Pocket Botanist
Check these first
- Inspect the undersides of the leaves for tiny insects.
- Check the soil to see if it is soggy or waterlogged.
- Look for silvery streaks on the leaf surface.
Likely causes
Thrips
PossibleHow to tell: You will see tiny, slender insects or silvery, streaky patches on the foliage. These pests suck sap and leave behind damaged, discolored tissue.
What to do: Spray the leaves with insecticidal soap or neem oil every 7 days. Ensure you cover the undersides of the leaves where they hide. Remove any heavily infested leaves to stop the spread.
Fungal Leaf Spot
PossibleHow to tell: Small, dark, or water soaked spots appear on the leaves during periods of high humidity or poor air circulation. These spots may grow larger if the foliage stays wet for too long.
What to do: Improve air circulation around your plants by spacing them further apart. Avoid overhead watering to keep the leaves dry. Apply a copper based fungicide if the spots continue to spread.
Find out what's wrong with your giant onion
The list above covers the usual causes. Pocket Botanist looks at a photo of your actual plant and tells you the likely cause, how serious it is, and what to try first. Identification and a health check are free.
Scan with your phone's camera to get the app
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When to worry
Worry if the spots are spreading rapidly or causing the entire leaf to collapse. Minor cosmetic marks on older leaves are often normal as the plant prepares for dormancy.
Stopping it coming back
Plant your bulbs in well draining soil and provide plenty of direct sunlight to keep the foliage dry.
Giant Onion: spots? Check yours with a photo
Every plant is a little different. Take a photo of your giant onion and Pocket Botanist tells you which cause is most likely, how serious it is, and what to try first. A few checks a day are free. Save the plant and you get care reminders timed for it.
Scan with your phone's camera to get the app
Or go to pocketbotanist.app/get on your phone
Get the app · Or check it from a photo now, free
Questions people ask
- Can I just cut off the spotted leaves?
- Yes, you can trim off damaged leaves to keep the plant looking tidy. Use clean, sharp scissors and sanitize them between cuts.
- Will these spots kill my bulb?
- Usually not, as long as you address the cause quickly. The bulb is the most important part, so focus on keeping the soil from staying wet.
Guides to these causes
More problems with Giant Onion
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- Why is my Allium giganteum drooping?
- Why is my Allium giganteum growing so tall and leggy?
This article was generated with AI on from the plant’s care profile and is general information, not horticultural advice. Read about how we build these guides.