Citruses are flowering trees and shrubs from the rue (Rutaceae) family and are native to subtropical and tropical regions of South Asia, East Asia, Southeast Asia, Melanesia, and northeastern Australia.
Plants in the genus Citrus provide a wide assortment of fruit types, including important crops such as oranges, mandarins, lemons, lime, grapefruit, pummelo, kumquats, and finger lime.
Most citrus varieties can be grown from seed, as they’re true to seed, meaning that they’ll grow into the same variety as the parent tree. That’s possible because the seeds are polyembryonic, they contain more than one plant embryo.
- Only one of these embryos in the seed is a zygotic embryo that is produced by fertilization (sexual reproduction) and will therefore show genetic variation, these are referred to as ‘off types’. They are often inferior in growth, are a different variety from the parent tree, and not true to seed (not true to type).
- The rest of the embryos in the seed are nucellar embryos, which are derived from the maternal tissue (nucellus) that surrounds the embryo sac, and are therefore genetic clones of the parent tree, and will produce exactly the same variety of fruit.
When these citrus seeds are planted and sprout, they produce multiple shoots, and the single fertilised shoot (which is not true to seed) is usually the weakest and is removed.
For anyone interested, there are three factors which contribute to nucellar embryos being stronger, and the zygotic embryo (that aren’t true to type) being weaker:
- The nucellar embryos grow more rapidly within the seed, getting a head start over the zygotic embryo
- The zygotic embryo is located in an unfavourable position in the seed, in the apex of the embryo sac, where it may receive less nutrients, and be more subjected to crowding pressure
- The zygotic embryos are usually genetically weaker than their nucellar counterparts because of inbreeding depression, the reduced biological fitness in the offspring produced by self-pollination.
Which Citrus Cannot Be Grown from Seed?
The following citrus are monoembryonic, the single and only embryo in the seed is the product of fertilization (sexual reproduction), so it’s genetically different from the parent tree, and will therefore not grow true to type:
- Clementine Mandarin
- Meyer Lemon
- Nagami Kumquat
- Marumi Kumquat
- Temple Tangor
The rootstock Trifoliate orange (also known as Citrus trifoliata, Poncirus trifoliata, Japanese bitter-orange, or Chinese bitter orange) also does not grow true to type.
How to Grow Citrus Trees from Seed
Seeds removed from citrus fruit will eventually grow into trees that produce fruit, but it may take quite a while for the juvenile trees to mature and become productive. A lemon tree planted from a seed will take around 15 years to bear fruit! Different citrus varieties grown from seed will vary in how long they will take to bear fruit.
It’s rather normal for seed-grown fruit trees to take a long time to produce fruit, while grafted trees produce fruit much quicker. By comparison, a grafted lemon tree will produce lemons in 2-3 years. For a more detailed explanation, see the article – “The Difference Between Seedling, Grafted and Cutting Grown Fruit Trees“
Grow citrus trees from seed in ten easy steps:
- Remove seeds – extract the seeds from mature citrus fruit.
- Wash seeds – rinse the seeds thoroughly with water to remove any pulp adhering to the seeds, and to wash off any sugars on the seed surface. This is to avoid any fungal diseases, which may then kill the young plant as the seed germinates.
- Sort seeds – separate the good from the bad seeds using the floating test. Place all the washed seeds in a glass filled with water. The seeds that float will not germinate (sprout), so discard them. The seeds that sink are the viable ones, remove them from the water as these are the ones to use for planting.
- Dry seeds – spread the seeds out evenly on non-stick paper or aluminium foil, place them in a location that is not exposed to direct sunlight, and leave them there for a day or two until they’re completely dry. Even though it’s possible to plant the seeds directly without drying them, it’s recommended to dry the seeds first for optimal germination.
- Sow seeds – plant the seeds in shallow pots, flat seedling punnets or other suitable containers that have drainage holes at the bottom, filled with good quality potting mix (growing mix). Sow the seeds around 6-12mm (1/4-1/2 inch) deep.
- Water pots– moisten the potting mix slightly, then cover the top of the pot with a piece of cardboard, some folded newspaper, a piece of plastic wrap or something similar to reduce evaporation and prevent the seeds and the potting mix from drying out really quickly. It’s a good idea to place the pots in a tray to catch the excess water that runs from the bottom of the pots when watering, but don’t let the pots sit in water, pour out any excess.
- Locate pots – place the pot with seeds in a warm location, such as the top of a refrigerator or other barely warm appliance, until the seeds germinate.
- Monitor seeds – check the pots regularly to ensure that they don’t dry out. As the soil begins to dry, add a little water to remoisten the potting mix, but don’t overwater and make it waterlogged, as the seeds may rot.
- Provide light – The seeds won’t need any light initially. Under ideal conditions, with sufficient warmth and moisture, the seedlings (baby plants) will emerge within 2–3 weeks after planting. As soon as they sprout, emerging from the potting mix, they’ll require light to grow. Remove the cover over the pot and move the pots to a location which receives several hours of bright light each day, such as a sunny window. Direct sunlight is not necessary, but bright light is necessary.
- Transplant seedlings – once seedlings grow to an adequate height, they will need to be transplanted into larger pots, such as 10cm (4″) wide plastic pots. Only transplant the strong and healthy seedlings, and discard the weak and stunted seedlings, as they’ll probably never catch up with the others.
Also, pay special attention to any seedlings which have a different leaf morphology (leaf shape/structure) or have an unusual appearance, as they’re likely to be the ‘off-types’ produced by fertilisation we mentioned earlier, that are not true to type, and are different to the parent plant.
These off-types must be rogued – in horticulture, roguing refers to the act of identifying and removing plants with undesirable characteristics. As the citrus plants grow in size, repot as required into larger pots, and eventually train them to a single stem by pruning off any branches within 15-20cm above the top of the pot.
How to Speed Up Citrus Seed Germination
Germination of citrus seeds will be delayed if seeds are planted without drying, and at least a short period of cold storage. After drying them, leave them in the refrigerator (the vegetable crisper would work well, not the freezer!) for a day or two before planting them to speed up their germination so they sprout sooner.
There are methods to speed up germination even more, which I’ve included here simply out of interest:
- Soaking dried seeds in aerated water (air bubbled through water, such as with an aquarium air pump) maintained at 30°C (85°F) for 24 hours (using a water heater with a thermostat, such as an aquarium heater) before planting will increase total germination, while shortening germination time.
- Scarifying the seed coats of the seeds, by mixing the seeds with sharp sand in a jar and shaking vigorously for 10 to 20 minutes to damage the hard seed coat which prevents water entering the seed, delaying germination. Seed scarification can also be done by gently rubbing the seeds across a piece of sandpaper to just scratch through the hard outer seed coat. After scarification, seeds can be soaked in water for about eight hours prior to planting to reduce germination time.
How to Store Citrus Seeds
It’s not necessary to plant all the cleaned and dried citrus seeds, they can be stored for use at a later date.
Dried seeds should be placed in sealed polyethylene plastic bags and stored in a refrigerator at 4–10°C (40–45°F), where they can be stored for several months with little loss in viability. Seeds must be completely dry before storing them away to eliminate any diseases.
Why Are Citrus Trees Grafted If They Can Be Grown from Seed?
Most citrus trees can be grown from seed, but they usually won’t survive for very long because they won’t develop a healthy root system. That’s because most citrus are grown outside of their native environment, in different soils to the types they’re adapted to grow in.
To get around this problem, citrus growers have resorted to grafting, a technique for combining two plants together, a rootstock cultivar (cultivated variety) that forms the lower trunk and root system, and a scion (a shoot, twig or bud from a different fruiting cultivar) on the top, that provides the fruit.
Grafting is a very old technique that has been practiced in China before 2000 BC. The Chinese author Jia Sixie who wrote the “Qi Min Yao Shu” (Essential Techniques for the Welfare of the People) in the 6th century CE, the earliest complete Chinese agricultural treatise to have survived, discusses the grafting of persimmons, as well as the grafting pear twigs onto crab apple, jujube and pomegranate stock.
A grafted citrus tree combines the best attributes from different citrus trees, the good fruit quality from the scion, with the healthy roots from the rootstock that can tolerate pests, diseases, and challenging abiotic (non-living) environmental conditions such as temperature and water.
What Are the Benefits of Grafted Citrus Trees?
All commercially available citrus trees are produced by grafting or budding (bud grafting) to confer various benefits, such as:
- To speed up fruiting – the scion wood is a cutting that has the same genetic maturity as the parent plant, so grafted trees fruit much sooner, reducing the time from planting to the first harvest of fruit.
- To boost vigour in the citrus variety – they type of rootstock used affects yield, fruit size and quality. Although the characteristics of the fruit are primarily determined by the variety of the scion used (Washington Navel orange for example), the fruit size, color, and sweetness can also be significantly affected by the rootstock.
- To increase resistance to soil pest and diseases – the variety of rootstock used determines how well a citrus tree is able to resist or tolerate infection by diseases such as citrus tristeza virus (CTV), citrus root and collar rot (Phytophthora nicotianae) and citrus greening (Candidatus Liberibacter asiaticus) (CLas), and soil pests such as citrus nematodes.
- To improve tolerance to adverse soil conditions – the hardier citrus rootstock varieties increase tolerance and adaptability to different soil types (such as clay), as well as various abiotic stress conditions such as salinity, heavy metals, nutrient stress, water stress, and alkalinity.
- To improve cold tolerance – citrus are cold tender evergreens that are highly vulnerable to freezes and are injured by temperatures between -2.2 to -13 °C, which can cause fruit and foliage injury or even tree death. Injury is caused by subfreezing temperatures after ice forms within the tree, which usually occurs between -2.2 to -6.7 °C. Freezing temperatures, in fact, are considered the largest environmental factor limiting citrus production worldwide. The ability of citrus trees to survive freezes is often associated with the cold hardiness of the rootstock. The less cold hardy the rootstock, the more freeze damage to the scion.
- To provide greater tree longevity – a hardy citrus rootstock that can better withstand pests, diseases, less-than-ideal soil types and adverse conditions increases the chances of a trees survival and ensures its longevity for a long and highly productive life.
- To limit tree size – citrus trees vary in size depending on the variety, but most grow quite large. Grafting them onto a dwarfing rootstock, such as the ‘Flying Dragon’ variety, limits the height to around 1.5m, which also makes it suitable for planting in pots and containers.
Growing citrus from seed is only really viable if you have plenty of time (many years) to spare, otherwise a grafted citrus tree is a far better option for obtaining a crop sooner.
- University of Florida, IFAS Extension Program, Citrus Propagation, HS1309, Ute Albrecht, Mongi Zekri, and Jeffrey Williamson https://edis.ifas.ufl.edu/pdf%5CHS%5CHS130900.pdf
- NSW Department of Primary Industries, Selecting citrus rootstocks, Reference number: PUB21/121, April 2021 <https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0017/1313711/Selecting-citrus-rootstocks.pdf>
- USDA Agricultural Research Service, Citrus Rootstock https://citrusrootstocks.org/
- Meng, Chao; Xu, Dong; Son, Young-Jun & Kubota, Chieri (2012). “Simulation-based Economic Feasibility Analysis of Grafting Technology for Propagation Operation”. In Lim, G. & Herrmann, J.W. (eds.). Proceedings of the 2012 Industrial and Systems Engineering Research Conference. IIE Annual Conference. Norcross: Institute of Industrial Engineers.
- New Mexico State University, College of Agricultural, Consumer and Environmental Sciences (ACES) Extension/Outreach, Southwest Yard & Garden, 2000 Archives, Issue: April 8, 2000 – Lemon Tree from Seed https://aces.nmsu.edu/ces/yard/2000/040800.html
- University of Florida, Institute of Food and Agricultural Sciences, UF/IFAS Extension, Citrus Propagation, Ute Albrecht, Mongi Zekri, and Jeffrey Williamson, Publication #HS1309, Date: 09/04/2021 https://edis.ifas.ufl.edu/publication/hs1309