Key lime (Mexican lime)
Citrus × aurantiifolia — the small, aromatic true acid lime
A Key lime is small, floral and full of sharp, fragrant juice. It has more character than a Persian lime, but is also one of the least cold-tolerant citrus.
Key lime is the tropics’ true acid lime, with the floral, intense aroma that defines classic lime flavour. An old hybrid with papeda and citron ancestry, it forms a thorny shrub carrying small round fruit that turn yellow when fully ripe and remain juicy and seedy. It is more distinctive than the Persian lime, yet far more cold-sensitive: this is an F0–F1 citrus. In Britain it needs bright, frost-free winter shelter.
Like yuzu or makrut lime, it is valued for its juice and zest, not eaten like dessert fruit. The contrasting choice is Persian lime: larger, nearly seedless and slightly less tender.
Reliable checks before you buy
Choose it for fragrance, then provide greenhouse care
Key lime earns its place through aroma: more floral and distinctive than Persian lime. That is why it suits drinks and tropical cuisines. The trade-off is a very tender tree that needs warmth to grow and crop. In Britain it is usually kept in a container with bright, frost-free winter shelter. Treating it like an ordinary patio lemon risks serious winter damage.
Do not confuse two separate effects: a rootstock changes the roots’ response to soil, vigour and lime tolerance, but cannot make the canopy frost hardy. No rootstock turns Key lime into a hardy citrus.
| Your situation | Rootstock option | Reason — and trade-off |
|---|---|---|
| Warm site, vigour, light soil | Volkameriana, Citrus macrophylla | Vigour in heat. ⚠️ Neither adds frost hardiness to the canopy. |
| Other conditions | Only a locally proven combination | Use another rootstock only when that exact pairing is supported locally. |
| Do not expect | A “frost-proof” rootstock | Any gain concerns roots, never the leaves’ resistance to freezing. |
| Remember | Winter protection is decisive | Survival depends on warmth and shelter, not rootstock alone. |
A Key lime calendar for Britain
Open-ground culture suits genuinely frost-free sites only. Elsewhere, overwinter a container in bright shelter; sustained warmth can extend flowering and harvest.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Plant / repot once warmth is established | ||||||||||||
| Keep bright and frost-free indoors | ||||||||||||
| Move outside after nights exceed 10–12 °C | ||||||||||||
| Flowering, repeated in sustained warmth | ||||||||||||
| Harvest green or as colour begins to turn | ||||||||||||
| Feed citrus while actively growing |
Help a Key lime survive winter and set fruit
Measure the warmth you can provide
Be realistic before buying: without bright, frost-free shelter this tropical citrus will not pass a British winter safely.
Leaves deteriorate in winter
Leaf fall or scorched tissue usually points to conditions that are too cold or dark, rather than disease or the wrong rootstock.
Increase light and safe warmth
Bring the container into bright shelter, reduce watering during slow growth, and wait for reliably mild nights before moving it out.
Do not rely on rootstock
A rootstock helps roots cope with soil; it does not protect leaves from frost. Winter shelter remains the decisive safeguard.
Read the symptom, trace the cause, correct and prevent
Key lime shares the usual problems of container citrus; its exceptional cold sensitivity is the first variable to check.
| Observed sign | Likely cause | Correct | Prevent |
|---|---|---|---|
| Leaf fall or winter scorch | Winter quarters too cold or dark (F0–F1 tenderness) | Move into bright, frost-free shelter; reduce watering | Use a suitable greenhouse or conservatory |
| Yellow leaves with green veins | Iron chlorosisYellowing of the foliage caused by poor iron uptake, often linked to chalky soil. from high pH, lime or excess water | Check watering; use iron chelate and a suitable rootstock | Free-draining compost and low-lime water |
| Cottony deposits or scales on stems and leaf backs | Mealybugs or scale insects | Clean by hand, isolate, then use an authorised control if needed | Ventilate winter quarters and inspect before moving indoors |
| Fine webbing and yellow stippling | Glasshouse red spider mite in warm, dry air | Rinse foliage and improve humidity without waterlogging | Avoid excessively dry winter air |
| Winding silvery mines in young leaves | Citrus leafminer | Remove only badly damaged shoots and conserve beneficial insects | Avoid excess nitrogen that drives repeated soft flushes |
Keep treatment proportionate: clean foliage, ventilated shelter and balanced feeding come first. Check current UK plant-health guidance before moving citrus plants or treating a suspected regulated pest.
Use the juice and zest rather than eating it whole
Too acid for dessert eating, Key lime excels as juice: a small amount gives food or drink a fragrant sharpness. Its zest holds the floral aroma. The peel can provide pectin in marmalade, while whole fruit can be salt-preserved or fermented and may also be dried. Green fruit taste especially brisk; fully yellow fruit become slightly rounder.
What the commercial standard actually says
These are marketing checks for fresh fruit, not targets for a home-grown crop.
- EU commercial minimum for Mexican lime: diameter at least 25 mm and juice content at least 40%; the standard uses size codes 1 to 5.
- Colour: fruit is commonly picked green, but a Key lime naturally turns yellow at full maturity; yellowing alone does not mean decay.
No universal yield is credible: rootstock, soil, plant age, light and temperature all matter. Record the crop from your own tree instead.
Practical answers about Key lime
Key lime or Persian lime?
Key lime is smaller, seedier and more aromatic but markedly more tender. Persian lime is larger, nearly seedless and slightly easier to manage. Choose by flavour and the winter shelter you can provide.
Why is my Key lime turning yellow?
That is normal ripening. Key limes are often harvested green or as colour begins to turn, but a sound yellow fruit is not automatically spoiled.
Can Key lime grow outdoors in Britain?
Not reliably year-round. Keep it in a container and provide bright, frost-free winter shelter; it is among the most cold-sensitive citrus.
Can a rootstock make it hardy?
No. Rootstock affects roots, soil response and vigour, not the canopy’s tolerance of frost. It cannot make Key lime hardy.
Evidence and limits
Evidence checked on 4 August 2026. Rootstock performance still needs an exact, locally tested scion–rootstock pairing.
- Royal Horticultural Society — Key lime: British plant profile for the thorny shrub and small green-to-yellow fruit (Citrus × aurantiifolia).
- EUR-Lex — citrus fruit marketing standard: current size, juice-content and colouring requirements for Mexican limes.