Tree · Rutaceae
Key Lime
Citrus × aurantiifolia
Give it full sun, warmth, and free-draining soil, and this thorny little citrus ripens small, sharp, deeply aromatic limes.
Full sun
Every 5–10 days
USDA 9–11
6–13 ft · 1.8–4 m
Overview
The Key lime is a small, thorny citrus tree grown for its tart, deeply fragrant fruit. Its slender branches bristle with sharp thorns and carry small leaves, and its limes stay round and green, only an inch or two across, with a thin rind and pale, juicy flesh crowded with seeds. That heavy seediness and sharp aroma are exactly what make it the lime behind a real Key lime pie.
It is an everbearing tree, flowering and fruiting over a long stretch rather than all at once. Small blossoms open heaviest in spring, and the little green-skinned limes ripen through summer into fall, picked while the skin is still green rather than yellow. Left to itself the tree reaches 6 to 13 feet (1.8 to 4 m) tall, its thorny canopy filling out, though it takes well to a pot and to pruning that holds it far smaller.
Key lime is far more sensitive to cold than to anything else. It wants heat, sun, and soil that drains fast, and it withstands a short dry spell better than most citrus. The greater threat is disease, since citrus canker and citrus greening, both of which mark the leaves and fruit plainly, have reshaped where and how this tree can be grown.
Care
The short version: full sun and warmth, soil that drains freely, and steady moisture once the tree settles in. Feed it lightly through the growing season, keep it clear of frost, and prune only to shape it or hold its size.
Light
Key lime wants full sun, and the more it gets, the more freely its small blossoms open and the sweeter its green fruit ripens. It tolerates partial sun, but the harvest thins and the thorny canopy stretches leggy in the shade. Indoors, give it the brightest window you have.
Water
Let the top half of the soil dry out between waterings, which works out to roughly every 5 to 10 days depending on heat and pot size. The tree handles a short drought better than most citrus, yet it sets the heaviest crop of limes with steady moisture. Never leave it standing in waterlogged soil.
Soil
Plant it in well-drained loam, silt, or sandy soil, since fast drainage matters to this tree more than anything else. It is easygoing about pH and grows across a wide 6.0 to 8.0 range. Heavy, soggy ground is the one thing it will not forgive.
Temperature
Warmth suits this tree from root to thorn tip. It sits at the tender edge of the citrus family, holding steady anywhere from 32°F to 100°F (0°C to 38°C), though frost burns it at the bottom of that range. A hard freeze kills the tree outright, so it belongs in the ground only where winters stay soft, with container specimens carried indoors before cold weather settles in.
Humidity
This tree likes the air on the humid side, around 50% to 80%, which keeps its small leaves from crisping at the edges. Outdoors in a warm climate that takes care of itself. Indoors in dry winter air, group it with other plants or set it on a pebble tray to lift the moisture around it.
Feeding
Feed through the spring and summer growing season with a balanced citrus fertilizer that includes micronutrients, applied sparingly rather than heavily. Citrus is prone to minor-nutrient shortages that show as yellowing between the leaf veins, so a feed made for it beats a generic one. Ease off as growth slows for the year.
Pruning
Key lime needs little pruning. Take out dead, damaged, crossing, or low branches and strip any suckers below the graft, the whippy shoots that rise from the rootstock rather than the lime. Beyond that, prune only to hold its size or shape the thorny canopy as a container tree.
Repotting
A container Key lime needs a fresh pot every 2 to 3 years. Move up when you see roots circling the pot or pushing pale through the drainage holes, water running straight through, or growth stalling. Step up one pot size and refresh the mix rather than drowning it in a giant container.
Size & spacing
Left to grow, the tree reaches 6 to 13 feet (1.8 to 4 m) tall and spreads its thorny branches 5 to 15 feet (1.5 to 4.5 m) wide. Give an in-ground tree room to fill out, or lean on regular pruning to keep a patio specimen compact.
When to plant
Key lime runs on warmth, not a calendar. Plant it outdoors in spring once frost is well behind you, and only in zones 9 to 11 where winters stay mild. Its small white-tinged blossoms open heaviest in spring, and the round green limes ripen from summer into fall, while an established tree flowers on and off all year.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter to early spring from seed in cold climates; often bought as a grafted nursery tree
- Plant: Spring after all danger of frost has passed; zones 9-11 only
- Harvest: Everbearing; main bloom in spring with fruit ripening summer to fall, picked while still green
Hardiness
USDA hardiness zones: 9–11
Temperature: down to -7°C
Troubleshooting
Most Key lime trouble shows itself plainly on the leaves, the twigs, or the small green fruit, and a few of the worst problems are bacterial diseases with no cure. Learn to read a fungus you can prune away from a quarantine disease you must report. Match the spots, mottling, or dieback you see below, then act before it spreads through the rest of the tree.
Corky raised spots ringed with yellow on leaves and fruit
Those rough, raised brown spots, each circled by a bright yellow ring on leaf, twig, and rind, are citrus canker, a bacterial disease that scabs the tissue and strips the tree of its foliage. There is no cure, so the infected growth is bagged, a copper bactericide goes on, and local quarantine and reporting rules are followed. A certified tree and water kept off the leaves are the best guard against it.
Fix:
- Remove and bag infected leaves, twigs, and fruit
- Apply copper-based bactericide per label
- Comply with local quarantine and reporting rules; there is no cure
Blotchy mottled leaves with small lopsided fruit that stays green
Leaves gone blotchy with uneven yellow mottling, alongside small lopsided limes that stay stubbornly green at the base, are the signature of citrus greening, or HLB, the most serious disease the tree can carry. The Asian citrus psyllid spreads it, a lab test confirms it, and no cure exists once a tree is infected. Infected trees are removed and destroyed, the psyllids are controlled, and the case is reported to the agricultural authority.
Fix:
- No cure; remove and destroy infected trees
- Control Asian citrus psyllid populations
- Report to local agricultural authority
Young shoot tips dying back with blighted blossoms
When the tender new shoot tips shrivel to brown and the pale blossoms blacken and drop, the cause is lime anthracnose, or withertip, a fungus that sinks small dark lesions into the young growth. The affected twigs are pruned back to healthy wood and a copper fungicide is applied. An open, airy canopy with the deadwood cleared out keeps it from creeping back.
Fix:
- Prune out affected twigs back to healthy wood
- Apply copper fungicide
- Improve air circulation and remove deadwood
Drooping, falling foliage above dark, sodden soil
Leaves that droop and fall while the soil stays dark and wet point to root rot, the fine roots turning soft, brown, and foul as overwatering or poor drainage suffocates them. Watering is cut back and the drainage improved, and a potted tree goes into a fresh well-drained mix with the mushy roots trimmed away. The cure holds only once the tree no longer stands in standing water.
Fix:
- Reduce watering and improve drainage
- Repot into fresh well-drained mix and trim rotted roots
Toxicity
The pale, juicy flesh and its sharp juice are perfectly fine for people, but the rest of the tree is not a pet snack. The green rind, the small leaves, and the seeds carry essential oils and psoralens that upset an animal's stomach and can make skin react to sunlight. Keep cats, dogs, and horses away from the foliage and prunings, and wash your own hands after handling fruit if you are heading into the sun.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Mild
A cat that bites into the glossy leaves or the oily green rind usually drools, then vomits, with diarrhea and a dull, low mood to follow. Those same rind oils can leave the skin touchy in sunlight.
Dogs Mild
A dog that chews the leathery foliage or the fragrant rind tends toward an upset stomach, drooling, and a flat, listless mood. It mostly stays mild, though the citrus oils can leave the skin sensitive to the sun.
Horses Mild
The peel and leaves can give a horse a sour stomach and a sluggish, off feeling. Horses seldom go after the sharp-scented citrus, so trouble is rare unless prunings land within their reach.
Rabbits Caution
The aromatic oils packed into the rind and the small leaves upset a rabbit's gut, so every part of the tree belongs well away from the hutch. Trimmings are off-limits rather than a treat.
Livestock Caution
Goats, sheep, and cattle may nibble fallen leaves or dropped fruit, and the citrus oils can leave them mildly out of sorts. It is rarely serious, yet the prunings are no place for them to graze.
Birds Caution
The essential oils concentrated in the green rind and the leaves make the foliage a poor choice around birds. A taste of the juicy flesh is far less of a worry than a beakful of oily rind or leaf.
People Flesh safe
The pale, juicy flesh and its sharp juice are a kitchen staple and safe to eat. The catch is in the oily rind and sap: handle the fruit or foliage and then step into the sun, and a blistering rash called phytophotodermatitis can raise across the skin, so the hands are washed first.
Lime juice and peel oils combined with sunlight can cause phytophotodermatitis (a blistering skin rash) in people; wash hands and avoid sun exposure after handling fruit or foliage.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Key lime does not come in many forms, but the few that exist differ enough to tell apart before you choose a tree.
Mexican (West Indian) Key lime
This is the classic Key lime of a true Key lime pie, a thorny tree hung with small, round 1 to 2 inch fruit that is heavy with seeds and sharply aromatic.
Thornless Key lime
A nearly spineless selection whose branches carry the same fruit as the original but without the bristling thorns, making it far smoother to handle and harvest.
How to propagate
Key lime is unusual among citrus in that its seed comes mostly true to type, so the choices run from a free seedling to a grafted nursery tree.
SeedEasy
Spring · 3-6 years to first fruit
Seed is the easy route with Key lime, since its seed is largely polyembryonic and comes fairly true to type, unlike most citrus. Sow the small seeds in spring and wait, because a seedling takes 3 to 6 years before the first green limes appear.
GraftingModerate
Spring to summer · 2-3 years to fruit
Budding or grafting onto a rootstock is the nursery's choice, since it brings disease resistance and even, reliable vigor. Worked from spring through summer, a grafted tree bears fruit sooner than a seedling, in about 2 to 3 years.
Stem cuttingsModerate
Spring to summer · 1-2 years
Semi-hardwood cuttings, the firm green-brown wood of the season, taken in spring or summer will root and stand a tree on its own roots within a year or two. It works, though most growers reach for seed or grafting first.
Air layeringModerate
Warm season · 1-2 years
Air-layering through the warm season gives a tree on its own roots in about 1 to 2 years. It is a workable option for citrus, just slower and more fiddly than starting from seed.
Myths
The Key lime is nothing more than a Persian lime in miniature.
The two are separate species, not one shrunk down. The Key lime is Citrus x aurantiifolia, while the Persian or Tahiti lime is Citrus x latifolia, a larger, seedless fruit that takes cold in better stride. They look related, but they are not the same fruit.
Because citrus ends up on human plates, it poses no danger to dogs and cats.
What sits fine on your plate does not sit fine in a pet's stomach. The ASPCA lists lime as toxic to cats and dogs, because the rind, leaves, and seeds hold essential oils and psoralens that bring on stomach upset and can leave skin sensitive to sunlight. The fruit being edible for you says nothing about it being safe for them.
Grow a Key lime from seed and the new tree will not match the one it came from.
Most fruit trees scatter when grown from seed, but the Key lime holds steady. Its seed is largely polyembryonic, so the seedlings come fairly true to type and usually match the tree they came from.