Citrus

(Citrus species)

Introduction: Citrus are prized for their delicious fruit, but their wonderfully fragrant flowers and dark green evergreen leaves also make them very popular ornamental garden plants for mild winter areas.

History: These species all originated in Southeast Asia, mostly in China, though they have long been grown all around the world. Various famous cultivars bear the names of the places they were developed (Seville orange, Lisbon Lemon, Mexican Lime and Tahitian Lime).

About Citrus: Size Standard – 18-30 ft tall x 20ft wide Dwarf – 8-12 ft tall x 10 ft wide Zone: 9-11 Winter chill hours: 0 Blooming period: Depends upon the species Fruiting period: (depends on species) Life expectancy: 10-50 years depending on whereabouts in the country and on what site. Bearing age: 2-4 years Days to harvest: 100 Spacing: Standard tree 15-25ft apart Dwarf tree 6-10ft apart (depends on species)

Ease of growing: The Citrus are easy to grow in the right climate, not so easy if it isn’t suited to them. All need full sun and hot weather if they are to produce sweet fruit. Lemons and Limes don’t produce sweet fruit of course so don’t need as much heat.

Climate: More than most tree fruit the right climate is critical for growing good citrus fruit and the area where they can be successfully grown is very restricted.

Citrus are native to warm climates and can’t tolerate much frost. Winter weather must be mild, with few night below freezing (even one night of hard frost can do serious damage). The coldest it should ever get is 20 deg F and even then you will need to protect them carefully (or grow them in pots and taken into a greenhouse in winter). I list the relative hardiness of each species below.

The plants also need lots of heat if they are to produce sweet fruit; if the summer doesn’t get warm enough the fruit won’t taste very good (lemons aren’t supposed to be sweet so they can be grown in cooler areas than other fruit. In marginal areas they can be grown against south facing walls to provide additional heat

Citrus growing in tropical climates (where day and night temperatures don’t fluctuate very much) don’t develop a colored skin and remain green even when ripe (coupled with year round bearing this makes it hard to tell when the fruit is fully ripe). The fruit is also sweeter and not so acidic, which makes it good for juice.

Soil

pH 5.5 – 8.0

Generally citrus trees prefer well drained, fairly good soil with lots of organic matter. It should also be water retentive, but not wet, which could encourage root rot. They can be grown on very light or heavy soils if you add lots of organic matter to improve their texture. They don’t like saline soils at all. If your soil isn’t very well drained you might consider planting on a hillside (or on a mound if the site is flat).

Site: Citrus really need heat to produce sweet fruit, so should be planted in the hottest and sunniest part of the garden. This will usually have a southern or western exposure, but should be protected from cold or drying winds. In marginal areas they can be espaliered against walls for maximum warmth (and winter protection). I hardly need add that you should avoid low lying frost pockets where cold air collects on cold nights.

Careful use of microclimate is important when growing citrus, because it can mean the difference between having to go out every time frost threatens (to cover the trees), or being able to forget about them. It really pays to study your garden carefully and put these tender plants in the warmest and most protected places you can find. I have mine on the south side of my house and under the southern and western drip line of a large oak tree (this gives them a significant amount of protection from frost).

Like most other fruit trees, citrus don’t like grass because it competes with the trees for water and nutrients (and is very competitive).

Site preparation: If the soil isn’t very good you may want to dig the entire planting area and incorporate 3” of compost or aged manure, as well as wood ashes and maybe a fertilizer mix. This works much better than simply amending the planting hole, as it encourages the trees roots to spread out. In areas with wet soil they might be grown on mounds.

What to look for when buying a tree: Before buying a Citrus tree you should examine it carefully for problems. The leaves should be large, dark green and have no significant signs of insect damage or infestation. It should not be flowering or bearing fruit and the roots shouldn’t be circling around the root ball (look inside the pot). These are all signs that the plant has been in the container for too long.

Planting

When: Citrus are usually bought as container grown plants, though they are sometimes available ball and burlapped, or even bare root. Container trees can be planted out at any time, but spring is best as it gives the plant a whole season to get established.

How: Dig a hole for the tree 2‑3 times as wide as the root spread and the same depth as the root (this gives the roots plenty of space). A tree should always be set in the ground at the same depth it was growing in the nursery (this is most easily measured by laying a stick across the hole to get the right height). The graft union should be at least 3” above soil level. In poorly drained soils you may have to plant on a mound, to keep the collar of the tree from too much moisture, which could potentially cause it to rot. Throw a couple of shovels of soil (and possibly organic matter) into a mound in the bottom of the hole, then spread out the trees roots evenly over the mound (trim off any damaged roots). Then refill the hole with the soil (you might want to add some rock phosphate, wood ash and lime to the soil). Pack the soil down into the hole firmly (shake the tree slightly to help soil settle as you add it) and then water well (this not only gives water to the plant, but also helps to establish contact between roots and soil). When you have finished planting the tree should be on a slight mound, so as the soil settles it becomes flat (if it starts out perfectly flat you may end up with a slight depression where water can collect). A final step is a mulch to conserve water and keep down weed competition.

If gophers are a problem in your area you will have to plant in a wire basket. This is an extra hassle and no one likes doing it, but it’s a lot less painful than losing the tree 3 years later, just as it starts to come into bearing.

Support: Newly planted trees were once routinely staked, but it is now thought that the trunk gets stronger faster if left unstaked. However a tree in an exposed position may have to be staked to stop it being continuously rocked by the wind.

Maintenance: Citrus are pretty low maintenance, the most significant task being to ensure the plants aren’t seriously damaged or killed by frost.

Fertilizing: Citrus are quite hungry plants and need a steady supply of nutrients to keep them producing well. This is best supplied in the form of a thick mulch of compost, though you can also give a top dressing of a fertilizer mix. You might give them a boost of nitrogen in late winter, so it is available when growth starts up. They also need plenty of iron, magnesium, manganese and zinc. The trees feeder roots are mostly in the top two feet of soil and extend out well past the drip line.

Watering: It is important that young trees are watered regularly, so the soil doesn’t dry out.  Established trees are fairly drought tolerant, but need a constant supply of water if they are to produce well. Give them an occasional deep watering (ideally down to 4 ft). Don’t give them too much water however as constantly wet soil can lead to root rot. Drip irrigation is best as it doesn’t wet the foliage or fruit.

Mulch: Citrus benefit from a deep mulch of compost or aged manure (though not one that contains much salt). Use it to prevent the evaporation of moisture from the soil, to suppress water robbing weeds and to add nutrients. Keep it at least 6” away from the trunk, as if this stays wet it can encourage disease. Mulch can harbor slugs and snails which like to eat the foliage, so be aware if these are a problem.

Pollination: The insect pollinated flowers generally appear in early spring, though this depends upon species and climate. Most citrus are self-fertile and fruit prolifically, though very high temperatures can result in poor fruit set. Indoor plants may be hand pollinated.

Pruning: Mature trees don’t need much pruning, except to remove suckers and dead, diseased, damaged, broken or crossed branches. Younger trees may need to be pruned to control their shape and to remove unwanted suckers and overly vigorous growth.

The evergreen leaves don’t just produce food, they also store it (as do the branches). This means that any time you prune and remove leaves and branches you are removing food stores, so it’s good to keep it to a minimum. Plants can be pruned at any time of year, except from late winter to early spring, which is when they have most stored nutrients.

Propagation: Citrus are usually propagated by budding or grafting on to seedling rootstock (though improved rootstocks are being developed). Many Citrus produce seed which is polyembryonic (contains more than one embryo that can grow into a plant – cut a seed and look at how many are inside) and these commonly produce vigorous seedlings almost identical to the parent. Citrus seed must be moist when planted, if it dries out germination will be severely reduced.

Frost: In most citrus growing areas of the United States there is a chance of a severe frost damaging your trees every so often (you are lucky if you can go 10 years without significant damage). Any time the temperature drops much below freezing there is a danger to the trees and you might have to think about protecting them. The grapefruits and limes are the most tender, followed by the oranges, while the mandarins and lemons are the most hardy.

The best way to protect the trees is to put them in the warmest protected microclimates in the garden. If you are lucky this may be all you need to do (while neighbors who planted their trees in less favorable situations have to run out in the middle of the night to protect them). If you don’t have a warm enough microclimate to protect them you will also have to take other steps. The commonest of these is to cover the plants with sheets when frost threatens and this can help them get through moderate frost without harm. If the frost seems likely to be severe you could put old fashioned Christmas lights or incandescent light bulbs under the trees as well. The small amount of heat they give off will be trapped by the sheets and can help a lot.

If your winters are just too cold for growing citrus, you could try growing them in large tubs and bringing them indoors for the winter.

How much damage the plants will suffer in a frost is determined by number of factors: How cold it gets, how long the cold lasts, whether the cold air can drain away, or whether it collects in a frost pocket and the location of the tree (is it out in open or near shelter of buildings or trees).

Frost damage is also related to the stage of growth (young foliage and flowers are most vulnerable) and whether the trees are actively growing. Trees stop growing when temperatures go below 55 degrees and are less damaged by frost when they are dormant. A frost that comes when the trees are actively growing in spring will be much more damaging.

The degree of damage to the fruit depends upon where on tree it is. Fruit exposed on the top of the tree will suffer more than those at the bottom, protected by a dense cover of leaves.

If a plant is hard hit by frost and all the leaves are killed don’t be in too much of a hurry to remove damaged branches (or even the whole tree). Wait until late the following spring and see how much of it recovers. In many cases it will grow new leaves and come back as good as new.

Pests: Citrus can be afflicted by a range of pests and diseases, especially when growing in adverse conditions.

Aphids

Mites

Scale

Mealy bugs

Slugs and snails

Gophers

Disease:

Root rot

Oak root fungus (Armillaria mellea)

Gummosis

Harvest:

When: This depends upon the type of fruit and the variety, some kinds fruit over a long period (sometimes for months), some hold ripe fruit on th3e tree until next years crop is ready and some bear a heavy crop all at once. Fruit will usually stay in good condition on the tree for quite a while, so doesn’t have to be harvested immediately it is ripe. However fruit that is to be stored for any length of time should be picked soon after it is fully ripe.

The fruit produced when trees are very young isn’t usually very good and is often removed while it is small, to prevent the tree wasting energy.

How: Fruit should be cut from the tree using pruning shears. Pulling it off is a bad idea as it can damage both the tree and the fruit.

Landscape uses: With their beautiful and strongly fragrant flowers, attractive fruit and dark evergreen foliage, the citrus are wonderful ornamentals for warm climates. It is a good idea to plant them close to the house where you can enjoy them. A warm sunny patio is a good place, as you can appreciate their scent and they are convenient placed if you have to protect them from frost. The lower growing types can be used as shrubs to create an evergreen hedge, while the bigger trees can be used as specimen trees.

Containers: In cooler areas dwarf Citrus are often grown in containers, so they can be brought indoors for the winter.

Dwarf citrus: There are standard and dwarf varieties of citrus. The dwarfs may produce half as much as a standard tree, but you can grow four dwarf trees in the same space as one standard. Trifoliate orange is often used as a dwarfing rootstock.

Food uses: Flowers, peel, fruit and juice are all used.  

Varieties: The Citrus species vary a lot in their hardiness and uses. The minimum temperatures are extremes and not guaranteed. They include:

Kumquat (C. fortunella) (Hardy to 20° F) – The hardiest citrus.

Tangerine or Mandarin (C. reticulata)

Owari Satsuma – This delicious seedless mandarin is one of the hardiest species and has been known to survive to 20°. The fruit starts to ripen in December, which is why they are sometimes associated with Christmas. This has been the most trouble free and dependable Citrus in my garden.

Meyer Lemon – This cross between a lemon and a mandarin is one of the hardiest of all Citrus and has been known to survive temperatures in the low 20’s F. It flowers and fruits prolifically over a long period and is almost everbearing, so you can get fruit almost year round (a wonderful asset in any fruit). It is also a natural dwarf and apparently can be grown from cuttings.

Sweet Orange (C. aurantiacum?)

Washington Navel – A fine flavored orange as well as one of the hardiest. It can survive down to 24˚ F.

Moro Blood Orange – Has dark purple flesh with a unique flavor. Hardy to 27° F.

Grapefruit

I haven’t tried to grow one of these but apparently they are hardy to 25° F.

Lemon (Citrus limon)

The lemons are a good choice for areas with cooler summers because they don’t need heat to develop sweetness for best flavor. 

Eureka – This is the classic commercial lemon.

Lisbon – The fruit is quite similar to the Eureka, but the plant is more vigorous, hardier and generally easier to grow.

Lime

Bears Lime

The hardiest lime (to 28° F)

Mexican Limes (Hardy to 32° F)

Citron (Hardy to 32° F)