Lifting and Storing Gladiolus Corms Over Winter

Decide whether you are lifting at all

Gladiolus corms survive winter in the ground where the soil does not freeze deeply and does not stay saturated, roughly in climates with mild winters and reasonable drainage. In colder regions they are killed either by frost reaching the corm or, more often, by sitting in cold wet soil for months and rotting. If your winter soil regularly freezes below the planting depth, or if the bed is heavy and wet through winter, lift them.

The middle ground is worth naming. Where winters are only marginally cold, corms left in the ground often survive but come up late, unevenly and smaller each year, because the corm is not properly ripened and loses condition through a wet dormancy. Lifting even in these conditions usually gives a visibly better display, and it also gives you the chance to sort the corms and separate the offsets, which is the only way a planting increases rather than slowly declining.

Left in the ground, mulch heavily with six inches (15 cm) of straw or leaves after the foliage dies down and pull the mulch off in spring once growth starts. Lifted, the sequence below applies.

Lift after the leaves yellow, not before

The corm you will store is built after the flowers finish, and it is built by the leaves. Wait until the foliage has yellowed naturally, which is usually four to six weeks after the last flower, or until the first light frost has blackened the tops. A light frost on the foliage does no harm to the corm at that depth and is a convenient signal. A hard freeze that reaches the corm is another matter, so do not leave lifting until the ground is starting to freeze.

Lift on a dry day. Push a fork in well outside the plant, six inches (15 cm) or more from the stem, and lever the whole clump up rather than pulling on the stem. Under each plant you will find the new corm sitting on top of the shriveled remains of the one you planted, with a cluster of small offsets, called cormels, around the base. Sift the loosened soil for those; they are the free stock for future seasons.

Cut the stem off to about half an inch (1.2 cm) above the corm straight away. A long stub is where thrips shelter and where rot starts in storage, and it comes away easily at lifting while it is still green.

Curing is the step that decides everything

Curing means drying the corms fast and warm for two to three weeks immediately after lifting, and it is the step most often skipped or done half-heartedly. Spread them in a single layer in shallow trays or on mesh, somewhere warm, airy and out of direct sun, at 75-85F (24-29C) if you can manage it. Good airflow underneath matters as much as the temperature, which is why mesh trays or slatted boxes beat solid ones.

A properly cured corm has a dry papery tunic, a hard firm body and, at the base, the old spent corm dried to a flat disc that snaps off cleanly with thumb pressure. If the old corm still holds moisture and tears rather than snapping, the batch is not cured and needs another week. Storing an uncured corm is how a whole box goes moldy.

Do not wash the corms. Water at this stage is exactly what curing is removing, and washed corms are far more likely to rot in the box. Rub the dry soil off with your fingers after curing instead.

1. Cure warm and airy for two to three weeks Single layer, mesh or slatted trays, 75-85F (24-29C), out of sun and rain. A garage, shed or spare room with a fan running works. Cold or damp curing is not curing.
2. Snap off the old corm and the roots When cured, the shriveled parent corm separates from the base of the new one with a clean break, taking the dried roots with it. Discard it. If it will not separate, cure for longer rather than forcing it.
3. Separate and keep the cormels Rub the cluster of small cormels off the base and store them in a separate labeled bag. Planted shallow in a nursery row in spring, they reach flowering size in one to two seasons and are how a planting grows without buying more.
4. Sort by size and discard the failures Keep firm corms over an inch (2.5 cm) across for the main display and bag smaller ones separately. Throw away anything soft, hollow, moldy, or showing the sunken brown scabby patches of rot, and anything with the sticky brown surface residue that means thrips.
5. Store just above freezing, dry and dark Paper bags, mesh sacks or open trays at 35-45F (2-7C) with air movement. An unheated but frost-free space is ideal. Sealed plastic traps moisture and rots the batch; a warm room breaks dormancy early and produces long pale shoots by spring.

Corms carry gladiolus thrips through the winter, and warm storage lets the thrips continue breeding on the stored corms until spring. Cold storage below about 45F (7C) stops them, which is the second and less obvious reason the storage temperature matters.

Checking through the winter and recording the result

Look through the stored corms once or twice over the winter. Discard anything that has gone soft, developed blue-green mold, or shows the fine silvery-brown sticky patches on the surface that indicate thrips feeding. One rotting corm in a closed bag will take several with it, and thrips left undisturbed will infest the whole box by spring.

Write down two numbers at lifting and keep them with the planting notes: how many corms you planted in spring, and how many flowering-size corms you lifted in autumn. Add the count of cormels saved. A season that returns more good corms than it started is one you can run indefinitely; a season that returns fewer, or corms visibly smaller than those you planted, means the spikes were cut with too much foliage, the feeding stopped too early after flowering, or the plants were lifted before the leaves had finished. Each of those has a different fix and only the record tells you which applies.