Lemon, orange and mandarin are probably the three most common fruiting trees in New Zealand backyards, and between them they cover most of what a home gardener needs to know about growing citrus generally. All three want the same basic conditions. Where they genuinely differ is cold-hardiness, how long they take to bear a useable crop, and how much space they eventually need, and getting those three things right for the specific tree you're planting matters more than any other decision you'll make.
All three need full sun and a sheltered spot, protected from frost and wind from the day they go in the ground. Citrus also dislikes wet feet: plant into free-draining soil, or build up a raised mound if your soil holds water, rather than a low spot that stays damp after rain. Give lemon around 2.5m between trees, mandarin 3m, and orange 3.5m; orange also grows into the biggest tree of the three, reaching around 4m tall at maturity against roughly 2.5-3m for lemon and mandarin, so it needs the most room to spread.
Planting windows depend on where you are. In Northland, Auckland, Bay of Plenty, Gisborne and Hawke's Bay, plant citrus either in autumn, from March to May, or in spring, from September to November. Everywhere from Waikato and Taranaki down through Manawatū-Whanganui, Wellington, Nelson and Tasman, stick to the spring window of September to November. In Canterbury and Otago there's no reliable outdoor planting window for any of the three; grow citrus in a large pot instead, and move it under cover over winter.
Cold-hardiness is the main thing that separates the three. Lemon is the most cold-tolerant of the common home-garden citrus and the most realistic choice for a cooler garden, particularly the Meyer and NZ Lemonade cultivars. Mandarin is the most frost-tender of the three and needs a genuinely frost-free position more than either of the others. Orange sits in between but is the least forgiving of a marginal, only-somewhat-sheltered site: it needs the warmest, most reliably sheltered spot of the three, and a cold snap during flowering will hit its fruit set harder than it would lemon or mandarin.
Citrus is a heavy feeder. Use a dedicated citrus fertiliser rather than a general all-purpose feed, and split it into several smaller applications through the growing season rather than one large feed. All three are self-fertile, so a single tree of any of them will crop on its own without a second tree nearby, though bees visiting the flowers still improve fruit set, so avoid spraying anything while the tree is in bloom.
The same handful of pests turn up on all three: citrus leafminer tunnelling through new leaves, scale insects and aphids clustering on soft new growth, and citrus rust mite on developing fruit. Sooty mould, a black coating on the leaves, isn't a disease in its own right; it grows on the sticky honeydew that aphids and scale leave behind, so treat the pest and wash the leaves rather than treating the mould itself. If a tree is dropping leaves rather than just picking up the odd pest, that's usually a different problem, most often watering or a sudden environmental shock, and it's covered in more depth in our citrus leaf drop guide.
Sooty mould, a black coating on the leaves, isn't a disease in its own right; it grows on the sticky honeydew that aphids and scale leave behind, so treat the pest and wash the leaves rather than treating the mould itself.
Lemon is the most precocious of the three, often producing a first useable crop from as little as 2 years after planting, and a mature tree yields around 15-30kg of fruit a season. Orange and mandarin both take longer to get going, typically 3-4 years to a first useable crop. Once mature, orange is the heaviest cropper of the three at around 25-40kg a season, reflecting its larger size, while mandarin yields a similar 15-30kg to lemon but from a noticeably more compact tree, which makes it easier to net or throw a frost cloth over when a cold snap is forecast.