often either missed, or mistaken for waxy deposits from mealybugs. The waxy appearance is due to Ficus sap including latex, turning it while.
The purpose is the same as EFN's, to supply food for ants to attract them and help them protect the plant from pests.
Another reason a ficus growing outdoors may not show the deposits is because of the abundance of ants eagerly getting a free feed.
You can help others
Before you go, if you came here because you currently have what you suspect is guttation, extrafloral nectaries or phenolic gland wax deposits, please consider sending in a photo.
Guides like this become even more helpful to others when you can see what these common symptoms look like.
DM me your photo via Instagram > or via Facebook > and I'll make sure to credit the photo to you.
ABOUT THE AUTHOR: Anna is the owner of plant store Love That Leaf which specialises in products and advice for indoor plants. She is also the resident indoor plants writer for NZ Gardener magazine. Her houseplant care articles appear in print in magazines in-stores NZ-wide, and online on Stuff, The Post, NineHoney and more.