Why brown spots on hydrangea leaves appear – and an expert trick to cure them

The question of how to treat brown spots on hydrangeas is one that many gardeners may face as the flowers blossom over summer. Hydrangeas are beloved for their blue, pink and yellow-green blooms, and there is nothing quite so diminishing as premature brown spots to ruin their aesthetic. 

Knowing how to grow hydrangeas correctly is among the most impactful ways you can prevent brown spots from arriving and prevent hydrangea flowers turning brown. However, infection from hydrangea diseases can still occur. Brown spots are (most frequently) caused by the fungus Cercospora that commonly exists in your soil. It usually reaches the plant through overhead watering or rainfall. 

So, while you may know when to plant hydrangeas for optimum growth, it’s equally important to know how to treat brown spots if they arrive. The secret? Compost tea, say garden experts. 

Treating brown spots on hydrangeas – with compost tea

Compost tea – the (homemade) liquid version of solid compost material – is an organic yet powerful way to treat brown spots on hydrangeas. So, it’s unsurprising that it’s so favored by experts. Here’s how they’re apply it.

How to make compost tea

'Compost tea is an easy-to-make natural remedy for brown spots on hydrangea leaves,' says  Jason White, the CEO of All About Gardening. To do so, he recommends using an old pillowcase or a burlap bag and filling it with compost.

'This will act as your tea bag as you place it in five gallons of water and leave it to steep for three days,' he says. After this time, Jason suggests pouring the liquid into a spray bottle [such as this one on Amazon] and applying it to the brown leaf spots of your hydrangeas.

Expert Jen Stark similarly encourages gardeners to try this compost tea remedy. 'Adding compost tea to a spray bottle and spritzing it on the brown spots on your plants can help remove them,' she adds. 

How does compost tea treat brown spots on hydrangeas?

How does this organic treatment fight this common problem? Jason explains that the compost tea has beneficial bacteria that will effectively fight the bacteria responsible for brown leaf spots in the hydrangeas.  

And, to protect your plant further, he suggests spraying the plant with diluted liquid kelp to boost its immune system so as to prevent the spots from recurring.

Why do my hydrangeas have brown spots on the leaves?

As the experts suggest, the most common cause of brown spots is the fungus Cercospora. However, certified master gardener, Gabriel J. Croteau suggests the problem could be down to its general growing conditions, too. 

'Hydrangeas need full sun to grow well and bloom. If the plant has been growing in the shade or partial shade, then you should move it into a sunnier spot,' he says. Therefore, when considering where to grow your flowers, and what to plant with hydrangeas, it is important to choose a spot that receives lots of sunlight. 

'Using compost tea is an accessible way to treat brown spots quickly and effectively,' adds H&G's garden expert, Rachel Crow. 'If you face these spots – whether from Cercospora or lack of sunlight – this trick will cure your plants back to health. The only thing left to know is how to prune hydrangeas to extend their lives further.'