One of the most common mistakes gardeners make in determining when to plant gladioli bulbs is forgetting to take their local climate into account. Fortunately, figuring out what may work best for you is pretty straightforward. You can enjoy a succession of blooms for staggered cut flower bouquets or, if you prefer, you can plan a massive show of gladioli flower stalks which come on simultaneously. Regardless of which one you’re aiming for, in this article, you’ll learn the best timing and techniques for success.
Meet the gladiolus
You may know gladioli by a different common name—sword lilies. Actually members of the iris family, gladioli are named for the sword-like shape of their leaves. (The word “gladius” is Latin for “sword” and “gladio” means “with a sword.”)
Very affordable and easy to find, gladioli bulbs are an outstanding value—particularly when you consider that individual flower spikes can produce more than 20 eye-popping flowers apiece. Beginning at the base of each spike, these florets usually open one or more at a time over a couple of weeks.
There are hundreds of gladioli varieties featuring an amazing range of floral colors from pink and yellow pastel shades to vibrant rainbow mixes and classic gemstone tones. You can also find early, early-mid, mid-season, late-mid, and late-flowering varieties. Early varieties require 70 to 74 days from the time you plant them to the time they begin flowering. Late varieties, by contrast, can take up to 99 days to reveal their blooms.
Incidentally, although you’ll frequently see references to gladiolus bulbs, gladioli actually grow from corms. A corm is essentially an extra thick, underground stem. Gladioli corms look slightly rounded with a protective covering akin to onion skin. (For simplicity’s sake, I’m using the terms gladiolus corms and bulbs interchangeably here.)
Why knowing when to plant gladioli bulbs is important for success
Knowing when to plant gladioli bulbs matters to your success for a few reasons. First, your bulbs are much more likely to rot than take root, if they are planted in soil that’s too cold for them. Bulbs planted too early are also still subject to the danger of frost. Similarly, gladioli bulbs planted too late may not have enough time to root and flower before the first frost.
Want to enjoy the lengthiest possible flower season for your area? To have gladioli blooms as soon as possible, you could plant a very early-blooming gladiolus variety such as white dwarf Gladiolus Albus several days before your last average frost date in early spring. (And if frost is forecasted, simply mulch any early gladioli sprouts to protect them temporarily.)
Then, every couple of weeks thereafter, plant additional bulbs for wave after wave of stunning flowering plants.
How hardy are gladioli?
If you live in USDA hardiness zones 7 or 8 and up, you can treat your gladioli like perennials. That means allowing them to stay planted outside year-round without losing them to cold temperatures. Zone 7’s low temperatures range from 0 to 10 degrees F (-17.8 to -12.2 degrees C). Lows in Zone 8 are 10 to 20 degrees F (-12.2 to -6.7 degrees C).
For gardeners living in the remaining zones? Sadly, gladioli generally aren’t winter hardy. As such, they must either be dug up and protected during winter or thought of as expendable annuals. Later in this article, you’ll learn how to dig up and overwinter the bulbs, as well as a mulching trick that can help them overwinter outdoors, even in moderately cold regions.
When to plant gladioli bulbs if you live in a cold climate
Knowing when to plant gladioli bulbs is especially important if you live somewhere cold. Remember, gladioli are only hardy to zones 7 or 8 and up. To get a jump on the growing season, you can either plant them in pots to keep in a frost-free location like a garage or greenhouse or plant outside a week or so before your average last frost date.
Another option is to “pre-sprout” your bulbs by putting them root-side-down into a container with a half-inch of water about 2-3 weeks before your last expected spring frost. The bulbs will start growing roots immediately and the shoot system will begin to grow. Replenish the water as necessary, and plant the sprouted bulbs into the garden when the danger of frost has passed.
When to plant gladioli bulbs if you live in a warm climate
As for when to plant gladioli bulbs if you live in a warm climate, you can plant gladioli during spring, summer, or much of the fall. Just don’t plant them too close to your first frost date because gladioli need soil temperatures of at least 55 degrees F (12.7 degrees C) in order to develop good roots. If soil temps dip below that 55-degree mark before your bulbs become established, they could rot.
When to plant glads in a pot
Wondering when to plant gladioli bulbs in pots? Provided soil temperatures are at least 55 degrees F (12.7 degrees C), potted glads in full sun can get growing. And because you can still move planted containers inside during spring’s unexpected cold snaps, planting bulbs in pots may enable you to get an even earlier start on your gladioli blooms.
You might also choose to plant multiple bulbs in a draining bucket or bulb basket which you then plant outdoors once your garden soil has warmed. At season’s end, you can easily unearth this perforated container for overwintering in the garage.
After examining and drying any corms to be overwintered, you might discover some baby corms—called “cormels”—which develop on healthy gladioli. You can gently pop these off of the parent corm, plant them in trays, and eventually put them out alongside your other glads.
The best growing conditions for gladioli
Think you’ve figured out exactly when to plant gladioli bulbs for ideal results in your planting beds or container garden? Next up, you’ll need to make sure to provide your bulbs with the best possible growing conditions, including:
- Full sun—Although gladioli can tolerate partial shade, they prefer full sun.
- Well-drained soil—For best results, plant in soil that’s rich in organic matter like humus and well-rotted compost.
- Consistent watering—Give gladioli at least an inch of water each week.
- The right pH—Your gladioli will shine if grown in soil with good fertility and a soil pH of 6.5 to 7.5.
How deep to plant the bulbs
You can plant most gladioli bulbs about four to six inches (10-15cm) deep. For accuracy, use a marked bulb planter or a trowel and tape measure. And to ensure a strong performance, include a little bone meal or slow-release bulb fertilizer in the planting area before planting each bulb.
How far apart to plant gladiolus bulbs
How far apart you should plant your gladioli depends in part on how many you’re growing and how you’ll use them. For instance, if you want to grow many rows of them for inclusion in bouquets, space your individual rows about a foot apart for easier harvesting. Otherwise, try spacing gladioli about six to 10 inches (15 to 25.4 cm) apart, depending on the expected size of your mature plants.
Overwintering gladioli bulbs outdoors
Even if you live in a cold climate, it might be possible to overwinter your gladioli bulbs where they’re planted. Some varieties are more cold-tolerant than others. Try planting them within a slightly warmer microclimate—like a raised bed near your home’s foundation or a sunny spot near heat-absorbing concrete. You can also plant these bulbs a little more deeply than you otherwise might and heavily mulch before winter.
But what if you want to guarantee that your gladioli make it through the cold winter? In that case, you’ll need to:
- Dig up corms to be stored.
- Use a sharp knife to trim away any remnants of old foliage and flower stalks.
- Spread the corms out to dry for a couple of weeks indoors.
- Once dried, you should be able to separate any soft or spent corms from the healthy corms.
- Discard spent corms and root material and store all healthy corms for next year at temperatures between 35 and 45 degrees F (1.6 to 7.2 degrees C). To optimize airflow, keep the corms in a breathable mesh produce bag.
Care tips for gladiolus plants
Besides figuring out when to plant gladioli bulbs, here are a few extra tasks you’ll need to undertake at just the right time:
- Staking—Both large and giant gladioli types often need extra support for their enormous flower heads. Before florets open, securely tie each flower stalk to a sturdy stake. This should provide support as flowers open, as well as during heavy rains and strong winds.
- Mulching—In advance of the season’s hottest months, add mulch to help suppress potentially competing weeds and lock in valuable moisture.
- Cutting back—Spent flower stalks should be cut after they’ve faded. (This enables the plant to direct more of its energy back to corm production for next season’s show.)
- Monitoring for pests—Gladioli aren’t impervious to aphids, thrips, Japanese beetles, and other pest insects, so routinely monitor for these, as well as assorted fungal diseases and, sometimes, even slugs. You can contain small outbreaks by hand-picking and discarding the offenders, but larger infestations may require an application of diatomaceous earth or insecticidal soap. (Just remember, these also can affect your beneficial insect populations.)
Now that you know when to plant gladioli bulbs, as well as how deeply and how far apart to plant them, you can give your glads the best possible start. You can also choose just the right varieties based on your specific preferences as well as the length of your growing season. Want extra-large flowers all season long? Just succession-plant a variety of early, mid-, and late-flowering giants.
Looking for something more subtle? You can always incorporate miniature gladioli varieties as border plants in perennial beds or container gardens. With so many colors, sizes, and bloom times to experiment with, your choices are nearly limitless.
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