Susan Jasan, MS, PLA is a horticulturist and landscape architect passionate about creating custom outdoor spaces that serve as an oasis for rest, renewal, and play.

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The Language of Plants: Yellow Leaves
by Susan Jasan       #Disease   #Irrigation   #Yellow

 

 

 

 

 

 

 

This tropical Pittosporum is showing signs of iron deficiency, with the yellowing leaves and the green veining.


Just as we communicate through facial expressions and the spoken and written word, plants communicate as well. When we learn a foreign language, it is to understand our neighbors, just as it is important for gardeners to understand the language of their plants. When leaves start turning yellow, that is a plant’s way of telling you that it needs your attention. This is true for treasured indoor houseplants and plants in the landscape.

The top 10 reasons for yellow leaves: Underwatering and overwatering top the list, others include temperature, lighting, soil condition, nutrients or lack thereof, pests, disease, transplant shock, and age.

Whatever the cause, remember that it may take weeks or even months for your plant to recover and return to normal growth. The reality is that if you can’t provide the proper environmental conditions for your particular plant’s needs (water, light, nutrients, and temperature) then it can’t thrive.


A Philodendron x evansii with signs of what appears to be potassium deficiency. Some of the leaves are showing a bit of tip burn on the edges and other portions of the leaves show more yellowing (necrosis) in the spaces between the main veins of the leaf. This typically presents on older leaves.

 

Hint:

I’ve learned that using rainwater, distilled water, or filtered water is best for watering houseplants. Tap water often includes chemicals (fluoride or chlorine) that can be detrimental to plants. Some plants, such as Dracaena, which generally is thought of as a pretty tough drought-tolerant plant, are actually particularly sensitive to the chemicals in tap water. And room-temperature water is best. None of us likes a cold shower or extremely hot water.

Understanding The Message
Both over- and underwatering cause yellowing leaves. Overly moist soil is often the result of well intended, but overzealous, watering. Overwatering leaves the plant’s roots saturated and unable to “breathe” because of the excessive soil moisture. In essence, you’re drowning your plants. If your plant is in a container, make sure it has drainage holes and water less frequently. If there’s a green crusty appearance to the soil surface, this is algae and it too is an additional symptom of overwatering. If the plant isn’t too far gone, you’ll probably want to repot it into new soil. Check the roots: white roots are an indicator of healthy plants; black roots that appear to be decomposing are a death knell. When repotting an overwatered plant with rotting roots, trim back those roots that are in decline leaving the healthy roots to recover.

If you feel that the cause is not enough water, the remedy is obvious. Make sure you’re watering your plants properly: wait until soil begins to dry, then drench it fully, and wait until the soil begins to dry out before watering again.

Temperature and light of course play a significant role in plant health. If your leaves appear more faded than truly yellow, this could be a sign of lack of sunlight. Plants need proper light for photosynthesis to occur – the lifeblood of any plant. For plants in containers, be sure to rotate the pots periodically so that all the foliage is exposed to sunlight.


Identifying causes of leaf damage involves a bit of detective work. Think about the environment, particularly changes in the environment: temperature, chemicals, watering, humidity, etc. This leaf on this arrowhead plant (Syngonium podophyllum) seems perfectly normal on one half, while demonstrating obvious, significant damage on the other half. It might seem odd to see only one-half of a leaf affected, but a change in the plant’s environment makes the cause obvious: The browning is a result of cold shock. The plant was only partially protected overnight from the cold temperatures. The side that was exposed to the chilling temperatures was “burned,” much like freezer burn. The protected side was unaffected.

Research the light needs of your specific plant and make sure you’re providing the right type of light. Some plants like bright, but indirect, light. Others prefer full sun, yet others thrive in low light. If your plant is getting too much or too little light it will let you know. Plants with too little light also tend to become “leggy” as they’re reaching for more light with stems becoming atypically long with the leaves growing undersized. Sometimes the leaves may even fall off.

Though seen more often in landscape plants than houseplants, a significant temperature change can leave leaf tips looking “burned.” This typically occurs in spring, when there is a late freeze after tender new growth has emerged. If this is the case, trim off the “burned” foliage and allow new growth to re-emerge.

Houseplants have preferred temperature ranges: Some like it cool, around 50-60 F, others prefer 70s and 80s. Some prefer high humidity, others not so much. Some plants will drop all their leaves as a stress response to change when moved from one location to another, only to subsequently rebound with new growth. Ficus are particularly known for this.


These yews (Taxus x media ‘Densiformis’) are showing signs of “burn” on tender new growth caused by freezing temperatures that followed a period of extreme warmth. The plant will survive and the damaged tender tips should be trimmed after the possibility of any additional freezes.
 

If yellow spots on your leaves appear along with tiny critters (be sure to check the undersides of the leaves) then you have an insect population that’s enjoying your plants a little too much. First identify the pest and then treat for that particular insect. Remember that often pests are so little they cannot be seen with the naked eye, but their damage is obvious. Typical insect infestations on houseplants are caused by one of the following: mites, aphids, mealybugs, thrips, scale, or whiteflies. Repeatedly washing the plants or applying an insecticidal or horticultural soap is one treatment is often effective as well as environmentally safe.
 

This dracaena was overwatered for some time and eventually was repotted in new soil in an effort to revive the plant. The yellowing leaves here are a natural response to transplant shock. The plant is dropping older leaves in order to conserve energy as it develops new, strong roots to support new foliage.

Nutrient deficiencies are often indicated by yellowing foliage.

Iron deficiency is very common. This causes yellowing, stunted growth and interveinal chlorosis. Have your soil tested and maintain a pH below 7.

Potassium deficiency can cause yellow leaves, sometimes the edges and tips of the leaves, sometimes it will be yellow between veins. Composted fruit and vegetables added to the soil can help provide the needed nutrients.

Nitrogen deficiency can be indicated by stunted growth, yellow edges on tips of leaves, the veins of the leaves may be yellow, and sometimes the entire leaf will be pale or yellow. Adding compost will help and adding coffee grounds to the soil is a great alternative.

Magnesium deficiency shows as interveinal chlorosis – between the veins. Treat the soil with Epsom salts or compost.

Calcium deficiency causes crinkled, mottled, or distorted leaves. Adjusting soil pH can help, though be aware that adjusting soil pH occurs slowly over time and the pH may eventually creep back up to its “natural” level.


In some cases, leaves simply just age-out, slowly yellowing over time – the plant’s natural life cycle. Probably the most dramatic example is the seasonal cycle of white pine (Pinus strobus): its older interior needles all turn brown in the fall and seemingly drop at once, usually in the spring. Many species of pines will retain their older needles for about three years, and then eventually the older interior needles of the tree will drop.

Remember our plants really do “talk” to us. We simply must understand their “leaf” language.

 

Confessions of a neglectful gardener:

After seeing a spectacular bird’s nest fern (Asplenium nidus) at a botanical garden in Milwaukee, and noting that I owned one at home, I told my sister that bird’s nest fern was a great plant and almost impossible to kill. I still stand on that statement, but a confession follows.

Note the light brown leaves at the base of this plant (left), and the light green shriveled leaves below: all signs of severe underwatering. Well, I resolved I would take “better” care of this beautiful plant and water more regularly. Well, a little too regularly and with too much water each time was the case, and the result, double damage … overwatered! The core of the plant began to turn blackish brown as it succumbed to rot (right).


Lessons Learned:

1. Remember that each plant has its own unique growing requirements that need to be met.
2. Listen to your plants early, when they first show signs of stress. Respond with moderation. This is where I failed miserably!
3. Overwatering will kill a plant faster than almost any other pest or disease – including underwatering.
4. Learn from your experience and try again.

 

A version of this article appeared in a June 2017 print edition of State-by-State Gardening.
Photography courtesy of Susan Jasan.

 

Posted: 06/01/17   RSS | Print

 

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