By Cindy Watter, UC Master Gardener of Napa County
Most of us UC Master Gardeners are accustomed to being approached by people who think we know everything about horticulture. While flattering, that is not the truth. Our training has taught us how to find the answers, however.
When a friend asked me why her lemon tree, despite diligent fertilization, had bright yellow leaves and was failing to thrive, I told her I would try to determine the problem.
UC Master Gardeners of Napa County operate a free help deskfor home gardeners. The help desk is staffed on Mondays and Fridays from 10 am to 1 pm. You can either walk in with your question (bring a photograph or plant sample if possible) or you can email your question to mastergardeners@countyofnapa.org. A Master Gardener volunteer will get back to you. Again, a photograph of the problem is helpful.
We also have many resources on our website. Click on the “Gardening Resources” link and you'll find a wealth of information, all University of California approved. There are Healthy Garden Tips, UC Master Gardener videos, a Napa County Vegetable Planting Guide and UC IPM's Plant Problem Tool. (IPM stands for integrated pest management, which is shorthand for a variety of sustainable ways to control plant pests and disease.) If you are completely at sea with what is troubling a plant, this is a good place to start.
I am a decidedly low-tech person, but even I managed the Plant Problem Tool. There was a series of boxes to explore—plant types, plant names, plant parts, damage. I dutifully clicked plant types (fruit tree); then plant names (citrus); then plant parts (leaves) and damage (discolored, necrosis, chlorosis). These last two terms mean “dead” and “lack of chlorophyll.” I also researched holes in citrus leaves because my own patio Meyer lemon has a couple of yellow leaves with chewed edges.
I received 16 possible causes of my friend's tree ailments, from aphids to wind injury. I decided aphids were my problem, so I am going to spray the leaves with water, buy some ladybeetles and advance to horticultural oil if needed. I thought my friend's issue was likely mineral deficiencies, as her tree looked like one of King Midas's accidents.
Citrus trees need regular irrigation and fertilization. Lemons can be fertilized with nitrogen at any time; early spring is a good time to start. Fertilizing three times a year is best practice. Citrus fertilizer, available at your local nursery, has twice as much nitrogen as phosphorus and potassium.
Iron deficiency is a possible cause of yellowing. Try adding a chelated fertilizer to the soil. A magnesium deficiency can also cause yellow leaves, and a home remedy for that is a good handful of Epsom salts scattered around the dripline of the plant. The dripline is where the water drips off the leaves onto the ground. The nutrients will reach the roots faster that way than if you fertilize and water around the trunk of the tree.
Mulching around citrus trees is advisable, because mulch inhibits weeds that compete for nutrients. Keep the mulch away from the trunk. Monitor your soil moisture. If the soil is dry two inches below the surface, the tree needs water.
Chlorosis (leaf yellowing) has many possible causes. Overwatering, underwatering, lack of nutrients and pests are just a few. My friend's lemon tree is in a pot, and she assures me that the potting soil is excellent. However, the pot has an open bottom.
I wonder if the soil below is packed hard or has a lot of clay. That could affect nutrient absorption if the plant roots have moved down.
Testing the pH of the soil might reveal the source of the problem. If the soil pH is off, the tree can't absorb nutrients, as I learned from a speaker from the Resource Conservation District at a recent Master Gardener class.
Lemon trees like a slightly acidic soil (pH less than 7.0)Alkaline soil will keep a lemon tree from absorbing nutrients and may cause chlorosis. You can buy a soil testing kit or use a local environmental testing lab.
I can look out my kitchen window and see a Meyer lemon tree across the street. It is loaded with lemons and its leaves are full and green. I have never seen it fertilized or watered, or covered up during a freeze, and it is perfectly healthy. I would be more jealous of the owner's success but she is always giving me lemons.
One reason her tree is so healthy is that it was planted about 40years ago, when Napa got more rain than it does now. Its roots are probably very deep, so it has access to moisture. It gets plenty of sun with its southwest exposure, and the soil in that yard is healthy, with lots of compost. The ladybeetles in the neighborhood eat plant pests. This is a lucky tree. It could live another 50 years.
Library Talk: Join UC Master Gardeners of Napa County for a talk on “Planning a Cut Flower Garden,” on Thursday, March 6, from 7 pm to 8 pm, via Zoom. This summer, your garden can include beautiful, colorful flowers to cut and bring into your home. Join the UC Master Gardeners of Napa County to learn about the best annuals, bi-annuals and perennials for cutting, along with soil preparation tips, starting seeds indoors, and direct seeding techniques. Click here to register!
Library Talk: Join UC Master Gardeners of Napa County for a talk on “Containing My Dahlia Enthusiasm,” on Thursday, February 6, from 7 pm to 8 pm, via Zoom. Dahlias produce dazzling flowers. However, rodents and other pests adore them as much as we do. Join the Master Gardeners of Napa County to learn how to best address these challenges and grow an abundance of show-stopping blooms in pots, gopher baskets and grow bags. Click HERE to register.
Help Desk: The Master Gardener Help Desk is available to answer your garden questions on Mondays and Fridays from 10 am until 1 pm at the University of California Cooperative Extension Office, 1710 Soscol Avenue, Suite 4, Napa. Or send your questions to mastergardeners@countyofnapa.org. Include your name, address, phone number and a brief description of the problem.