Is your citrus tree dropping its fruit before they reach full size, or even worse, dropping the flowers before fruit even begin to form?
Trees photosynthesise to produce carbohydrates such as sugars which they store as their energy source. They can only store a finite amount, which they use to drive the growth of new leaves, branches, roots and stem. Fruiting requires a tree to divert its finite energy resources away from these vital activities into the production of flower buds, flowers and fruit.
Fruit development involves pollination followed by fertilization, growth, maturation and ripening. This process takes between 6-7 months in warmer areas, requiring a considerable amount of the trees energy resources as a result.
Trees are capable of managing their own resources, and will re-divert them in cases of emergency…
Fruit Drop and Plant Stress
In regions where citrus produce a single crop each year, they go through a specific sequence of growth phases as follows:
- Bud formation and flower initiation (mid winter)
- Flowering and fruit set (early spring)
- Fruit growth – cell division (late spring to early summer)
- Fruit growth – cell expansion (mid summer to early autumn)
- Fruit maturation (late autumn to winter)
The exception to this is in regions where multiple citrus crops are produced throughout the year. In these cases, different growth phases will be occurring simultaneously.
A citrus tree can manage its crop load quite effectively, and will only carry as much fruit as it can support. It’s natural for all citrus trees to drop excess small fruit and young blossoms in early spring to prevent overproducing. There’s no need to be too concerned about flower drop, as a citrus tree only needs 1% to 2% of the blossoms to produce a good crop, and sometimes even less than 1% is enough.
If a tree gets stressed because it doesn’t receive enough water during hot, dry windy weather, of if it is starved of nitrogen because it hasn’t been given adequate fertilizer, the fruit drop will be much heavier in spring. There can also be a minor amount of fruit drop in summer under stressful conditions.
Solution: Provide sufficient water, and water more often during hot weather and strong winds. Feed citrus with a balanced fertiliser at the start of spring (September in the Southern hemisphere, March in the Northern hemisphere)
Fruit Drop and Potassium Nutrient Deficiency
Where citrus trees are bearing heavy crop loads, fruit drop can be exacerbated by low potassium levels.
Potassium, also known as potash (represented by the chemical symbol K on fertiliser labels which state an N-P-K or nitrogen-phosphorus-potassium ratio) is a primary macronutrient which is necessary for flowering and fruit formation. Next to nitrogen, plants absorb potassium in greater quantity than any other nutrient. Without potassium, trees stop flowering and fruiting, that’s what happens when they’re not fed regularly.
Before any smug horticulturist proclaims to you that potassium is not directly responsible for flowering and fruiting, let me say that as a horticulturist and biochemist, plant chemistry, like the chemistry of all living things is complex, but certain inputs are necessary to produce certain outputs, and the references I’ve cited at the end of this article support my statements!
To get into the science, Potassium (K) is Important for water, nutrient and carbohydrate movement in plant tissues. It also activates enzymes which facilitate complex chemical reactions within the plant – such as the production of starch, protein and adenosine triphosphate (ATP). The role of ATP is to transports the energy necessary for all cellular metabolic activities in all living organisms, and in plants the production of ATP can regulate the rate of photosynthesis.
Potassium also helps regulate the opening and closing of the stomata, the openings or pores on the underside of leaves by which oxygen and carbon dioxide are exchanged with the atmosphere, and the loss of water vapour through transpiration is controlled. It Increases root growth which increases nutrient uptake and improves drought resistance, and also increases resistance to frost, insects and diseases. Optimum levels of potassium levels produce uniform ripening and growth rate of fruit, as well as better food quality, flavour and grade.
Agricultural extension agencies advise that large amounts of potash are needed by most plants, and that a deficiency of potassium or inadequate amounts of the macronutrient lead to stunted plant growth and reduced yields, and that potassium levels affect not only yield, but fruit size, juice quality and shelf life.
Since potassium plays such a key role with water regulation, a deficiency would clearly increase the stresses associated with fruit drop discussed in the previous section.
Solution: Ensure that citrus trees are fertilised with a balanced fertiliser as previously discussed. Additional potassium can be supplied by using seaweed extract (which is not fertiliser but contains a good amount of potassium), wood ashes (used in tiny quantities only as it’s very alkaline), or sulphate of potash (potassium sulphate) – all these are certified as acceptable in an organic garden.
Don’t ever use potassium chloride as it can be toxic to plants, it’s the cheap nasty alternative that fertiliser manufacturers substitute to save money!
Even if there are sufficient potassium levels in the soil, they may not be accessible to plants, unless we make certain improvements.
Improving Plant Uptake of Potassium
There are several factors that can affect potassium uptake by plants:
- soil moisture
- soil aeration and oxygen levels
- soil temperature
Higher soil moisture levels increase potassium availability to plants by enhancing the movement of potassium to plant roots.
As soils get wetter and closer to saturation, where they become waterlogged, potassium uptake decreases, because air is excluded from wet soils and oxygen levels become very low. Air is necessary for both root respiration and potassium uptake.
All plant physiological activity, including root activity, increase as soil temperature increases, leading to increased potassium uptake, with the optimum soil temperatures being between 16-27°C (60-80°F)
Solution: Water enough but don’t overwater, and mulch the soil in late spring to keep the soil temperatures in the optimum range during very hot weather and to reduce water loss to evaporation.
Sometimes there may be several factors causing increased fruit drop in citrus. How does the fruit drop mechanism work in fruit trees?
Fruit drop (also known as fruit abscission) is regulated by the balance of two endogenous (meaning from within) plant hormones, auxin and ethylene. When the ratio of ethylene to auxin is higher, it induces the enzymes which dissolve cell wall components in the abscission zone between the fruit and stem (peduncle) at the button, which separates the fruit from the tree.
Ethylene is produced in response to stress factors such as water stress, physical injuries, frost damage, and decay of the fruit. When the fruit is injured, ethylene gas production is triggered, which may cause fruit to drop.
If citrus trees are planted in poorly drained soil, extended hot, rainy weather in late late summer to early autumn may lead to root root and cause excessive fruit drop in mature trees
Additionally, if the lower branches of the tree canopy are shaded out and don’t receive adequate light, the fruit is quite likely to be shed from those branches. Prune citrus to an open vase shape to ensure good light penetration through the canopy, which is important for even fruit ripening.
- University of Arizona Cooperative Extension – Diagnosing Home Citrus Problems, AZ1492 April 2009 – John Begeman, Glenn Wright
- University of Georgia Extension – Citrus Fruit for Southern and Coastal Georgia, Bulletin 804, 20154 – Gerard W. Krewer, Bob Westerfield
- LSU AgCenter – Citrus Problems, 3/21/2015 – Daniel Gill
- University of Florida Institute of Food and Agricultural Sciences (IFAS) Extension – Citrus Problems in the Home Landscape, Publication #HS876 – Mongi Zekri and Robert E. Rouse
- University of Florida Institute of Food and Agricultural Sciences (IFAS) Extension – Citrus Nutrition Management Practices, HS1292 – J. D. Burrow, T. Vashisth, M. Zekri, S. H. Futch, and A. Schumann
- Virginia Cooperative Extension – VCE Publications / 426 / 426-613, Environmental Horticulture: Guide to Nutrient Management – Diane Relf, Extension Specialist, Horticulture, Virginia Tech
- University of Minnesota Extension – Potassium for crop production
- New South Wales Department of Primary Industries – Growing lemons in Australia- a production manual
- Potassium Nutrition in Plants, Fact Sheet. A&L Canada Laboratories Inc.