A hybrid plant is the offspring of two organisms from different biological taxa (e.g. different genera, species, subspecies, etc.). Some hybrids are sterile and lack the ability to produce viable seed.

Naturally occurring hybrid species are found throughout the world and have the potential to exist wherever the ranges of the two parent species overlap. 

Artificial hybrids are crosses that originally occurred in cultivation. These can be created intentionally via hand pollination, tissue culture, etc. or occur entirely by chance as the result of cross-pollination by animals, the wind, etc.

Hybridization is largely important to horticulture and agriculture as a means to create and manipulate the traits of both ornamentals and food crops. For example, prior to the late 1700s, the garden strawberry as we know it did not exist; humans only cultivated wild strawberry species (e.g. Fragaria vesca, F. virginiana, F. chiloensis) which bore smaller, less abundant fruits. It wasn't until 1776 that imported strawberry species (F. virginiana and F. chiloensis), grown side by side in a French garden produced the hybrid species, Fragaria ×ananassa (the modern garden strawberry). Since then, it has been continuously bred and developed into numerous cultivars with longer-fruiting periods, larger fruits, sweeter fruits, etc.

The International Code of Nomenclature for algae, fungi, and plants (1)  provides guidelines on how hybrid formulas are written:

Article H.1 . Indication of Hybrids

H.1.1. Hybridity is indicated by use of the multiplication sign × or by addition of the prefix “notho-”1 to the term denoting the rank of the taxon.

Article H.2 . Hybrid Formulae

Article H.2 - H.2.1. A hybrid between named taxa may be indicated by placing the multiplication sign × between the names of the taxa; the whole expression is then called a hybrid formula. 

Recommendation H.2A - H.2A.1. It is usually preferable to place the names or epithets in a formula in alphabetical order. The direction of a cross may be indicated by including the gender-denoting symbols (♀: female; ♂: male) in the formula, or by placing the female parent first. If a non-alphabetical sequence is used, its basis should be clearly indicated.

Article H.3 . Nothotaxa

H.3A.1. In named hybrids, the multiplication sign × belongs with the name or epithet but is not actually part of it, and its placement should reflect that relation. The exact amount of space, if any, between the multiplication sign and the initial letter of the name or epithet should depend on what best serves readability.

Note 1. The multiplication sign × in a hybrid formula is always placed between, and separate from, the names of the parents.

H.3A.2. If the multiplication sign × is not available it should be approximated by the lower-case letter “x” (not italicized). (1) 

Hybrid formulae:

A hybrid Origanum plant whose parents were O. laevigatum and O. rotundifolium would read as follows, (the formula does not specify the male and female parents):

Origanum laevigatum Boiss. × Origanum rotundifolium Boiss.

OR

Origanum laevigatum Boiss. × O. rotundifolium Boiss.

In scientific literature, etc. the name of the author who first published/described the species is also included after the species name as additional verification. In horticulture, this practice is not widely used or very practical and generally reads as follows:

Origanum laevigatum × Origanum rotundifolium 

OR

Origanum laevigatum × O. rotundifolium

Named Hybrids:

Many naturally occurring hybrid species (and some artificial hybrids) have been assigned their own epithet. These are referred to as named hybrids.

Examples:

Cistus laurifolius × Cistus parviflorus = Cistus ×pagei 

Cistus monspeliensis × Cistus salviifolius = Cistus ×florentinus

Lavandula angustifolia × Lavandula lanata = Lavandula ×chaytoriae

Lavandula angustifolia × Lavandula latifolia = Lavandula ×intermedia

1. Turland, N. J., Wiersema, J. H., Barrie, F. R., Greuter, W., Hawksworth, D. L., Herendeen, P. S., Knapp, S., Kusber, W.-H., Li, D.-Z., Marhold, K., May, T. W., McNeill, J., Monro, A. M., Prado, J., Price, M. J. & Smith, G. F. (eds.) 2018: International Code of Nomenclature for algae, fungi, and plants (Shenzhen Code) adopted by the Nineteenth International Botanical Congress Shenzhen, China, July 2017. Regnum Vegetabile 159. Glashütten: Koeltz Botanical Books. DOI https://doi.org/10.12705/Code.2018

"A cultivar, as a taxon, is an assemblage of plants that (a) has been selected for a particular character or combination of characters, and (b) remains distinct, uniform, and stable in these characters when propagated by appropriate means" (Article 2.3, Cultivated Plant Code, Ninth Edition, 2016)

There are two primary types of cultivars in the nursery trade 

• Cultivars that are propagated clonally via stem cuttings, root division, tissue culture, etc. This method ensures genetic duplicates of the original plant so that growers and gardeners know exactly what to expect when it comes to growth, flower color, etc. This method is common for just about any plant that can be cloned relatively easily and has been used by humans, in some manner, for over 4,000 years. It is important to note that any seedlings of a clonal cultivar no longer represent that cultivar and should be assigned a different name to avoid confusion in the nursery trade, sadly this is not always the case. 

• Cultivars that are propagated by seeds "may form a cultivar when it meets the criteria laid down in Art. 2.3 [above] and when it can be distinguished consistently by one or more characters even though the individual plants of the assemblage may not necessarily be genetically uniform." Seed cultivars are most common for plants such as vegetables, basils, milkweeds, and anything else that is difficult or impossible to propagate clonally. Unfortunately, many cultivars that were originally propagated clonally are now, or at some point, have been propagated by seed and were never renamed to indicate such; this has caused much confusion in the nursery trade.

Not all plants in cultivation have cultivar epithets (names). For example, many plants such chives (Allium schoenoprasum) and butterfly weed (Asclepias tuberosa) are often seed-grown and genetic variances between plants are mostly negligible in terms of their overall use, be it culinary or ornamental. That's not to say that there can't be cultivars of chives or butterfly weed if certain desirable traits (e.g. a white-flowered chive) are found among them. Just about any species of cultivated plant could, theoretically, have a cultivar.

A cultivar name is designated by single quotation marks, and is normally preceded by the genus and species of the plant. For example:

• Lavandula angustifolia 'Twickel Purple'

• Salvia officinalis 'Tricolor'

If the plant is of hybrid origin, cultivar names are preceded by the hybrid species. For example:

• Lavandula ×chaytoriae 'Ana Luisa'

• Symphytum ×uplandicum 'Bocking 14'

In some instances, the species of a particular cultivar is unknown, disputed, or of unnamed hybrid origin. In such cases only the genus name precedes the cultivar name. For example:

• Salvia 'Christine Yeo'

• Sisyrinchium 'Devon Skies'

Cultivars are generally introduced by reputable plant breeders, nurseries, institutions, etc. The term "introduction" or "introduced" indicates that a cultivar has been marketed and published as such in a nursery catalog, website, or something of that kind. One way to think of it is that the cultivar "has been introduced to the plant trade" (and is accessible to gardeners and growers alike). Theoretically, anybody could introduce a new cultivar but if there is no way for them to market it then it will probably not be around very long.

Before an introduction can occur, a cultivar needs to be selected. There are several common ways this is done. For example (but not limited to):

• A batch of seedlings is sown and raised by a grower. Lo and behold, one of them is truly remarkable and different from the rest. This seedling would then be evaluated (hopefully) on performance and other metrics. 

• In a garden setting, a "chance seedling" (a plant that was not planted intentionally) occurs and it seems to be unique from everything else around it. Sometimes the parent plants can be determined and sometimes not. The plant could be a hybrid species, combining the favorable traits of two nearby plants, or it could be the result of two of the same species breeding. In 40 years, our nursery and gardens have had thousands of chance seedlings occur and we've introduced only about 10 that we decided were truly remarkable after selecting and evaluating them.

• A branch mutation occurs on a plant and it becomes easily distinguishable from the rest. This is how many variegated cultivars originate as well as unusually-colored flowers. These are generally worth evaluating and will for the most part mirror the growth habits of the original plant. Not all of them are stable and oftentimes the new clones will revert (partially or entirely) back to the original one. This is why you will sometimes see partially variegated plants or plants with two simultaneous flower colors. Needless to say, branch mutations should be evaluated before introducing them as a new cultivar.

Whatever the means of selection, a cultivar should be rigorously evaluated before being introduced into the wider trade. Many selections are, in fact, never introduced into the trade after being evaluated.

Unlike introducing a cultivar, it is relatively easy for just about anyone to select a plant in any of the above examples. It is quite common that cultivar selections are made (and sometimes named) by gardeners and then introduced via a reputable nursery on their behalf. This also contributes to a fair amount of cultivars being named after people (hopefully the one who selected it or someone of their choosing). For example:

• Plant enthusiast, Neil Bell, found a chance seedling of a grevillea in his garden that he was quite impressed with and thought that it should be shared. Lacking the means to distribute it himself, he then gifted stem cuttings of that plant to Xera Plants Inc. of Portland, OR. The fine folks at Xera, who know their grevilleas very well, evaluated it and agreed that it was a remarkable plant. With Neil's permission they named it after him (Grevillea 'Neil Bell') and introduced it into the trade. 

There are several other ways to select a cultivar via large breeding programs and chromosome manipulation, etc. but that is a bit beyond our scope. 

Coming soon.

The term "ex" is generally used in two ways in horticulture:

1) to indicate that a plant is a descendant of a specific cultivar but not genetically identical or "true".  For example, Gladiolus ex 'Ruby' signifies that the plant is a seed-grown descendant of the original cloned cultivar Gladiolus 'Ruby'. Though it may share many of the same traits as the original, a seed-grown descendant may differ in a number of ways and should not be sold as the true cultivar (except in the cases of seed-grown cultivars). 

2) to indicate the origin of a wild-collected plant. For example, Salvia rosmarinus ex Crete is an unnamed cultivar of a rosemary collected in Crete.

This should not be confused with an x or × in front of a cultivar name which used to indicate hybrid origin.

A trade name (trade designation) is a device used for marketing a specific cultivar that is (and must be) different than the cultivar name. Sometimes cultivar names are named for specific people but also have a catchier trade name to better sell a plant. For example:

• Lavandula angustifolia TWO SEASONS 'Irene Doyle'

In the above example, the cultivar 'Irene Doyle' was named for the breeder's mother-in-law but for better marketability it was sold under the trade designation of TWO SEASONS to highlight that it bloomed in early summer and again in autumn, hypothetically increasing its marketability.

Trade names are generally written in capitals to avoid confusion with cultivar names. "When citing a trade designation together with an accepted name, whilst it is preferable to list the accepted name first, the order in which they are quoted may depend on the circumstances; it is recognized that users of a name may be more familiar with the trade designation. Thus Dianthus ‘Londaison’ FANTASIA or Dianthus FANTASIA ‘Londaison’ can both be used. Placing the second element within brackets is another accepted style of presentation e.g. Dianthus ‘Londaison’ (FANTASIA) or Dianthus FANTASIA (‘Londaison’)" (Appendix X, Cultivated Plant Code, Ninth Edition, 2016

Recently, it has become popular among some organizations to register a trademark on part or all of a plant's trade name so that nobody else can use that name unless they purchase a license to do so. In this way, the owner of the registered trademark owns the rights to the trademarked portion of the name but not to the plant itself. For example:

• RED MOUNTAIN® FLAME ICE PLANT is a marketing name for the cultivar Delosperma 'PWWG02S'. It can be alternatively written as Delosperma RED MOUNTAIN® FLAME ('PWWG02S'). In this case, RED MOUNTAIN® is a registered trademark and FLAME ICE PLANT is not; the cultivar name, 'PWWG02S' is, intentionally, not very catchy on its own. 

• A grower can only use a trademarked trade name if given license to do so. However, it is legitimate to sell a non-patented plant under another, non-trademarked name without penalty. In the above example, one could sell this as Flame Ice Plant or simply as Ice Plant, but the botanical name and cultivar should always be listed on the label as well (e.g. Delosperma 'PWWG02S'). 

• As a consumer, it is important to remember that various marketing names could refer to the exact same plant cultivar. The cultivar name, always written in single quotes, is the best resource for growers and consumers to identify a plant.

Subspecies, variations, and forms are all taxons that are ranked below the species level.

subsp. The rank of subspecies (subsp. or ssp.) applies to populations of plants of the same species that differ in morphology (size, shape, etc.) and do not interbreed in the wild due to either geographic isolation or due to morphologically different reproductive systems. However, it is possible for some subspecies to interbreed in cultivation (especially those that are geographically isolated).

var. The rank of variety (var.) is below the rank of subspecies and describes morphological variations that occur throughout the geographic range of a species. Unlike subspecies, different varieties of a species will interbreed freely in the wild. In legislation, marketing, etc.  the term variety is often used interchangeably with the term cultivar and this is botanically incorrect; cultivars are plants that are intentionally bred to have specific traits or combinations of traits.

f. The rank of form (f. or forma) is below the rank of variety and describes noticeable morphological differences of groups within a species. Forms can be found throughout the range of a species and, unlike varieties, don't necessarily need to be closely related to one another. This is most commonly used for flower colors that are uncommonly found within a species. For example, f. alba or f. leucantha usually indicates a white-flowered form whereas f. rosea would indicate a pink-flowered form of a plant.