Tag: Iris Family (Iridaceae)

Adding bold swaths of color and elegant, fanned foliage to the late season garden, gladiolus (Gladiolus spp.) are beloved for their showy, ruffled flowers held on upright stems.

The tall spikes of fluttering petals bloom in saturated shades of burgundy, coral, mauve, pink, purple, rose, white, and yellow as well as bicolor combos, adding dramatic appeal to beds, borders, and cutting gardens.

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They can be grown as annuals or perennials, depending on your region, and are generally pretty low maintenance, but if you have problems with your gladiolus not blooming, it’s usually a result of a few common growing mistakes.

In our guide to growing gladiolus, we discuss how to cultivate these flowers in your landscape.

In this article, we’ll unearth the reasons why your gladiolus may fail to bloom and suggest some easy fixes.

Here’s a preview of what’s ahead:

Gladiolus, or “glads,” as they are sometimes known, grow from corms, which are underground storage organs similar to bulbs.

These frost-tender plants can be grown as perennials in USDA Hardiness Zones 8 to 11, although there are some varieties suitable for colder regions.

Outside of their hardiness range, they grow as spring-planted annuals, blooming about two to three months after planting. Then after they’ve finished flowering the corms are lifted in autumn and stored for the winter.

For the most exuberant floral displays, the corms should be grown in a full sun location with humus- and nutrient-rich soil that’s well draining. And protection from the wind is also beneficial.

They can grow in light shade but you can expect their flowering performance to be somewhat diminished.

To promote multiple large blooms, enrich the planting site with plenty of organic matter such as compost, well-rotted manure, or worm castings.

And mix in a teaspoon of bone meal for each corm to promote strong, healthy root growth and vigorous bud set.

Corms typically produce one to three flower stalks per season and deadheading does not encourage more blooms – although you may want to remove the spent stalks to improve appearances.

Always leave the foliage in place and don’t be tempted to cut it back – the plant requires the leaves to produce energy for the corms to develop next year’s buds.

If your plants aren’t performing the way you’d like them to, here are the most common issues and how to fix them.

1. Corm Age and Size

The size and age of the corm will determine how abundantly it flowers – if it blooms at all.

Corms that have matured – and flowered – for a few years produce new corms that piggyback on top of the old one, eventually replacing the parent which shrivels up and dies.

Along with the new main corm, tiny baby corms, called cormels or cormlets, are produced at the same time, emerging from the seam between the old and new sections.

First year corms are smaller than the parent and typically won’t bloom – they only produce flowers in their second year of growth, and cormlets can take up to three years to bloom.

If you’re dividing an existing clump, make sure you replant only the largest corms – these are the ones that produce the most buds.

First year corms and cormlets can be planted up in pots then placed in an out of the way spot to mature for a year or two before planting out.

When you purchase commercial stock, choose only the largest and healthiest corms for the most abundant blooms.

2. Corm Planting Depth

With any spring or summer flowering bulbs, one of the most common reasons for seeing plenty of foliage but few flowers comes from planting them too deeply.

Gladiolus corms follow the bulb-planting rule of thumb: plant to a depth that’s about twice the height of the corm.

For large corms, this can be four to six inches deep. Medium sized ones are planted three to four inches deep, and small ones just two to three inches deep.

Gardeners often plant bulbs and corms deeper than is recommended to protect them from winter’s chill.

If you have perennial clumps and worry about damage from freezing temperatures or hard frosts, adding a winter mulch is a better option than burying them too deep.

In mid-autumn, layer on two to four inches of bark mulch, shredded leaves, or straw then remove it in spring.

Your corms will stay safely snug and produce more flowers than when they’re planted too deep.

3. Fertilizing Needs

For a profusion of blooms, gladiolus plants need regular fertilizing and do best with a bloom-boosting feed.

As mentioned, start them out strong by amending the planting site with a shovelful of organic material and mixing in some bone meal when you put them in the ground.

Once the second leaf emerges, feed with a product that has an NPK ratio a bit higher in phosphorus than nitrogen, such as 5-10-10 or 5-10-5. Nitrogen is the first number (N), phosphorus is the second number (P), and potassium is the third (K).

Use slow release pellets scratched into the soil around the plants in early spring and reapply in midsummer.

Alternatively you can use a liquid or water-soluble fertilizer instead, but this should be applied monthly throughout the growing season to promote healthy growth and plenty of blooms.

4. Overcrowding Issues

Another frequent reason for declining flower numbers comes from overcrowding. This is most often an issue with perennial clumps but it can also impact annual plantings.

In the case of perennials, overcrowding happens as the corms mature.

With each growing season, the bulbs increase in size. And at the same time, they’re producing new corms and cormlets – and all are vying for their share of available light, nutrients, and water.

A clump consisting of a dozen or so corms can produce hundreds of tiny cormels and dozens of first year corms, effectively choking out the larger ones and stunting development.

And stunted corms means fewer blooms or flowers that are small and underdeveloped.

To promote vigorous plants with ample flowers, divide your perennial clumps every two or three years in fall. Wait until after blooming is finished, but be sure to do it before hard frosts arrive.

Keep only the mid-sized to large corms for replanting and discard the majority of cormlets – but not in the compost bin, they’ll grow there too!

With annual plantings, overcrowding happens as a result of planting the corms too close together. This can quickly deplete available nutrients which are needed to produce big bulbs for big blooms.

Exhibition plants are grown with a six- or eight-inch spacing, but in the garden, planting corms three to four inches apart is adequate for energetic growth and attractive blooms.

5. Pest Problems

Pest infestations can reduce or prevent flowering as well as contribute to the spread of plant disease. To reduce the risk, always start with top quality corms that have a crisp hull and are free of blemishes, soft spots, and odors.

The most likely culprits to infest your gladiolus plant are aphids, mites, and thrips. If you notice that pests are attacking your glads, apply a natural insecticide like neem oil to get rid of them.

Avoid possible disease issues from botrytis blight, mosaic virus, and rot by controlling pest infestations and planting corms in well-draining soil.

Our guide to managing common gladiolus pests has more information.

6. Planting Too Late

Corms should be planted out after all danger of frost has passed and the soil has warmed up to 55° F.

Since it can take up to three months for them to bloom, if they’re planted too late or you live in an area with a short summer season, the window for buds to develop can close before they have a chance to flower.

To ensure a showy display even in areas with short summers, start your plants indoors four to six weeks before your average last frost date, then transplant out once the ground is warm enough.

To start them indoors, plant several corms at the appropriate depth, but closer together than in the garden – about half to one inch apart in a one gallon nursery pot.

When it’s time to transplant outdoors, they’ll still be small enough to easily separate for proper spacing in garden beds.

Our guide to starting gladiolus indoors for early flowering has all the details.

Another smart option for short seasons is to grow dwarf types, cultivars of G. nanus, which bloom earlier, usually in about 60 days. They’re hardy down to Zone 5 and are particularly well-suited for areas with short summers.

7. Shade and Sun

How much sun your plants receive is another important aspect that can affect flowering.

Gladiolus need full sunlight, at least six hours daily, to produce robust growth and sturdy stalks with plenty of blooms.

They’ll grow in light shade, but plants are typically smaller, producing fewer blooms and the risk of planting in shady locations is that they may not bloom at all. Full sun will promote prolific flowering.

If your bulbs are underperforming because they’re in too much shade, move them into a full sun location to improve flower development.

8. Water Issues

To develop large, uniform flowers in abundance, gladiolus plants also need consistently moist soil.

Avoid boggy, wet, or waterlogged sites which can create issues like root rot, but don’t let the soil dry out entirely either.

Aim to supply one inch of water per week in the absence of rain, and avoid overhead watering to minimize disease problems and damage to stalks.

9. Winter Care

To keep your bulbs safe from freezing, lift them in fall and store them in a cool, dry, and dark location with temperatures in the range of 40 to 50°F.

If you’d like to learn more about lifting corms, our guide on how to prepare and store bulbs in winter has the info you need.

In areas with mild winters, they can be left in the garden.

Most standard sized gladiolus are hardy in Zones 8 to 11, but many gardeners in Zones 7 and even parts of Zone 6 overwinter their corms in-ground – but you’ll need a cozy winter mulch for them to survive cold spells.

Without an insulating winter mulch, the corms can heave in freeze and thaw cycles, exposing them to frigid temperatures that can rot the bulbs or cause stunted growth and hardly any blooms the following season.

Even in Zones 8 and 9, a thick winter mulch should be applied for protection, laying on at least three inches of organic materials such as bark mulch, conifer leaves, shredded leaves, or straw.

Remove winter mulch materials once spring arrives.

Showy Floral Displays

Gladiolus are easy-care flowering corms that add welcome color to the late summer garden.

If your gladiolus plants fail to bloom, a few simple adjustments are usually all that’s needed to quickly restore their showy floral displays!

Remember that small corms usually won’t flower until their second or third year, and always ensure they aren’t planted too deep – set them in the ground at a depth that’s twice the height.

Grow them in full sun in moist, well-draining soil and fertilize regularly for the largest blooms.

When growing as annuals, give the plants ample space, and divide perennial clumps every few years to keep them vibrant and prevent overcrowding.

Then all you have to do is enjoy their colorful, impressive blooms!

Are you growing gladiolus? Have you had problems with them failing to bloom? Drop us a note in the comments section below.

And for more information about growing summer-flowering bulbs, be sure to read these guides next: