Gladioli come in a wide range of sizes from the miniatures with florets of less than two and half inches in diameter, the graceful primulinus hybrids with the hooded florets, the small flowered, the versatile medium flowered through to the large and giant flowered types with florets up to six and a half inches in diameter and spikes growing up to six feet tall. Colourwise, gladioli are now available in the purest white, the brightest green, the palest yellow to brilliant orange, the lightest pink to blackest red, the palest blue to deepest purple and even light tans, dark browns and greys. Many of these cultivars also have contrasting coloured blotches, throats, stripes and picotee edges. Summer flowering gladioli originate from corms, a ready made food store, and to be successful it is best to begin with the best quality stocks. Large flat corms with a large root scar should be avoided. Look for corms which have small root scars and are plump and high crowned, a diameter of 12cm-16cm is ideal.
Never plant dormant corms, always look for the evidence of swelling of the root nodules around the basal scar. The fact that the corm may have developed shoots from the top of the corm should not be taken as a sign that they are ready to plant, always look at the root scar. Dormancy can usually be broken by holding the corms at a temperature of 70-75 °F for a few days. Planting should never commence until deciduous trees in the locality are well into leaf unless protection can be applied to the soil. Prepare the soil in advance by digging well to a depth of eight to ten inches incorporating well rotted organic matter. It is crucial that the material is well rotted as too high a Nitrogen content may cause problems later on in the plant's development. Gladioli appreciate well drained soil but can be grown in heavier soils if planted within a pocket of sharp sand or gravel-this will assist the drainage. Corms can be planted in trenches or individual holes and should be planted around six inches deep, a little shallower in heavier soils. The spacing will depend on final use i.e exhibition or cut flower, size and type. Although spacing depends on type, a minimum of four inches between each corm is a good starting point. Remember gladioli are gross feeders so for exhibition purposes allow a greater spacing.
Depending on soil temperature and weather conditions, some two or three weeks after planting the corms, check for signs of emerging shoots. At this stage it is important that the shoots are not impeded as they break through the soil surface. Ensure that stones, clods of soil or caked soil as a result of excess drying cannot hinder growth. Throughout the whole growing season ensure that plants do not suffer stress as a result of lack of water, nutrients or oxygen. Regular shallow hoeing around the plants will reduce weed growth and allow oxygen to penetrate the soil. From the emergence of the first shoots a sharp look out should be kept for pests and diseases and it may be advisable to begin a preventative spraying programme from the second leaf stage onwards. Virus diseases can also be damaging to gladiolus and these are usually quite difficult to detect in early growth, therefore plants that develop yellowing or mottling of the foliage or which grow in a curve rather than upright should be removed and destroyed by burning as soon as possible. When the plants have made five or six true leaves, thickening of the foliage sheath at soil level is an indication that the embryo flowerhead has formed, and during the next few weeks will appear from between the central leaves. From this stage onwards the flowerhead elongates very rapidly and taller growing cultivars will require some method of support. Bamboo canes with string or cord ties are the easiest method of support, however ties should not be too tightly placed as this will result in the still elongating flowerhead crooking. Always ensure the stakes are inserted away from the base of the plant to avoid damaging the corm below ground level.
Large gladiolus corms have a lot of food stored within them but will require additional feeding throughout the season. This feeding can take either the form of chemical feeds or more natural organic fertilisers, in either case a balanced feed given at regular intervals should suffice with a higher potash feed being applied as the flower spike develops. This will strengthen the spike and enhance the flower colour.
As important as soil nutrients to gladioli is water and to perform at their best they must never suffer from the lack of it. To produce a good flower spike gladioli need around an inch of water per week throughout the season, until flowers are cut. For all types of gladioli water, especially during the formation of the stem and flowerhead is all-important, as lack of it at this stage will result in shorter flowerheads,smaller florets and poor opening and holding qualities. Many growers believe that in well drained areas it is impossible to over-water gladioli.
If the daughter corm is not to be spoilt for future growth, flowers should be cut with a minimum of damage to the foliage leaves. This may often cause problems if a long stem is required for exhibition spikes, however a minimum of four leaves will secure the daughter corm for a further year. These leaves will be capable of taking in food and water thus maturing the new corm underground.
Corms should be lifted around four to six weeks after flowering, earlier if browning of the leaves has begun. After lifting, a batch of corms with its identification label should be placed in a ventilated tray for drying, but only after the leaves have been removed to within an inch of the top of the corm. If the corms were planted in a pocket of sand or gravel they should be relatively clean, if planted into bare soil they may need washing before drying. Corms should be placed in a warm room with good air flow, around 70°F for two to three weeks to allow curing to take place, this time will vary with corm size etc. When fully cured (dry) the old corms can be twisted off to leave a clean basal scar, if the separation is not clean or the corms look diseased they should be discarded. As removal of husks encourages loss of moisture, only a dirty outer layer should be removed. Cormlets- the small offsets around the new corm can also be removed and dried for putting into store and will be used to build up stocks of a particular cultivar for future years.
After completion of the curing and cleaning process corms should be stored root scar downwards in shallow trays in a cool room with good air circulation. Periodically during storage corms should be examined for signs of fungal disease or rot. Any suspect corms should be disposed of.