Why Are My Mum’s Leaves Turning Yellow?

Noticing your mums’ leaves turning yellow can be a bit alarming, but don’t worry - there’s usually a simple explanation! In this article, gardener and biologist Lindsay Miller explains the common reasons behind this issue and how you can get your mums back to their vibrant best.

The vibrant pink chrysanthemums bloom brightly against the backdrop of their fading yellow leaves, creating a striking contrast in the garden.

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By October, your fall garden decor is likely in full swing.

Pumpkins? Check.

Ornamental kale? Check. 

Mums? Yikes. Those once-vibrant flowers you purchased a few weeks ago are now sporting pale yellow leaves and looking a little frail. What’s a gardener to do?

You’ll recognize a healthy mum when you see it. These are compact, dense little plants that are typically mounded in shape. Stems should be bushy and chock full of leaves (no leggy mums, please). Leaves should be a lush, deep green, with no signs of wilting, or ahem, yellowing. The flowers are bright tufts of yellow, orange, copper, and burgundy.

The yellowing of plant leaves is referred to as chlorosis. The leaves appear yellow because they lack chlorophyll. In some cases, it’s just a part of the plant’s natural aging and growth cycle. However, it can also be indicative of nutrient deficiencies, disease, or overwatering.

So grab your garden gloves and let’s get to the root of this problem—your mums will thank you with fresh foliage in no time.

Poor Drainage

A close-up of a gardener in gray gloves adding fresh soil to a newly transplanted chrysanthemum in a wicker pot, which displays clusters of vibrant pink flowers against a backdrop of dark green, deeply lobed leaves that enhance its texture and contrast.
Keep them moist without overwatering for vibrant blooms.

Despite their hardiness and cold tolerance, mums are actually a bit picky when it comes to water. They like to be kept moist. They do not like their roots to dry out. These classic fall-blooming plants have shallow roots and therefore, they need regular, consistent watering.

Even so, these plants do not do well in saturated soil. Roots actually need a bit of oxygen, and soil that is heavy, compacted, or doesn’t release water will eventually “drown” the roots. This can cause mum leaves to yellow and eventually blacken and drop.

What To Do

Avoid soggy roots in the first place! Make sure your planter, container, or hanging basket has at least one good-sized hole in the bottom. If your planter is sitting in a dish to collect spills, empty the dish after watering so the mum isn’t just sitting in a puddle. Found a super cute pot that you *must have*, but it has no drainage? That’s ok – it’s incredibly easy to drill a drainage hole!

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Mums should be grown in soil that is light, rich in organic matter, and well-drained. The soil should not stay soggy for a prolonged period of time. Any basic, quality potting soil that’s got plenty of perlite or sustainably-sourced peat will stay loose and aerated.

Do you have a mum showing signs of being overwatered? If it’s already planted in high quality potting soil and the container has plenty of drainage, simply reduce your watering frequency. If you’re unsure about the mum’s origins, you may want to repot your mum in fresh potting soil

Disease

Mums are not without their fair share of fungal and bacterial-borne diseases. Diseases can significantly affect their health and appearance, often leading to stunted growth, reduced flowering, and yes—yellowing foliage.

Leaf Spots

Chrysanthemum leaves affected by leaf spots exhibit irregular brown and black lesions surrounded by yellowing tissue, giving them a distressed and unhealthy appearance.
Remove infected foliage promptly to prevent disease spread.

This disease is characterized by yellowish spots on the foliage that eventually darken in color and expand to cover most of the leaf surface. Typically, the lower leaves are infected first and will wither and die as the disease progresses up the plant.

Several different kinds of fungi cause leaf spot, including Septoria chrysanthemi, Septoria chrysanthemella, Alternaria species, and Cercospora chrysanthemi. These fungi overwinter in plant debris, so try to pick off infected leaves as soon as you see them and regularly scoop up any leaves or flower buds that have dropped from the plant. Sanitize any garden tools you prune with between uses.

Fusarium Wilt

Chrysanthemums in a pot on the windowsill affected by Fusarium wilt display wilting and yellowing leaves, along with stunted growth, while the stems exhibit dark vascular discoloration near the base.
Discard infected plants promptly to prevent further spread.

Fusarium wilt is caused by the fungi Fusarium oxysporum f.sp. chrysanthemi and f. sp. Tracheiphilium. It spreads easily through infected soil and plant cuttings. Yellowing foliage is the first sign of fusarium wilt, followed by wilting, stunted growth, and potential death of the entire plant.

Unfortunately, there is no single easy treatment for home gardeners to cure a mum of fusarium wilt. Your best bet is to toss the infected plant in the trash and start fresh. Apply a fungicide to protect healthy plants, and avoid highly susceptible mum cultivars such as ‘Bravo’, ‘Cirbronze’, ‘Illini Trophy’, ‘Orange Bowl’, ‘Royal Trophy’, and ‘Yellow Delaware’.

Verticillium Wilt

Close-up of a plant affected by Verticillium wilt, showing wilting leaves that have turned brown and stunted growth.
Wilting and discoloration may signal a fungal infection.

This fungus-borne disease is usually only seen on mature mums, after flower buds have formed. Those infected with Verticillium albo-atrium experience wilting and yellowing foliage, typically only on one side of the plant. As the disease progresses, the plant may completely lose its foliage and flower buds.

Dispose of the infected plant since full recovery is unlikely. Mum cultivars susceptible to verticillium wilt include ‘Bright Golden Ann,’ ‘Echo,’ ‘Glowing Mandalay,’ ‘Mountain Peak,’ ‘Puritan,’ and ‘Wedgewood’. However many of the newer cultivars have at least some resistance to this disease.

Nutrient Deficiency

White chrysanthemums in a black hanging pot are complemented by yellowing leaves, creating a striking contrast against the backdrop of an autumn balcony.
Insufficient nutrients can hinder plant growth and vitality.

Nutrients play crucial roles in energy transfer, root growth, photosynthesis, flowering, and a whole host of other biological functions in plants. Yellowing leaves may be a sign that your mums aren’t receiving enough nutrients; either the plant cannot absorb nutrients or there simply aren’t enough nutrients available in the soil.

Take a look at your mum. Which leaves are turning yellow? If the young leaves on the ends of the stem are turning yellow first, that can be indicative of an iron deficiency. However, deficiencies in nitrogen, magnesium or zinc typically develop on inner, older leaves first, and then progress outward. If you’re growing hardy perennial varieties in the ground, you may want to start with a soil test to determine soil pH and nutrient levels.

But what about a potted annual that you will likely be composting in a few weeks once winter hits? Keep it simple. Potted varieties purchased at the store will have already been fertilized by the nursery or grower, but it won’t hurt to give them a little extra boost!

Some potting soils have granular fertilizer mixed right in or consider applying a well-balanced, liquid fertilizer that has equal parts nitrogen (N), phosphorus (P), and potassium (K). Feed every four weeks or so, up until 2-3 weeks before your first anticipated frost date.

Final Thoughts

Don’t let yellowing leaves throw shade on your fall mums! Regular watering and perhaps a little boost of fertilizer might be just the thing they need to look fresh. With a little care, your mums can continue to brighten your garden all fall.

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