How to properly feed your citrus trees?

Growing citrus trees in pots and in the ground

To fertilise effectively, you need to understand that the tree does not live in the same world depending on whether its roots are in the soil of your garden or in the confines of a pot. The major difference lies in the citrus tree’s ability to access nutrients.

In open ground, the citrus tree develops a powerful root system, composed of a deep taproot and numerous surface roots that extend well beyond the drip line of the foliage. It explores a vast volume of soil. In a pot, the tree is confined. Its roots circle around the pot walls. It can only draw on what is contained in the few litres of available potting compost.

In addition, in open ground, minerals are not washed away by rain. In contrast, in a well-drained pot, with every watering, the water that drains through the bottom holes carries away part of the fertiliser.

Finally, in open ground, soil temperature and acidity (pH) vary very slowly. But in a pot, the medium is unstable. In midsummer, the temperature of the potting compost can rise very high, accelerating the breakdown of the fertiliser and risking scorching the roots.

Spring: the start of citrus trees

From March onwards, citrus trees come out of their winter dormancy and begin to grow. Although, unlike deciduous trees, citrus trees do not have complete dormancy. They are in a ‘vegetative rest’ triggered by the drop in temperatures below 10–12°C and reduced daylight. As soon as the soil warms up and night-time temperatures stabilise, sap rises, transporting sugar stores from the roots to the branches. That is why fertiliser application should be relatively early.

Citrus trees in pots

The substrate is often depleted after winter. As soon as temperatures exceed 12–15°C, fertiliser applications should resume, about every two weeks. Note that, with the small volume of soil, the substrate warms up very quickly. The potted tree typically breaks dormancy 2–3 weeks earlier than those in the ground.

• How to do it? Use a Citrus trees liquid fertiliser rich in nitrogen (N) and iron. Always water with clear, non-calcareous water, for example rainwater, before applying the fertiliser to avoid scorching the confined roots.
• The little extra: A top-dress by replacing the first five centimetres of soil with well-rotted compost, or, if unavailable, fresh potting mix, is ideal in March.

Citrus trees in the ground

The garden soil takes much longer to warm up; it heats up more slowly than the pot substrate. The tree seems to be dormant even though the air is already mild. The strategy, therefore, is to get ahead a little by applying foundational organic amendments from the end of February. They will take a few weeks to mineralise under the action of soil microorganisms, becoming available by the time the sap reaches the tree’s crown.

• How to do it? You can thus spread at the base of the tree, directly beneath the crown, 3 to 5 kg of well-rotted manure or a granular organic fertiliser. Lightly fork in to incorporate.
• The little extra: Mulch generously to retain moisture and feed soil life.

Summer: fruit formation

Summer is the season of transformation: flowers become fruit, and these fruits must swell without exhausting the tree. This transformation is one of the most demanding for the tree, as it must simultaneously support the growth of its new shoots and direct a substantial amount of sugars and minerals to developing fruits. If nutrient supplies are insufficient or irregular at this stage, the tree will trigger a survival mechanism to abort part of its production to avoid exhausting itself prematurely. During these hot months, regular fertilisation becomes essential.

Potted citrus trees

With the heat, watering is almost daily. Watering leaches minerals from the pot. So keep up the fertilisation schedule to prevent fruit drop, every 10 to 15 days.

• How to proceed? Alternate between the usual liquid fertiliser and a supply of trace elements (magnesium/zinc) if the leaves show signs of fatigue.
• The extra tip: If you’re going on holiday, do not use long-lasting fertiliser sticks just before you leave; they can be too aggressive if the irrigation is not running.

Citrus trees in the ground

The tree draws on its reserves and on the spring soil amendment. You will probably need to apply a small dose of fertiliser again in June or July to support fruit formation.

• How to do it? Use a granular fertiliser rich in potassium (K). This helps with drought resistance and with the concentration of sugars in the fruits.
• The extra tip: In a heatwave, the tree takes a break. There’s no point fertilising if temperatures exceed 35°C, as it won’t be absorbed.

Autumn, the season of consolidation.

In autumn, we no longer aim to make the tree grow, but to strengthen its wood and bring the fruits to ripeness. Stopping nitrogen fertilisers halts the production of fragile young shoots, thereby redirecting all of the plant’s energy toward the lignification of the shoots that will become sturdier to withstand winter’s rigours. At the same time, a targeted application of potassium promotes the concentration of sugars and essential oils in the fruits, ensuring optimal ripeness and a robust skin for harvest.

The citrus tree in a pot

The tree must be prepared for its return to the conservatory. We therefore gradually reduce fertiliser applications to once a month.

• How to do it? Drastically reduce nitrogen. Use a poorer fertiliser to avoid late-season shoots that would attract aphids indoors.
• The little extra : remember to check for the absence of deficiencies that manifest as foliage chlorosis before bringing the pot indoors.

The citrus tree in the ground

The tree must store reserves in its roots to get through the winter and harden. A final fertiliser application should be made in late September.

• How to do it ? An application of potassium sulfate or wood ash (very modestly) helps the wood to lignify (become woody) to better resist frost.
• The little extra : Remove fruits that have fallen to the ground to prevent cryptogamic diseases that could profit from residual fertiliser.

Winter, rest

For potted citrus

• Total rest: If the pot is kept in a cold greenhouse (5-10°C), stop all fertilisers. The plant is in dormancy.
• Exception: If you have a four-season lemon tree that continues to flower in a heated conservatory, maintain a very light feeding (¼ dose) once a month, only if the tree shows active growth.

For citrus in the ground

No action is required. Let mulch decompose. Nitrogen applied in winter would not be absorbed by the cold roots and would pollute groundwater.

Common mistakes to avoid

En croyant bien faire, certains jardiniers commettent des erreurs qui peuvent avoir de graves conséquences :

• Over-fertilising: applying too much fertiliser can burn the roots and make the tree more vulnerable to aphids that love shoots that are too tender and rich in nitrogen.
• Forgetting to water: fertiliser acts a little like salt. Without water to dilute it and carry it, it becomes toxic to the plant.
• Ignoring soil pH: Citrus trees prefer soils that are slightly acidic to neutral. In soil that is too calcareous, iron becomes unavailable, and the tree will suffer deficiencies even if you fertilise.
• Fertilise a diseased tree: if your tree loses its leaves due to excess water or a parasitic pest, do not fertilise to the “boost”. First address the health problem.