I have written this to help people understand and recognize the differences between the species of European Gladiolus most commonly cultivated and seen in their natural habitat. All my considerations are based on Flora d'Italia (1982) by Prof. S. Pignatti, which is the reference on the Italian flora used in all universities in Italy. We are awaiting a new edition of this book which is close to publication. You might not know that in Italy there are eight of the so-called Eurasian Gladiolus species, which are rather similar in appearance, all having a more or less pink-magenta color. Without close observation of some details it is easy to confuse them. In Southern Italy there are five species: Gladiolus italicus (which is present throughout Italy), Gladiolus byzantinus, Gladiolus communis, Gladiolus dubius, Gladiolus inarimensis. All the taxonomic keys are based on features that are often very difficult to check, for example, length of filaments compared to anthers, size and shape of seeds, shape of tunics of the corm. The observer can't have all the keys present at the same time; that is, if you are checking length of filaments on a fresh flower, you can't check the seeds at the same time. So for positive identification one needs to wait a whole year in order to check them all.
I will start with the lesser-known species and say just a few words about them.
Gladiolus dubius as the specific name indicates, is in fact a doubtful species. The Kew List of Monocots reports a Gladiolus × dubius Guss., Fl. Sicul. Prodr., Suppl.: 8 (1832) as a hybrid between Gladiolus communis and Gladiolus illyricus. This is quite unlikely because Gladiolus illyricus is found in Italy only in the foothills of the Eastern Alps in Friuli near the Slovenia border, while Gussone (1787-1866) described the species from samples collected in Sicily, which is more than 1,000 kilometers (about 621 miles) away. Apart from the geographic distance, Pignatti reports valid taxonomic keys, so while Gladiolus illyricus is surely a valid species, the Gladiolus dubius described by Gussone could even be an ecotype of Gladiolus communis/byzantinus or an aberrant form, as well as a genuine species. I don't have any material, so I can't compare it and make any conclusion.
Another lesser-known species, Gladiolus inarimensis , is endemic to Sardinia and some small islands in the Thyrrenian Sea (Ischia, Capri, Pianosa). Although Kew lists it as synonymous with Gladiolus italicus there are at least two taxonomic keys to separate these species, one of them being seeds that are not winged. Because of the narrow habitat, it's quite possible it has never been introduced into cultivation. I have wild-collected samples still to check, so I will skip further consideration of this species.
Our main discussion concerns the great confusion that exists around Gladiolus byzantinus and Gladiolus communis. As these species are rather showy, they have been in cultivation for a long time in Europe and worldwide, but not by the Italians, who often ignore their presence as elements of our native flora. The species names byzantinus and communis are often given as synonyms and you often see Gladiolus byzantinus ssp. communis, Gladiolus communis ssp byzantinus, and Gladiolus communis var. byzantinus- a true nightmare!
In his Flora d'Italia Prof. S. Pignatti treats them as separate species and as he is a well-known "lumper" I have always thought if he mentions them as true separate species, he's probably very sure of this. I have always tried to understand these confusing species, checking all material available, both from my collections in habitat, and from commercial sources.
The first consideration is the difference between Gladiolus italicus and Gladiolus byzantinus/communis. This is quite straightforward: observing the flowers makes it rather easy and looking at seeds and corms makes it absolutely certain. Gladiolus italicus is the only species that doesn't make winged seeds; they are like small pepper grains with three to four angles. Its flowers are easy to tell apart from those of the other two species as they have an "open" shape, the lateral tepals are oblong, narrow and well spaced, not overlapping the upper tepal and the two lower ones. This species is highly bulbilliferous, it increases by cormlets at an alarming rate and a single corm will turn in a clump in a few years. This is the reason that all vendors selling cheap Gladiolus byzantinus are actually selling the more prolific Gladiolus italicus.
About the color there's more to say. Many claim that Gladiolus byzantinus is a rich dark purple while Gladiolus italicus is a pale, dull copy, but actually that isn't so. I have seen some photos of Gladiolus italicus that show really ugly forms, pale and small, almost deformed, and I wonder who introduced them into cultivation, seeing that in nature there are many nice, strong, colorful plants.
Gladiolus italicus grows mainly in corn fields and it's not known to grow wild naturally, being always connected with human presence. We speculate it came into Italy with seeds (durum wheat that we grow in Apulia) in ancient times, probably from the Middle East. It has a reproductive cycle shorter than wheat so it doesn't resent harvesting and subsequent deep plowing, which actually helps it to spread better. It is considered a pest in agriculture ("Cyperaceae, Iridaceae and Commelinaceae as weeds," by M. Colasante, Gladiolus Corazzi, and R. Mortellaro, 5th International Conference, Ecology of Invasive Alien Plants, 13-16 October 1999, La Maddalena, Sardinia, Italy).
A white form is in cultivation, wandering around as a white Gladiolus byzantinus that we should call forma alba, which would be really a gem if it existed. Although I find references in some Italian gardening literature, again as white byzantinus, it seems that no one has ever seen a white Gladiolus italicus and, to be honest, I have never found any, having seen thousands of plants in their native habitat.
More subtle is the difference between Gladiolus communis and Gladiolus byzantinus. Both species grow only in southern Italy, the latter being subendemic. Their habitats are arid lawns, garigues (low open scrubland found in poor or dry soil in the Mediterranean region) and maquis (scrubland composed primarily of leathery, often aromatic broad-leaved evergreen shrubs or small trees).
In spite of the specific name Gladiolus communis, this species is not at all common. I rate it as quite rare, difficult to meet if you don't go to the right habitat. Both species don't increase by cormlets and are solitary in habitat; it's just in cultivation that they make offsets thanks to good soil and feedings. They both make a ring of very small cormlets at the base of the new corm, but they stay dormant and entangled in the older tunics unless the main corm is damaged or dug by wild pigs, mice or porcupines.
The flowers of both species are very similar, so close examination is required to tell them apart, but once you understand the key this will be easy. They have larger flowers than Gladiolus italicus and plants are taller. Gladiolus italicus averages 20 to 23 inches (50-60 cm), while Gladiolus communis and Gladiolus byzantinus grow up to 40 inches (100 cm). The main way to tell them apart is the by the shape and size of the three lower tepals. In Gladiolus communis they are more or less of the same length and width, while in Gladiolus byzantinus the central one is visibly longer and even wider. The upper central tepal is hooded in Gladiolus communis, but it points up in Gladiolus byzantinus. The median tepals have a "spoon" shape in the former and "rhombus" shape in the latter. Color is also a bit different: Gladiolus communis often has a slight two-tone effect on the lower tepals, but this is not a reliable feature, as many individuals are a solid color, while Gladiolus byzantinus is usually a dark rich purple. I see a strain or form called "cruentu" in the United States, but I am not aware of any reference or the origin of it and, in any case, it looks to me as just a true good form of byzantinus. Plants of Gladiolus byzantinus have broader leaves and are usually more robust, while plans of Gladiolus communis are usually slimmer and taller, hence the common name of "greater gladiolus."
The other true difference is the seed shape and size. Both species have winged seeds, but Gladiolus byzantinus has large seeds of about 1 cm, while Gladiolus communis seeds are 3 to 5 mm.
Even the corms have visible differences in fibers. Gladiolus italicus has a peculiar net on the tunics, especially on the top of the corm, and a rather light brown color. It's especially easy to pick out even in a batch of corms of mixed species. The other two species are more similar, but Gladiolus communis has tunics with very close parallel veins, while Gladiolus byzantinus has tunics that dissolve (separate) at the apex with parallel but much wider veins and this is a another valid key.
I grow all the species in the same place and they have distinct and well-defined flowering times. Gladiolus italicus is the earliest (late March), followed by Gladiolus byzantinus (late April) and finally Gladiolus communis (late May, but in habitat mid-June, as I always find it at 1,600 feet, or 500 meters, and higher).