Gladiolus

Gladiolus gandavensis is the tall, funnel-flowered garden hybrid sold every spring as gladiolus or sword lily, and it is the founding cross behind the thousands of large-flowered gladiolus cultivars grown today. It exists only in cultivation: raised in Belgium in 1837 from a cross of southern African species, it has no wild population of its own. Grown from a corm rather than a true bulb, a single corm sends up one tall spike that opens its lowest flowers first and works upward over one to two weeks, then dies back to a dry corm that has to be lifted before hard frost in most gardens. This guide covers spacing out plantings for a long cutting season, keeping the top-heavy spikes upright, the thrips and corm rot problems that follow this species specifically, and lifting, curing and storing the corms and their cormels over winter.

A Ghent Hybrid With No Wild Home

The cross was raised in 1837 by H. Bedinghaus, gardener to the Duke of Aremberg at Enghien, and introduced to the trade in 1841 by the Ghent nurseryman Louis van Houtte, whose name the hybrid still carries. The epithet gandavensis comes from Gandavum, the Latin name for Ghent. GBIF records the accepted name as Gladiolus x gandavensis Van Houtte, with Gladiolus glandavensis Van Houtte and two forms of Gladiolus x sulphureus de Graaf listed as synonyms. Van Houtte himself described the parentage as Gladiolus cardinalis crossed with Gladiolus natalensis (then called psittacinus), though other accounts credit Gladiolus natalensis crossed with Gladiolus oppositiflorus; both candidate parents are native to southern Africa. Because it is a hybrid of garden origin rather than a species collected from the wild, Missouri Botanical Garden's own record lists its native range simply as "Garden origin", and older nursery catalogues sometimes filed this same line of large-flowered hybrids under the broader garden name Gladiolus x hortulanus rather than gandavensis specifically.

Succession Planting for a Long Cutting Season

A single corm produces one main spike that stays in flower for roughly one to two weeks, so a single planting date gives a short burst of bloom rather than a season of it. Growers who want cut flowers over months rather than weeks plant a new batch of corms every two weeks, starting once the soil has warmed in spring and continuing into early summer, so later batches are still coming into bud as earlier ones finish. Each batch takes about eight to ten weeks from planting to the first open flower, which means a spring-planted batch flowers by midsummer while a batch planted in early summer is still flowering as the season turns toward autumn.

Planting Depth, Spacing and Staking

Missouri Botanical Garden recommends planting corms about 4 inches (10 cm) deep and spacing them 4 to 6 inches (10 to 15 cm) apart, set out in groups of at least five to seven rather than single spaced plants, for a fuller display. The RHS gives a similar depth for large-flowered hybrids, roughly the height of three corms stacked, deeper for large corms and shallower for small ones, and recommends a sunny, open spot sheltered from strong wind. Planting deep gives the developing corm and roots better anchorage, but a tall, top-heavy spike in an exposed bed can still fall over in wind or rain, so taller stems are usually tied to a cane at intervals as they grow, using a soft tie positioned to avoid piercing the corm below.

Full Sun and Well-Drained, Fertile Soil

Missouri Botanical Garden's culture notes call for average, medium-moisture, well-drained soil in full sun, with tolerance for some afternoon part shade; the plant does best in light, organically rich, well-drained soil and performs well in sandy loam. Keep the soil evenly moist through active growth, particularly as the flower spike is forming, since a corm sitting in waterlogged, poorly drained ground is far more prone to the rots covered below than one that simply dries out between waterings.

Thrips, the Pest Behind Streaked Flowers

Gladiolus thrips are the pest growers of this species run into most often, and they are active on both the growing plant and the stored corm. The RHS describes the first symptoms as white flecks on the leaves and flowers, with heavier infestations causing brown discoloration on the petals, buds that fail to open properly, and rough, grey-brown patches on the surface of the corm itself. North Carolina State Extension notes that a single female can lay 100 to 200 eggs over a five- to six-week life, that a generation completes in two to four weeks, and that nine or more generations can occur in a single year, which is why populations build quickly through summer. The thrips overwinter concealed on stored corms, and the RHS notes they can keep reproducing there as long as storage stays above about 10°C (50°F). Reducing the top growth before the corm is fully dried at lifting time removes some of the overwintering population, and North Carolina State Extension describes cleaning corms and soaking them in a mild bleach solution, about 4 teaspoons per gallon of water for three hours, before drying and boxing them for storage.

Corm Rot and Other Storage Diseases

Besides thrips, Missouri Botanical Garden lists aphids and spider mites as occasional pests, and corm rot, mosaic virus, gray mold and rust as diseases that can turn up on this hybrid. Most of these losses trace back to how the corms were handled at lifting and in storage rather than to anything happening in the border during summer: any corm that looks soft, discolored or damaged should be discarded rather than stored, since rot spreads from one corm to its neighbors in a closed storage container. Where these diseases have been a recurring problem, dusting the cleaned, dry corms with a general garden fungicide immediately before storage is the standard precaution.

Lifting, Curing and Storing Corms and Cormels

Missouri Botanical Garden's general procedure is to dig the corms after the foliage yellows and before the first significant fall frost, cut the stems and leaves off close to the corm, separate the small cormels that have formed at its base, dry the corms and cormels in a single layer for curing, discard anything diseased or damaged, and store the rest through winter in a dry medium in a cool, frost-free place. Missouri Botanical Garden rates the species hardy in USDA zones 5 to 9 and notes that plants can be left in the ground over winter in a protected site with a mulch of hay, straw or evergreen boughs, but adds that many growers lift the corms every autumn regardless of zone, since it tends to give better flowering the following year than corms left in place. The small cormels, separated at lifting, can be grown on in their own nursery row the following spring; they take about one to two years to build up to a corm large enough to flower.

Toxic to Cats, Dogs and Horses

The ASPCA lists Gladiolus as toxic to cats, dogs and horses. The toxic principle has not been identified, and reported clinical signs from chewing or ingesting the plant include salivation, vomiting, drooling, lethargy and diarrhea. The ASPCA notes that the corm carries the highest concentration of the toxic compound, which makes stored or freshly dug corms the highest-risk part of the plant for a curious cat or dog, well above the cut flowers in a vase indoors.