In winter it's a normal process to see yellowing leaves and leaf drop. When we see this in other seasons, it indicates a problem. When we see the yellowing leaves on our citrus, we often jump to the conclusion there is an iron or a nitrogen deficiency, but there can be a variety of causes.

Before adding nutrients such as iron or nitrogen, consider things such as the time of year, watering practices, soil pH, general care practices, as well as the pattern of yellowing on the leaves.

Things to check first before adding nutrients:

√ Is it winter with cold soil temperatures?

√ Are rains or irrigation practices causing soil to be waterlogged?

√ Is soil pH alkaline, greater than 7.5?

√ What fertilizers have been applied?

√ What is the pattern of yellowing on the leaves, symptoms in older or younger leaves?

Certain conditions often make nutrients unavailable to plants, even though the nutrient may be present in the soil.

Alkaline soil pH. Soil pH is a measure of soil acidity/alkalinity, with 7.0 as the neutral point. Citrus, like other plants that are adapted to slightly acidic soil, are especially prone to iron deficiency when soil pH is above about 7.5 (alkaline). Alkaline soils hold onto iron, making it inaccessible to plants, even though typically iron is plentiful in most California soils. A pH of around 6.5 is ideal for citrus, but it can tolerate a range from 6.0 to 7.5. Adjusting soil pH needs to be done carefully and slowly to avoid unwanted effects in the garden.

Cold soil and soggy soil limit microbial activity important in getting nutrients to the plants. Microbes need porous soil and a certain temperature range to thrive, which is why chlorosis (yellowing leaves) is more common in winter. Porous soils have spaces in the soil that allow water and air to move easily in the soil. Compacted or clay soils are especially prone to a lack of porosity in cold, wet weather which may persist into spring.

Nutrient imbalances, especially an overabundance of zinc, copper, and manganese, can make iron less available. Causes vary, but imbalances can be due to over-application of certain fertilizers.

Identification: Iron deficiency symptoms: yellow leaves, green veins – primarily in young leaves. Nutritional deficiencies in citrus can be identified by the pattern of leaf yellowing and by noting whether it occurs on old or new foliage. The pattern of yellowing leaves between the small, darker green veins occurs primarily in young leaves. This happens as soils cool, reducing root activity. Other possible causes of that pattern of yellowing include zinc and manganese deficiencies. See the UC Integrated Pest Management website for pictures of various patterns of chlorosis.

Iron deficiency symptoms on young citrus leaves ©UC IPM Jack Kelly Clark

What to do about iron deficiency?

Improve cultural practices and the soil environment.

• Make sure to irrigate with sufficient time between waterings to allow soil to dry but still provide enough water. Deep watering once a month is often sufficient, especially in summer.
• Make sure drainage allows roots to have air and water, avoiding waterlogged soil.
• Regularly add compost on top of roots, away from the crown of citrus. The addition of organic matter helps with iron deficiency as the organic matter decays and soil gradually becomes more acidic.
• If there has been a history of regular application of fertilizers, a soil test may be helpful to determine potential nutrient imbalances before applying more fertilizer.
• Iron, manganese and/or zinc deficiencies can be corrected with a foliar (leaf) application of a liquid micronutrient solution as the new growth emerges in the spring. You can also apply micronutrients, such as zinc sulfate or iron sulfate, to the soil. Microelements such as iron and zinc though, need an actively growing root system for uptake. Their soil application should be limited to the summertime.

Identification: Nitrogen (N) deficiency symptoms- leaves pale all over (light green or yellow), especially in older leaves. Nitrogen deficiency starts with older leaves near the bottom of the tree and foliage turns a uniform yellow. Slow growth and uniform yellowing of older leaves are usually the first symptoms of nitrogen deficiency. The causes include a true deficiency of nitrogen, adverse soil conditions, or unhealthy roots.

Nitrogen deficiency symptoms in Robertson navel orange citrus by Judy Quan

What to do about nitrogen deficiency in citrus?

Most citrus require regular fertilization with nitrogen. However, there are still conditions that would mimic nitrogen deficiency that would not be fixed by adding nitrogen.

• Wait until the soil is warmer, moist but not soggy, to apply nitrogen. Cold and soggy soil inhibits nitrogen uptake.
• Make sure to water with sufficient time between waterings to allow soil to drain well but still provide enough water to citrus. Deep watering once a month is often sufficient, especially in summer.
• Make sure drainage allows roots to have air and water, avoiding waterlogged soil, which often leads to rot and crown damage.
• Regularly add compost on top of roots, away from the crown of citrus. The addition of organic matter helps with iron deficiency as the organic matter decays and soil gradually becomes more acidic.
• Consider doing a soil test, either from a commercial laboratory or a home soil test kit. If there has been a history of regular application of fertilizers, a soil test may be helpful to determine potential nutrient imbalances and to know the current levels of nutrients such as N,P, K and pH.
• Remember it is possible to over-fertilize, potentially causing problems such as a delay in maturity and a reduction in fruit quality; so when in doubt, fertilize less.

When should citrus be fertilized?

Most mature citrus require regular fertilization with nitrogen.

• Begin nitrogen fertilization in February or March depending on the temperature, when soils are warmer and drier, just before pre-bloom.
• Fertilize only in the spring and summer months.
  • Choose more frequent/small applications of nitrogen that allow more efficient nitrogen uptake, so less fertilizer is required.
  • Or choose the more convenient schedule of three nitrogen fertilizations to be in February, early May and June – Valentine's Day, Mother's Day, Father's Day, with a last application perhaps in early fall.
• Stop applying nitrogen fertilizer if citrus leaf miner severely damages new foliage growth in late summer.
• Aim to go into winter with adequate nitrogen stored in the plant, so when spring comes, the nutrients are already there ready for the blooming period.
• Avoid late-season fertilization
  • It may affect fruit quality, delay fruit coloring and make the rind rough.
  • It stimulates new growth and makes trees more susceptible to frost injury. Subtropical plants, including citrus, develop some resistance to cold damage by going through quiescence, a resting time of no or slow growth when cooler temperatures arrive. Quiescence is induced several weeks after 40-50°F temperatures arrive. But that quiescence can be changed to active growing by pruning or nitrogen fertilization in early winter, increasing the possibility of cold damage.
• Avoid adding too much nitrogen, which can damage plants and cause other problems associated with nitrogen excess. Follow the directions on the package. If in doubt, fertilize less.

What fertilizer should be used?

Abnormally dark green grapefruit foliage from excess nitrogen (left), compared, with uniformly pale citrus leaves due to nitrogen deficiency UC IPM by Robert G. Platt

• Nitrogen is the chief nutrient required by citrus and should be applied each year. Fertilizers commonly contain nitrogen (N), phosphorus (P), and potassium (K), listed as NPK on the fertilizer label. Look for a fertilizer where the first number (nitrogen) is the highest of the three, e.g., 7-3-3; fertilizers formulated for citrus typically have higher nitrogen with lower levels of phosphorus and potassium. Avoid balanced fertilizers such those labeled 10-10-10. The relatively high concentrations of phosphorus and potassium are not needed since they are held in the soil much longer than nitrogen.
• Choose the type that best fits your needs. Nitrogen can be provided as organic matter, inorganic compounds, or commercial slow-release (e.g., suffer-or polymer-coated urea). Some fertilizers add nutrients only; other fertilizers are also amendments that indirectly affect plant growth by improving the physical properties of the soil. You can find more information about fertilizers and amendments from The San Luis Obispo Master Gardeners in their pdf handout.

When we see yellowing leaves in our citrus, we want a quick easy answer. But by taking a more comprehensive look at our citrus, the soil and the moisture levels in the soil, as well as the seasonal temperatures, we are more likely to correctly diagnose the problem and then effectively adjust factors that will result in healthier citrus trees. And fertilizing with the right amount of nutrients, mainly nitrogen, at the right time will promote the health and productivity of citrus while minimizing problems associated with overfertilization.

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Have a gardening question?  We'll help.  You  can reach us by:  

• Emailing acmg@ucanr.edu. Please include a photo of the problem, if you can, plus your name, phone number, city and a description of the problem.

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• In person at our Hayward office, during our office hours, only by appointment.

Resources 

• Adjusting Soil pH in California Gardens, a free downloadable PDF 
• A Guide to Citrus Nutritional Deficiency and Toxicity Identification 
• Citrus for the Home Gardener  2008 Vincent Lazaneo, Horticulture Advisor 
• Diseases and Disorders of Leaves and Twigs
• Fertilizing Citrus in the Foothills by Cindy Fake
• Iron deficiency UC IPM
• Nitrogen deficiency UC IPM
• When to Fertilize Avocado and Citrus in California

Nectarines on tree - photo by Jim Farr

Planting fruit trees is a rewarding and sustainable endeavor that brings the joy of fresh, homegrown fruit to your household. However, selecting the right fruit trees for your garden requires careful consideration of some key factors to ensure a bountiful harvest for decades. To help you embark on this fruitful journey, I have compiled a comprehensive checklist that covers essential factors to consider when picking out fruit trees from a nursery.

√ Choice of Type of Fruit

Plums - photo by Jim Farr

Choose fruit that you and your family enjoy eating. This may seem obvious but keep in mind that there will be times when you have an abundance of fruit. This will likely be far more fruit than your household can eat even if it's your favorite. My experience is that neighbors are very grateful to receive free fruit. Excess fruit can also be donated to food banks and churches.  Look here to identify potential food donation localities near you.

√ Climate Compatibility

Your climate is a key consideration. Different fruit trees thrive in different climatic zones. Consult the USDA Hardiness Zone map to determine the climate zone of your region and choose fruit trees that are well suited to that zone. Review the USDA 2023 Hardiness Zone by zip code to check your neighborhood.

Checking the hardiness zone ensures that your tree can withstand the local temperature extremes. This is also important when considering chill hours which I will discuss below.

√ Soil

Most fruit trees thrive in deep, well-draining, granular soil with a slightly acidic to neutral pH (6.0-7.5). Amend the soil as necessary. Adding compost to the soil will help provide a slow-release fertilizer and help build the soil structure. However, do not replace all of the native soil, which may cause the tree to be root-bound. Use this blog as your guide to amending your soil to provide the optimal environment for your fruit tree.

It is important to recognize that some fruit trees have unique soil requirements. For example, blueberry bushes prefer more acidic soil in the pH range of 4.5-5.5. The normal neutral pH for soil is pH 7. Amendments can be added to the soil to make it acidic, such as sulfur. However, planting blueberries and other acid-loving fruit in containers allows for easier control of the soil pH.

√ Space Availability

Consider the available space in your garden. Fruit trees vary in size. Ensure you have enough room for the tree to grow to its mature size without overcrowding or competing with nearby plants. Fruit trees come in dwarf, semi-dwarf and standard (full) size. Look at the tag carefully for size information. Even a dwarf fruit tree can often still grow more than 8-10 feet, semi-dwarf trees up to 15+ feet tall and standard fruit trees up to 20-25+ feet. Dwarf or semi-dwarf varieties are suitable for smaller spaces, while standard varieties will be appropriate for larger areas.

√ Pollination Requirements

Cross pollination - plum trees - photo by Jim Farr

Some fruit trees require cross-pollination with another compatible tree to produce fruit. The tag will indicate whether the tree is “self-fruitful” or requires cross-pollination (sometimes referred to as “self-unfruitful”). For trees requiring cross-pollination check carefully which varieties are better for cross-pollinating than others. Sweet cherry fruit trees often require cross-pollination. The Bing cherry, for example, will require cross-pollination with another variety such as Stella or Sunburst varieties (which are self-fruitful).

Research the pollination requirements of the specific fruit tree you're interested in and ensure you have enough space for at least two trees if a pollinator is necessary. Alternatively, self-pollinating varieties are often available for those with limited space.

√ Chill Hours

Chill hours refer to the cumulative hours a tree requires at temperatures between 32°F and 45°F during the winter to break dormancy and produce a normal crop of fruit. With insufficient chill hours, trees will leaf out late in the season, blossoming can be prolonged, buds may deteriorate and/or drop, and few if any flowers are produced. It's a critical factor for fruit development for many fruit species including apples, cherries, nectarines, peaches, plums, apricots, and pears. Choose fruit trees that match the average chill hours of your region to ensure successful fruiting.

To find the average chill hours in your region see the link below that provides current and past chill hours data for various regions in the Bay Area. Some fruit tree varieties may require more than 1000+ chill hours while other fruit tree varieties may require much less, for example, 200 hours. Avoid fruit tree varieties that have chill hours on the borderline for the averages in your area. Average chill hours will vary from year to year and this will avoid years of marginal harvests.

Find the chill hours for your county.

√ Pest and Disease Resistance

Investigate the susceptibility of your chosen fruit tree(s) to common pests and diseases in your area. Choose varieties known for their resistance to common diseases to minimize the need for chemical interventions and ensure a healthier, more sustainable home orchard. The tags on fruit trees will indicate any disease resistance of a particular fruit tree variety. Some types of fruit trees have fewer natural diseases than others. For example, pomegranates and persimmons have fewer disease issues than other species of fruit such as cherries, plums, and peaches. For information on the common fruit tree diseases for a particular type of fruit, the University of California Integrated Pest Management website is very useful.

√ Fruit Ripening Time

Nectarine Fruit ripening - photo by Jim Farr

To enjoy a continuous harvest throughout the season, choose varieties of fruit trees with different harvest times for your home orchard. This ensures a steady supply of fresh fruit rather than a glut all at once. Tags on fruit trees will indicate whether the particular variety of fruit tree is “early”, “mid-season” or “late”.

For example, if one wanted to extend the time when plums are ripe, planting the plum varieties Beauty, Santa Rosa, Burgundy, and Emerald Beauty will extend the harvest time from a typical 2 weeks for any one of these varieties to 3 months collectively. This can ensure a longer period of fruit harvest, as well as more flavor variety in the plums.

By timing the type and variety of fruit ripening dates, it is possible to have some fresh fruit available from May / June (apricots) all the way into October / November (apples).

√ Quality of Nursery Stock

Quality of Nursery stock - photo by Jim Farr

Purchase fruit trees from reputable nurseries. Inspect the quality of the nursery stock, looking for well-developed root systems, disease-free foliage, and a straight, healthy trunk. Avoid trees with signs of stress, damage, canker sores, and pest infestations. Often people look for fruit trees already bearing fruit. Keep in mind that once planted, it is usually recommended to remove any existing fruit from the tree to minimize shock during planting.

√ Planting, Pruning, and Training Needs

Owning fruit trees is a decades-long commitment to care for the trees. Use the information provided by the UC California Backyard Orchard site on planting your fruit trees.

The amount of care a fruit tree requires after planting will vary with the fruit type. In general, fruit trees will require some annual pruning. Research the specific needs of your chosen trees to ensure proper care to maximize fruit production. If your tree size is beyond your reach, you will require professional help pruning your fruit tree. Review information on pruning fruit trees from UCANR to give you some idea of pruning considerations.

Conclusion

Apricots - photo by Jim Farr

Selecting fruit trees for your garden involves carefully planning and considering various factors. By following this comprehensive checklist, you'll set the foundation for a thriving home orchard that will reward you with delicious, home grown fruit for years to come. Lastly, remember that the Alameda County Master Gardener Help Desk is only an email or call away if you have any questions about fruit trees or any other gardening issue.

References

• The California Backyard Orchard
• UC Davis Fruit and Nut Research and Information Center Chill Hours

Alameda County Master Gardeners Help Desk

This blog post is brought to you by the Help Desk of the UC Master Gardeners of Alameda County.
Have a gardening question? We'll help.

You can reach us by:

• Emailing acmg@ucanr.edu. Please include a photo of the problem, if you can, plus your name, phone number, city and a description of the problem.
• Using our online form.
• By phone, during our office hours, 10 am to noon Wednesday and 11 am to 1 pm Thursday: 510-670-5645. At other times, please leave a message and we'll return your call during our office hours.
• In person at our Hayward office, during our office hours, only by appointment.