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One of the most frequently asked questions about houseplant care is “why do my plants get yellow leaves?” There are many reasons for yellow leaves, but more often than not, there is only one root cause - and that is often the health of the roots.
Here, we explain how you can reduce the risk of your indoor plants getting yellow leaves, and what to do if they occur. We also take a brief look at some of the less common causes of yellow leaves.
When plants get yellow leaves, it is usually a sign of a problem at the other end of the plant: in the soil. Apart from ferns, which don’t have true roots, all of the houseplants we supply have a root system, and that is responsible for extracting water and nutrients from the soil. The nutrients are soluble, and are taken up with the water.
Getting water and nutrients out of the soil is not a passive process. Roots are not like sponges that soak up moisture, but are highly complex, fragile organs that affect the vitality of the plant more than any other part of the plant.
It is often easy to forget about the roots of a plant as they are hidden from view, but if they are unhealthy, the effects are usually seen as a lack of vigour in the parts of the plant that grow above ground, and especially the leaves.
<image: healthy roots by Kenneth Freeman>
The leaves are where some of the most complex biochemical reactions in a plant take place. The act of converting carbon dioxide and water into complex carbohydrates takes place in complex structures and requires different proteins and other substances to make the reactions happen. If the leaves are starved of water and the nutrients that make up the proteins, then tissues will start to die. When the chloroplasts (the structures that contain the green pigment, chlorophyll) die, or can no-longer be renewed, the green pigment disappears and the leaf turns yellow.
Over watering plants
If the roots cannot extract water, then, as we have seen above, the leaves will eventually turn yellow and die. So, why not just make sure that there is always plenty of water?
The issue here is that when soils are saturated, the air which provides the oxygen that roots need to live) in tiny air-filled cavities in the soil, is squeezed out. Without oxygen, the roots start to die and cease to function. Without the water delivered by the roots, the plant experiences drought conditions and leaves turn yellow and die.
Some plants are better able to cope with high levels of moisture than others - their roots are able to extract more dissolved oxygen than in other plants. It is also true that many plants can adapt to extreme levels of moisture over time - this is how hydroponics work. But for conventional plants in a soil-like substrate, too much water is a problem.
Find out how much you should water your houseplants in a separate post on our blog, or refer to the specific plant care tips for each plant sold at foli8.
<images: three heavily over-watered houseplants>
Under watering plants
Underwatered plants can also suffer from yellow leaves, for exactly the same reason - a lack of moisture getting into the plants. This time, however, it is not due to the roots dying, but simply the lack of water for them to extract from the soil and deliver to the rest of the plant.
Often, wilting will be the first sign of a problem - especially with herbaceous plants. Wilting is much less obvious in plants with woody stems and which remain rigid, even when dry.
<image: yellow leaf of Ficus ‘Natasja’ caused by under watering (Kenneth Freeman)>
Fortunately, under watering is easily cured - just give your plant a drink.
Be aware, however, that if the soil is really dry, it can be difficult to re-wet it, so take it steadily and, if possible, water from the bottom up. Re-wetting dry soil is much easier if your plant is in one of our foli8 coir pots. Stand that in some water in a decorative pot, or even in your sink, and allow the water to soak through into the soil from the outside.
As well as water, oxygen and carbon dioxide, plants need several other chemicals to make proteins and other substances used for growth and maintaining health. These are the components of fertilizers for plants. Most plant nutrients are obtained from minerals in the soil, whether they are there naturally, or added in the form of artificial fertilizers. To be useful to the plant, the nutrients are all water soluble and are taken into the plant by the roots.
Deficiencies in plants
<images L-R: interveinal chlorosis typical of magnesium deficiency (Jim Conrad, Public domain, via Wikimedia Commons), iron deficiency in Rhapis excelsa (American Phytopathological Society), Zinc deficiency in Dypsis lutescens (American Phytopathological Society)>
When nutrients are in short supply, the leaf cells furthest away from the veins that carry the nutrients are the first to die off, which is why nutrient deficiency is often characterized by yellowing of the tissues between the veins. If the leaves are otherwise still healthy, a dose of fertilizer, often applied direct to the foliage, will have a rapid effect. Only use fertilizers recommended for foliar application, at the recommended rate, otherwise the nutrients might not be absorbed.
Toxicity in plants
Nutrient toxicity can occur when fertilizers are applied at too high a rate, or if other contaminants get into the soil. When applied in excess quantities, the nutrients become toxic and can damage plant tissues. Unfortunately, dealing with toxicity is much harder than dealing with deficiency as once the nutrients are inside the plant, they cannot be removed. Often the only remedy is to cut off the affected foliage and repot the plant in fresh compost with no fertilizer in it all. The roots may recover and new growth might be less affected, but there is a risk that the damage caused will be permanent and toxic levels of plant nutrients remain in the plant.
As with watering, it is always easier to cure a shortage than an excess, so whenever you use fertilizer, do so at a very low rate. Houseplants do not need much in the way of fertilizer at all.
<images: plant leaves showing yellowing and browning due to boron toxicity (Kenneth Freeman)>
Pests and diseases in houseplants
Spider mites and thrips can also turn leaves yellow. This is due to their feeding habits. Both two-spotted spider mite and thrips (tiny insects) feed in the same way. Unlike sap-sucking insects, which extract nutrients by inserting their mouthparts directly into the sap stream of the plant, these pests crawl over the leaf surfaces and graze directly on the leaf cells. The pest uses its mouthparts to cut directly into the plant cells on the leaf surface and then suck out the contents. The cell dies and the damage is permanent.
<image: yellow leaves on a palm frond caused by two-spotted spider mite grazing on the leaf surface (Kenneth Freeman)>
Further information on the biology and management of plant pests can be found on our blog in an article on common plant pests and how to deal with them.
Some plants are supposed to have yellow leaves
There are some plants, of course, that have yellow variegation on their leaves - some of the Dracaenas, for example, have markedly yellow stripes running along the length of their leaves, and one plant in our collection, Epipremnum aureum “Neon” (Neon Pothos) has entirely yellow leaves.
By Kenneth Freeman