Plant Speak: THE YELLOW LEAF
Master Gardeners answer a lot of questions of the “what’s wrong with my …” variety. A typical exchange begins “My Hosta leaves are yellow. What can I do?”
The first step is to understand basic plant chemistry, in this case photosynthesis, which is how plants turn the sun’s energy into food for growth and health. A yellow leaf is a strong message that it no longer has sufficient chlorophyll, the pigment that keeps leaves green and is a by-product of photosynthesis. This chlorosis state is a precursor to necrosis or death of the leaf.
What is the yellow leaf telling us? Plants do speak and as gardeners we need to listen and interpret the signals. Master Gardeners are trained to ask probing questions, gather clues and assess the situation before venturing an opinion.
Natural Causes
The most obvious question would be what is the plant’s natural cycle? Hostas are herbaceous perennials which annually withdraw into winter dormancy, leaves gradually fading and dying. In addition to seasonal change, plants are constantly shedding the old to make room for new growth. Nutrients are withdrawn from older leaves which yellow and eventually fall. Last, plants often drop leaves in response to transplanting. It’s wise to wait and observe before looking further than shock for the reason.
Growing Conditions
Too much or too little of anything stresses plants, whether it’s water, light, or temperature. All contribute to plant health, but the balance needs to be right. An imbalance may produce yellowing leaves and indicate a problem for the plant.
Water: are the leaves at the base of the branches turning completely yellow?
Poor drainage and over-watering can leach nutrients from the soil so that the plant does not get what it needs. Leaves then yellow and drop. Roots can literally rot if water cannot percolate through the soil. Steps must be taken to improve soil drainage and also to water with care. Alternatively, too little water will quickly produce yellowing and leaf drop. Plants flourish when water goes deep into their roots, so make sure to water deeply rather than frequently. In addition, soil can be amended to retain water. Finally, it’s sometimes best to move plants so that their water needs are met in a new location.
Light: are the leaves and veins at the tips of branches yellowing?
Damage to this younger growth could indicate that too much light is causing the leaves to yellow. The plant will probably do better in filtered shade, either by providing shelter under a taller plant or by moving it to a spot where it can thrive. However, new transplants may shed leaves at first, then grow a new crop able to tolerate the different light level.
Temperature: yellow leaves and leaf drop?
Excessive heat and cold can both move a healthy plant from green to yellow very rapidly. Again, protection or relocation may be necessary.
Root problems: older leaves yellow and dropping?
New plantings may fail to establish and thrive simply because their roots were not given enough space to grow or because pot bound root balls were not untangled and spread out adequately. Established plants may also suffer if insects have chewed the roots, thus impeding the flow of nutrients. If a root problem is suspected, it may be necessary to dig down to the roots to investigate.
Of course, the ideal way to prevent plant problems is to make sure you provide the best growing conditions before planting. Always consult good gardening advice from books or local experts to make sure you are planting the right plant in the right place with the right care.
Soil Nutrients
If natural cycles and growing conditions are not the problem, there may be a nutrient imbalance in the soil. All plants require the right fix of nitrogen for green growth, phosphorus for root development and potassium for all round health. Consistent use of compost, mulches and a good all round organic fertilizer go a long way in building and maintaining nutritious soil. When planting in a new garden, professional soil testing can be worthwhile before you start to add nutrients.
Fertilizers
Over and under fertilization are both plant stressors. Too little, and the plant looks for more nutrients and fails to thrive. Too much and chemical imbalances can result in leaf burn. Natural fertilizers such as raw, uncomposted manure must not come into contact with roots for the same reason.
Soil pH
Soils can vary widely from acid to alkaline and plants are adapted to specific “pH” balances between the two. An imbalance can interrupt the supply of nutrients that the plant needs to thrive. For example, an excessively alkaline soil can create iron and manganese deficiency in acid loving rhododendrons. This results in yellow new leaves with green leaf veins, again that striped look. The solution is to increase the acid content of the soil with specific fertilizers or a long term acid mulch such as conifer needles. Soil can also be tested for pH balance for an accurate reading if problems persist.
Nitrogen & Magnesium
If older leaves at the base of the plant are yellow, and the leaf veins are green, the problem could be a deficiency of Nitrogen or Magnesium. Both elements are highly mobile as plants move them around from older to younger leaves. Water soluble, both are easily leached from soil by rain and watering. Protect soil from leaching by mulching with fall leaves for example. And restore the depleted elements regularly with compost, manure and organic fertilizers.
Plant signals are a fascinating topic. We hope we have shared with you the importance of looking for solid clues and weighing evidence. Plants do indeed speak!
Written by: Anne Wheeler and Lise Chartrand, Master Gardeners, Victoria Chapter
For more information, see the following sources used in this article:
David Deardorff, David and Kathryn Wadsworth. What is Wrong With My Plant? (And How Do I Fix It?). Portland: timber Press. 2009. (Book available from Glendale Gardens, HCP, Victoria)
Wong, Melvin. "Mature Leaf Chlorosis and Necrosis" , Landscape. L-15 (May 2006): n. pag. Web. 15 Oct. 2011. University of Hawaii Cooperative Extension Service
