When you tell someone you’re going to teach them how to “read their plants’ leaves,” they think you’re trying to turn them into some mystical soothsayer.
But really good gardeners learn to look closely at their plants and to observe subtle changes long before problems get out of control. I’ve made a list of symptoms, and I’ll explain what each of them means.
Perhaps these readings will be useful to you.
Edge or tip burn. Marginal or tip burning of leaves is due to moisture stress. However, it’s not that precise. It can be due to the plant’s getting too dry, of course, but it can also be due to excessive sunlight, drying winds, trunk or root damage, chemical injury such as excessive fertilizer and a host of other problems.
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Whatever the cause, though, the points farthest from the roots dry out first and get water last, so they are where the browning shows up most prominently. You must determine the cause and work to correct it.
Leaves misshapen, elongated or contorted. Aphids that suck tender new growth can cause distorted shapes. Hormonal type weedkillers such as 2,4-D certainly do, too. They are quite volatile and can blow even in light breezes for many hundreds of feet.
Leaves much smaller than normal (often happens when leaves are also quite sparse). These plants are struggling due to root loss or trunk damage. They aren’t pumping enough moisture and nutrients to the tops of the plants to supply a normal load of foliage. And so all leaves suffer in the process. You must determine if there is a challenge that you can address, then take steps to correct it.
Leaves much larger than normal. You won’t see this as often, but when it does show up it usually tells you that that plant isn’t getting enough light. Those leaves will also usually be loose and floppy.
Moving the plant to brighter surroundings may solve the problem, or you might have to do a bit of light pruning to get more light down to it.
Leaves lighter green overall. In most cases this indicates the need for more nitrogen. It could be because of some type of root or stem problem that is preventing flow of soluble nitrogen into the plant, but usually a feeding of a high-quality nitrogen fertilizer will correct it.
Leaves pale green or yellow with dark green veins, primarily on newest growth. These are textbook symptoms of iron deficiency. The convincing facts are that the veins remain green the longest and that the older leaves remain green. Iron is a part of the chlorophyll molecule, so once it is in place in a leaf it doesn’t move.
Leaves pale green or yellow with dark green veins, primarily on oldest growth. You’ll see this most commonly on fast-growing trees such as mulberries. They produce thousands of leaves during spring’s cool, moist weather.
Then it turns hot and dry, and suddenly, they can’t keep up with those leaves’ needs. The older leaves yellow and fall. Frankly, there’s not much way to stop it. It comes with having those trees in your landscape.
Older leaves yellowing, dropping prematurely in summer. This is a more severe form of the drying just mentioned. This can happen to almost any tree or shrub species, and it manifests with yellowed leaves sprinkled throughout the plant.
This often is an early warning of some impending problem that’s brewing in the trunk or roots. Borers may be weakening an ash or peach tree, for example, and it reacts by shedding leaves randomly.
All leaves suddenly turn brown and do not fall from plant. In our area, in almost all cases this is going to be the fatal soil-borne fungal disease called cotton root rot. It lies dormant in the alkaline soil, often for many years. Then suddenly conditions are prefect for it to rise up and kill a mature tree or shrub.
More than 80 percent of the plants that we grow are susceptible to one degree or another, and there is little we can do to prevent or control it other than to avoid planting highly susceptible species.
That list includes apples, pears, Indian hawthorns, roses-of-Sharon, forsythias, lacebark elms, silver maples and a host of other common plants.
The resistant list is shorter, but it includes some great types: hollies, nandinas, junipers, oaks, cedar elms, pecans and magnolias.
Leaves wilted, even though soil is wet. Plants that show these symptoms have been too wet for prolonged periods of time. Their roots are rotting due to the lack of oxygen in their soil.
This happens in low spots when you plant in heavy clay soils. It also happens in large pots, when the weight of the pot pushes it down against the ground, sealing off the drainage hole.
You must improve the drainage around these plants at once if you’re to have hope of saving them.
Purple leaves fading to purplish green in the summer. This is entirely normal for most red and purple variegated trees and shrubs. The colorful pigments turn pale in summer’s hot weather and are covered up by chlorophyll.
In spring and again in fall they predominate, so the red or purple will return.
Variegated shrubs suddenly turning solid green. This happens with goldspot and golden euonymus and with variegated privet. Both of these plants revert back to their original green foliage.
You must prune out the green shoots or they will soon crowd out all the variegation.