Ask the Plant Doctor! How to prune houseplants and deal with yellowing leaves

Pruning indoor plants plays a crucial role in their upkeep and care. Photo: 123rf.com

Do you have a question about plants or how to maintain your garden? Send your questions to the Plant Doctor!

Email your questions to [e-mail] with "Plant Doctor" in the subject field. Questions may be edited for brevity and clarity.

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Dear Plant Doctor,

I’ve been trying to nurture my heliconia plants, whose leaves constantly turn yellow or dry up. I fertilise with organic coffee fertiliser once every two weeks and have changed the soil to burnt soil so that less water is trapped. I’ve also tried to fertilise them with chicken poop and compost but even that didn’t work. I suspect that the clay layer below may be the problem. But the crossandra is growing well on the same plot with the same type of soil. Kindly advise. – Frustrated

Many factors can cause yellowing leaves, but your problem might be related to light exposure. Heliconia can tolerate direct sunlight, whereas crossandra flourishes in bright but indirect light. Consider moving the heliconia to a brighter spot and watching for changes.

Overwatering can also lead to yellowing leaves. To assess this, carefully dig out a handful of soil from the top 10-20cm and hold it without squeezing.

If water drips, it is overwatered, but if it drips only when squeezed, it is adequately watered. If the soil feels dry and no water drips even when squeezed, it is underwatered. This technique is just a rough estimate when a soil moisture sensor is unavailable.

Additionally, avoid adding to your soil coffee-based fertilisers, coffee grounds, or tea leaves, regardless of whether they are organic. Caffeine in coffee and tea leaves has been shown in several scientific studies to have detrimental effects on plant and soil health. It exhibits an allelopathic influence, inhibiting plant growth and seed germination, and is also antibacterial, which could harm useful soil bacteria.

Finally, be cautious with burnt soil. It is best to avoid it unless it has been prepared carefully through controlled burning to conserve nutrients. Most burnt soils are sold without providing nutrient analysis information, which typically indicates a lack of controlled preparation.

Additionally, using burnt soils is environmentally unfriendly due to the substantial energy needed for the burning process.

I recently adopted a puppy. He loves running in the garden but he also urinates in the garden, which has not been great for my plants. Is there something I can do about this? – SI

Urine, whether derived from humans or animals, has a bad or ‘icky’ reputation. However, many governments and private sectors have been working to manage wastewater more effectively, including recycling urine as plant fertiliser.

Since the 1990s, researchers from various countries have independently reported positive results from using urine as a fertiliser for various vegetable crops, sometimes outperforming conventional fertilisers.

Urine has several drawbacks, though.

It is low in phosphorus, an essential plant nutrient, and may contain high salt levels depending on the food source, which can harm plants and soil. Moreover, repeated application of urine in the same area for long periods have been found to adversely affect soil fauna.

So, the issue with dog urine in your garden is not the urine itself, but rather the repeated application over a small, consistent area. This high urine concentration may cause “fertiliser burns”.

Additionally, no one relishes urine-contaminated vegetable leaves if you have such crops in your garden. To address this problem, consider fencing off sensitive areas in your garden or training your dog to relieve itself only in designated spots. You can also water areas where your dog urinates to dilute the urine concentration.

How do we prune indoor plants to ensure they continue to thrive? – Dr Mabel Tan

Pruning indoor plants plays a crucial role in their upkeep and care. You may prune whenever you see that the stems have become overgrown. Make sure to use tools that are both clean and sharp, such as scissors or pruning shears.

Pinpoint the parts of the plant that require pruning, like yellowing or dead leaves, stems that are rubbing against each other or crossing, or branches that have grown too long or towards an unwanted direction.

With a clean and precise cut, remove the undesired portions of the plant, cutting just above a node or leaf to promote new growth.

After pruning, keep an eye on the new growth and trim it back again if necessary, ensuring you do not over-prune.

Finally, properly discard the pruned plant pieces, as they can be toxic if ingested or harbour pathogens if not immediately disposed of. By adhering to these guidelines, you will foster healthy growth and help your indoor plants maintain an attractive appearance. – Dr Sarah Baharudin, Crop Science Dept, UPM