What Is Oedema And How Do I Treat It?

by Sarah Gardiner

Sarah is a horticulturist with a passion for edible and cottage gardens. 

Red spots on new growth of Ficus lyrata is a result of inconsistent watering, leading to oedema. 

Have you ever wondered why the new, emerging leaves on your Fiddle Leaf Fig look perfectly green one day, then the next they’re covered in red spots and you’re not sure what you did wrong? Or maybe you’ve grown a Peperomia that’s been happily thriving in your care, only to notice that the undersides of the leaves look to be covered in bubbly scabs which you’d never seen before. Although it might manifest in different ways across varying species - the problem here is called oedema, commonly known as moisture stress.

What is oedema?

Oedema occurs in plants when leaves take up more water than they can transpire. It results in the excess water rupturing the cells, particularly on the underside of the leaves, leading to blistered and scarred-looking patches. When transpiration happens, plants draw water through the stem into their leaves and the water is returned to the atmosphere. To put it simply, transpiration is similar to how we would sweat, and oedema could be described as sweat bubbles that could occur if we were incapble of sweating profusely!

Signs and symptoms

As we already mentioned, signs of oedema vary between different susceptible species. You might notice a number of symptoms in plants suffering from oedema. Watery blisters will appear on the leaves at first. These will become raised like pimples or warts. These blisters can then rupture and have a scaly appearance, then becoming quite scabby or corky in texture as they harden.

Causes of oedema

Oedema is most commonly caused when conditions in the atmosphere or soil are excessively moist, giving the plant a higher water content than normal. When air temperatures drop but soil moisture levels remain high, plants are at risk of oedema. Another common cause of oedema is inconsistent watering, often occurring after overwatering a plant too frequently.

How to treat

As oedema is caused by environmental issues, it is important to treat the situation by figuring out what is causing the plant to retain too much moisture. Adjusting your watering regime is a good place to start and try to only water your plant after the soil has dried down, particularly as we move into cooler months of the year. Providing your plant is otherwise healthy and temperatures aren't too low, you can take the opportunity to repot your plant in a more free-draining potting mix, if the current soil is retaining too much moisture. Be sure not to remove too much of the affected foliage as oedema will move to the remaining leaves on the plant. Moving your plant to a brighter position helps plants with oedema as it will ensure they dry out faster, but be sure to do this gradually so they don’t suffer from sun stress or wilting.

Water pressure builds up in the internal cells of this Peperomia, eventually bursting and leaving blister-like damage on the underside of the leaf. 

Prevention

Oedema is common in greenhouse growing as temperatures can vary across the seasons and airflow is often limited. Good airflow is essential in preventing oedema. Make sure during spells of high humidity that plants have good airflow and are not crowded too closely together. Inconsistent watering contributes to oedema so grow your plants on the dry side and only water when the soil is dry. Always make sure to use a free-draining potting mix or add perlite to your soil so it doesn't retain too much moisture. 

So, now that we've explained oedema, simply follow the practices above to prevent or correct this problem in your plants. If you do notice any signs or symptoms, start by decreasing the water you provide and increasing both light and air circulation for best results to avoid the problem getting worse. 

Oedema on the velvety leaves of a Philodendron melanochyrsum manifests in the form of a watery blister. 

Oedema is the likely cause of small, rough and irregular brown spots on succulents - like this Echeveria.