Why do plant's leaves turn translucent after watering? (what is edema and how to fix it)

Ever noticed see-through patches on your plant’s leaves? Or brown or black dots that definitely aren't pests? This freaked me out the first time I noticed it, and of course it had to happen to my prized Stromanthe Triostar back when they cost a small fortune. Eeek. 

Leaves turning translucent or brown spots appearing after watering are signs of oedema. Pronounced 'edema' (and sometimes spelled that way also). More often seen in Stromanthe, Peperomia, Syngonium, Monstera, Hoya and Succulents, but it can happen to almost every indoor plant. It's more noticeable in variegated sections of leaves, and along leaf edges like these examples below.

Yes, edema is the same thing that causes those black rusty speckles in Ficus leaves, normally on new growth. In other plants such as Hoya and Orchids it can show up as small black dots, which can sometimes become raised bumps, often under the leaves. On others edema can present with multiple droplets 'oozing' out of the underside of leaves.

Edema presents differently in different plants and leaf types. If you've ever noticed your plants have water droplets along the edges or tips of the leaves, that's called guttation, which is a sign edema might be next. 

Find out below what causes translucent patches on leaves, if you should be worried about edema, what to do when you notice it on your plants, and how to prevent it from happening to your plants again.

What causes the clear patches and dots?

The patches and dots aren't always clear. The often look darker than usual, like the different examples above and below. But clear or darker, those patches are burst cells, almost like blisters, caused when roots take up water faster and retain more water than they can ‘breathe out’ and lose through transpiration.

The photo above shows 3 of the 5 most common plants you often spot edema on. On Stromanthe, Peperomia and Monstera the burst cells typically show as translucent patches. Syngonium is another one, like this chap below, where edema causes translucent patches. You can see signs of guttation below too where water droplets have appeared overnight following watering.

However in some plants edema shows as rusty, brown specks, usually on newer leaves first, like this Fiddle Leaf Fig below.

And on others like Orchids and Hoya it can look like small black or brown spots often mistaken for fungal disease or insect damage, like this leaf showing signs of both guttation and edema below.

Why does it happen after watering?

Over-watering often gets blamed because both guttation and edema show up after you water, but how can that be when you didn’t water any differently than usual? and it didn't happen last time?

That’s because a change in conditions your plants live in is more likely to be to blame, however you won't see that when the roots are dry, only when there is plenty of water available. After watering, roots get to work, doing their job transporting water to the rest of the plant, but guttation and edema happen when the plant can't use up all the water the roots are supplying.  

What causes edema?

If your plant isn’t getting enough airflow, is lacking enough hours of or intensity of light, or it’s been colder than usual lately, those can all reduce the normal  rate of water loss through transpiration. 

The move to a colder room, such as shifting a plant to a bathroom, or being too close to a window during cold weather (windows are one of the coldest areas of the house overnight), are common culprits.  

Roots take water in, but leaves don't 'breathe it out' as fast as usual, but that water has to go somewhere. On same plants you get drips on leaf tips, called guttation, others you get translucent patches where cells have literally burst from being too full of water.

How long will leaves stay translucent?

Don’t panic, but don’t ignore it either. Odema in this form normally goes away the same day after watering, no damage done. However do take it as a sign something needs to change 

If you don’t change anything and it keeps happening, it can cause permanent damage

How do you prevent edema?

As soon as you notice it, move your plant. Try somewhere brighter or warmer, but keep in mind the #1 cause is a lack of airflow. If it's warm enough, consider opening a couple of windows, or in a closed environment like a grow tent or prop box, add a small fan for airflow. Stagnant air is the main reason edema is more common in indoor plants than outdoors.

If it's not due to your plant's conditions needing improvement, another cause of edema relates to a deficiency in essential minerals. Edema can also be caused by a lack of calcium or potassium, so make sure you’re not starving your plants! Check the label on your fertiliser. It should list potassium, but not all fertilisers include calcium.

If your fertiliser doesn’t include calcium, you could change to one that does, or simply add calcium with a sprinkle of dolomite lime once a season. Don’t waste your time with eggshells or watered milk!

If conditions are great, and you're confident you're giving your plants all the essential minerals they need in the right amounts, then also check soil pH. If your soil is too acidic, or too alkaline, that gets in the way of your plant taking in the essential minerals it needs. Calcium for example gets ‘locked out’ when soil pH becomes too acidic, so even if you're adding calcium, it may be blocked from being absorbed.

You could get your soil tested, or there are also fertilisers available that are pH buffered to help maintain soil pH in the 'healthy zone' for nutrient uptake. For most indoor plants you want to aim for a lightly acidic pH of 6.

The good news is edema is normally temporary, it will go away, and with a change to better conditions, that’s often all your plant needs to stop it happening in the future.

Happy growing!

Anna @lovethatleaf