Calcium boron is one of the fertilisers we recommend most during flowering, fruit set and fruit growth. Many problems such as flower drop, blossom-end rot in tomatoes and fruit cracking are mainly caused by calcium or boron deficiency, not disease.
This article explains what each nutrient does, why they are applied together, when to spray calcium boron, and how to avoid mixing mistakes.
What does calcium do for plants?
- It builds cell walls, giving fruit its firmness and longer shelf life after harvest.
- It reduces blossom-end rot in tomatoes and peppers.
- It reduces fruit cracking and bitter pit in apples.
- It strengthens roots and new growth.
Important: calcium moves with the water the plant draws up, so most of it goes to large leaves and fruits receive only a little. That is why foliar spraying during fruit development makes a difference.
What does boron do for plants?
- It is essential for pollen tube growth; without it pollination is incomplete and flowers drop.
- It improves fruit set and fruit shape.
- It helps move sugars into the fruit.
- It helps calcium move inside the plant.
Why calcium and boron together?
- The plant needs both at the same stage: flowering, fruit set and early fruit growth.
- Boron helps calcium move through tissues, so the effect is stronger together.
- Many formulations add amino acids to improve uptake and reduce stress.
Signs of calcium and boron deficiency
| Sign | Most likely cause |
|---|---|
| Black rot at the tip of tomato fruit | Calcium deficiency |
| Burnt tips on new leaves | Calcium deficiency |
| Flower drop and poor fruit set | Boron deficiency |
| Deformed or cracked fruit | Boron or calcium |
| Death of the growing tip | Boron deficiency |
| Hollow or cracked stem | Boron deficiency |
To tell deficiency from disease, see Yellow Leaves and Nutrient Deficiency.
When to spray calcium boron
- Before flowering: prepares the plant for pollination.
- At full bloom: supports fruit set.
- Right after fruit set: reduces drop.
- During fruit growth: sprays every 10 to 14 days as per the label, especially in hot weather.
Best time of day: early morning or evening, so leaves do not burn and the spray stays wet longer.
Crops that benefit most
- Tomatoes, peppers and aubergines: for blossom-end rot. See Tomato Diseases.
- Cucumbers, courgettes and watermelons: to improve fruit set and shape.
- Strawberries: for firmer fruit.
- Citrus, mango and grapes: to reduce flower drop and improve set.
- Date palms: during pollination and early fruit set. See Date Palm Fertiliser.
Common mistakes with calcium boron
- Mixing calcium with phosphates or sulphates in the same tank, because it precipitates and blocks nozzles.
- Too much boron: the gap between enough and toxic is small, and excess burns leaf edges. Follow the label.
- Spraying in the heat, which burns leaves.
- Relying on spraying alone with irregular irrigation, a major cause of blossom-end rot.
- Spraying leaves only and missing the fruit, which needs to be covered.
Calcium by spray or by soil?
- Soil fertilising with calcium nitrate covers the plant's general needs. See Potassium Sulphate and Calcium Nitrate.
- Foliar spraying delivers calcium directly to the fruit when it needs it.
- Best is both together: regular soil feeding + sprays at fruit set and fruit growth. For more on spraying, see Foliar Fertiliser.
Products from Orkida
- Tradebor boron 11% by Rovensa
- Codabor liquid boron by Coda
- Kalborn Plus calcium and boron
- Amino Aga Cap calcium and boron with amino acids
- Bocal Plus calcium + boron + amino acids
- Dolphin boron
Calcium boron FAQs
Does calcium boron stop flower drop?
It reduces it when the cause is boron or calcium deficiency. But flower drop has other causes such as extreme heat, water stress and excess nitrogen, so look at the whole picture.
How often should I spray calcium boron?
Usually 3 to 4 sprays per season: before flowering, at bloom, after fruit set and during fruit growth, at the interval on the label.
Can I mix it with pesticides?
Do a jar test first. Avoid mixing with phosphate and sulphate fertilisers and copper products.
Is it suitable for home plants?
Yes for flowering and fruiting plants at home, such as potted tomatoes, peppers and lemons, applied the same way at the diluted rate on the label.