Our Master Gardener Help Line has received several calls this past month from gardeners concerned about citrus trees turning yellow. An article entitled “Yellow Citrus Leaves in Winter Usually Not a Disease,” written by Craig Kallsen, retired Citrus Farm Advisor in Kern County, does such a good job explaining the problem that I asked for and received his permission to reprint his entire article, presented below.

For many in California, the backyard orange or grapefruit tree is almost a member of the family, and any negative change in its appearance elicits concern. One such change in appearance is leaf yellowing and drop that often occurs during the winter in citrus. Citrus leaves can remain on the tree for as long as three years depending on tree vigor, but disease, inadequate or excessive nitrogen fertility, excessive salt or born in the soil, poor irrigation practices, freezing temperatures, pest pressures and low light levels significantly reduce leaf longevity.  Excessive leaf drop during the growing season is more likely to indicate a serious problem than leaf drop during the winter. Winter leaf drop normally reflects nothing more than a momentary swing in the natural balance between the natural elimination of old senescing leaves and their replacement.

Citrus trees are evergreens. New leaves are normally produced when shoots elongate.  Periods of shoot elongation are called “flushes.” Typically, an orange, mandarin or grapefruit tree in California will produce two to five flushes during the growing season from March through October. Lemons tend to be more vigorous and may produce leaves nearly continuously through the growing season. Unlike deciduous plants, citrus sheds older leaves throughout the year  Leaf yellowing and drop in the winter is probably attributable to excessive or too little water in the root zone or dry winds in combination with cool soils. Citrus is a subtropical plant and does not have a true dormancy period like the stone fruits. Generally, winter temperatures are too low in many citrus-growing regions in California for vegetative growth. Leaves continue to fall but none grow to replace them.  Excessive leaf drop manifests itself rapidly under these conditions.  Periods of high wind can remove large numbers of old leaves tree in a day, leaves which would have fallen naturally over a longer period of time, giving the impression that the tree is rapidly succumbing to a disease.

Reduce winter water applications to citrus trees that have defoliated or that have a significantly thinning canopy. Trees like this require little, if any, supplemental irrigation.  Even a mature tree with a full leaf canopy will require less than 10% of the water that it would require during the summer.

The best indicator of tree health for a defoliating citrus tree during the winter will be how well it produces the first flush of new growth in the spring. A decision to keep or remove a citrus tree based on tree health should not be made during the winter. Even trees that lose most of their leaves during winter are capable of replacing leaf canopies with the spring flush of growth, usually with little loss in fruit production. Trees that do not produce a vigorous flush in early spring may have a more serious problem. March is an excellent month to begin applying fertilizer to encourage new leaf growth and fruit production and to help keep an old friend of the farm or family backyard around for years to come.

Ed Perry is the emeritus Environmental Horticultural Advisor for University of California Cooperative Extension (UCCE) in Stanislaus County where he worked for over 30 years.  

The La Loma Neighborhood is sponsoring the third annual Modesto Pollinator Festival at the La Loma Native Garden, located at 1805 Encina Avenue. The event will be held on Saturday, April 1, 2023 from 10:00-2:00 pm. A costume pollinator parade will be a highlight at 10:15 a.m.

Several local agencies will have booths with educational information about pollinators and how we can help them thrive. Come spin the wheel/guess the pollinator at the UC Cooperative Extension Master Gardener Program table and take home a sticker or a bookmark.

This is a free family event with many activities for children. Crafts and games will also be included. A demo beehive will give children a close up look at bees. Food trucks will be on site to provide lunch. This will be a fun day for kids, so put this on your calendar!

Rhonda Allen has been a UC Cooperative Extension Master Gardener since 2020.

Here are seven tips on how to protect your plants from freezing temperatures in California:

1. Water your plants. Make sure your plants (not succulents) are hydrated before a freeze. Water the soil around the base of the plant, this will help to insulate the plant and keep it from freezing.
    
2. Cover your plants. Use blankets, fitted bed sheets, or frost cloths to cover your vulnerable plants. Make sure the cover reaches all the way to the ground and is securely fastened to prevent cold air from seeping in. Do not use plastic covers, as they can trap moisture and damage the plant. Make sure to remove the covering when the temperature rises.
   
   
3. Add warmth for your plants. Use incandescent miniature lights (not the LED type) to provide warmth and raise the temperature around your plants. Position a 100–150 watt light in the center of the tree and cover the plant with a sheet. 
   
   
4. Bring potted plants indoors. If you have potted plants, move them to a sheltered area (ie. Porch, indoors, under an awning), when temperatures are expected to drop. If possible, cluster potted plants close together in a sheltered spot close to the house. If plants are brought indoors, place them near a window that gets sunlight.
   
   
5. Mulch your plants. Apply a layer of mulch around the base of your plants. This will help to insulate the roots and keep the soil warm.
   
   
6. Prune your plants. Prune any dead or damaged branches from your plants before a freeze. This will help to prevent further damage from occurring.
   
   
7. Monitor the weather. Stay up-to-date on the latest weather forecasts and take action if freezing temperatures are expected. If possible, move your plants to a warmer location or cover them before the freeze sets in.

It's important to note that different plants have different levels of cold tolerance. Some plants are more susceptible to freezing temperatures than others. If you're unsure about how to protect a particular plant, consult with your local UC Master Gardener Program for advice, mg.ucanr.edu/FindUs.

By taking these steps to protect your plants from freezing temperatures, you can help ensure that they stay healthy and vibrant year-round. With a little extra care, you can keep your garden looking beautiful no matter what the weather brings. Stay warm this week!

Additional Resources:

Frost Protection for Citrus and other Subtropicals (UC ANR publication 8100)
https://anrcatalog.ucanr.edu/pdf/8100.pdf

UC Master Gardener Program of Marin County, Problems, Extreme Conditions – Frost  
https://marinmg.ucanr.edu/PROBLEMS/EXTREME_CONDITIONS/Frost_853/

Are you interested in attracting native insects and pollinators to your garden? If so, join the UC Cooperative Extension Master Gardeners for a free class at a local Stanislaus County Library to learn more about California native plants.  

Sat, 3/4 at 2:00 p.m. – Empire Library
Tues, 3/7 at 6:00 p.m. – Salida Library
Wed, 3/8 at 2:30 p.m. – Patterson Library
Sat, 3/11 at 2:00 p.m. – Riverbank Library
Mon, 3/13 at 6:00 p.m. – Ceres Library
Tues, 3/14 at 6:00 p.m. – Oakdale Library
Wed, 3/22 at 6:00 p.m. – Turlock Library
Mon, 3/27 at 5:30 p.m. – Modesto Library    

Don't see your local library on our list? Contact them to request our classes.