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Irises: How to Plant, Grow, and Care for Iris Flowers | The Old Farmer's Almanac

Irises

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Pixabay
Botanical Name
Iris spp.
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How to Plant, Grow, and Care for Irises

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The tall, beautiful iris, named after the Greek goddess who rode rainbows, comes in many magical colors. Despite its divine origins, this beautiful flower is rugged, reliable, and easy to grow. Learn all about planting, growing, and caring for iris flowers. 

About Irises

More than 250 species make up the genus Iris. The most familiar irises are the tall bearded irises (Iris germanica), which reach 2 to 3 feet in height. Their distinctive six-petaled flowers have three outer hanging petals (called “falls”) and three inner upright petals (called “standards”).

Irises may be a bearded or crested (aka “beardless”) type. Bearded iris are so-called because they have soft hairs along the center of their falls. The hairs on crested types, like the Siberian iris, form a comb or ridge instead.

Most irises flower from late spring to early summer. Some—mostly bearded hybrids—are remontant, meaning they may flower again later in the summer. The blooming period of Siberian irises tends to follow that of the bearded types.

Irises attract butterflies and hummingbirds, and make lovely cut flowers. For iris companions in the garden, look to roses, peonies, and lilies.

Planting

Irises will bloom best in full sun, meaning at least 6 to 8 hours of sunlight per day. They can tolerate as little as half a day of sun, but it’s not ideal. Without enough light, they won’t bloom well. Bearded irises must not be shaded out by other plants; many do best in a special bed on their own.

Provide well-draining, fertile, neutral to slightly acidic soil. Loosen soil to a depth of 12 to 15 inches, then mix in compost or aged manure. Good drainage is critical: Irises prefer “wet feet, but dry knees.” They will not tolerate wet soil in winter. Learn more about preparing soil for planting and organic soil amendments.

When to Plant Irises

  • Plant most irises in late summer to early fall, when nighttime temperatures remain between 40° and 50°F (4° and 10°C) or above. This gives them plenty of time to get established before the coming winter.
  • Tall bearded iris varieties are best planted closer to fall because they tend to go dormant in early to mid-summer.
  • If you receive bare rhizomes or irises in a container at some point earlier in the year, go ahead and plant them as soon as convenient. It’s better to get them in the ground rather than wait until the “ideal” time.

How to Plant Irises

  • Plant bare-root rhizomes (the thick stems) horizontally, with the top exposed and only the roots underground. In areas with particularly hot summers, set the rhizome just below the soil surface.
  • Dig a hole 10 inches in diameter and 4 inches deep. Make a ridge of soil down the middle and place the rhizome on the ridge, spreading the roots down both sides. Fill in the hole and firm it gently, leaving part of the rhizome and the foliage uncovered.
  • Plant singly or in groups of three, 1 to 2 feet apart, depending on the fully grown plant’s size.
  • Soak Siberian iris rhizomes in water overnight before planting, then set them 1 inch deep (2 inches, if the soil is sandy), 2 feet apart. Over a period of years, they will form clumps; divide when blooms get smaller and vigor declines.
  • Do not mulch around the rhizome as this may encourage rot.
  • Water thoroughly.

Striped iris flower

Growing

How to Grow Irises

  • In the early spring, remove winter mulch and any old foliage to allow for fresh, new growth.
  • Fertilize in early spring, scratching an all-purpose fertilizer around the plants. Avoid high-nitrogen fertilizers. Reblooming irises perform best if fertilized again after the first wave of flowering is finished.
  • Do not overwater irises; too much moisture in the soil can cause the rhizomes to rot. Water consistently and deeply, especially during summer drought.
  • Keep rhizomes exposed. Unlike bulbs, which thrive deep underground, bearded iris rhizomes need a bit of sun and air to dry them out. If they’re covered with soil or crowded by other plants, they’ll rot.
  • Taller irises may need staking or they will fall over. 
  • Deadhead (remove spent blooms) consistently. Bearded Irises flower sequentially on buds spaced along the stems.
  • After blooming is finished, cut flower stems down to their base to discourage rhizome rot, but do NOT trim the iris’ leaves. The plant’s foliage carries on with photosynthesis and generates energy for next year’s growth and flowers. Only prune off brown leaf tips, if desired.
  • After a hard frost in the fall, cut foliage back hard, remove any foliage that appears spotted or yellowed, and dispose of all debris in the trash. 
  • If iris foliage is hit with heavy frost, remove and destroy it to eliminate borer eggs. See your local frost dates.
  • For winter protection, cover the rhizomes with an inch or two of sand topped with a light layer of evergreen boughs, applied after the ground freezes and removed when the forsythias bloom in the following spring.
  • Iris borer, a common iris pest, overwinters as eggs in spent leaves. Signs include vertical streaks in the leaves. If apparent, look for the pests and squash them! If you see rot in the rhizome, dig it up and remove the affected parts. See pest tips below.

Dividing Irises

Over time, it’s not unusual for plantings of iris to become overcrowded, which causes the rhizomes to lose vitality and stop blooming. When this happens (usually every 2 to 5 years), it’s time to divide and replant healthy rhizomes in fresh soil. 

When to divide? Do this task after flowering finishes and then trim the foliage back to six inches. 

  1. Shortly after blooming (usually around mid-summer), carefully dig up the clump of irises. You’ll find that the original rhizome that you planted (the “mother”) has produced several offshoot rhizomes.
  2. Separate these rhizomes from the mother with a sharp knife and discard the mother, as it will no longer produce blooms. 
  3. Inspect the rhizomes for any rotting tissue, soft spots, or other signs of disease, removing and discarding infected parts or entire unhealthy rhizomes.
  4. Plant these fresh rhizomes in a new bed, replant them where they were before (after adding new soil), or share them with friends and spread the joy of irises!

irises-3530372_1920_full_width.jpg

Harvesting

Irises as Cut Flowers

  • Cut flowers for arrangements when they are just showing color.
  • Vase life is 3 to 7 days.
Wit and Wisdom
  • The iris is depicted in the French royal standard fleur-de-lis and is also the symbol of Florence, Italy.
  • Oral root, taken from the dried roots of Iris ‘Florentia’, was considered a cure for blood and lung diseases, and teething babies were encouraged to gnaw on a “finger” of dried root for its natural fluoride.
Pests/Diseases

Iris Pests and Diseases

Pest/Disease Type Symptoms Control/Prevention
Aphids Insect Misshapen/yellow leaves; distorted flowers; leaf drop; sticky “honeydew” (excretion) on leaves; sooty, black mold Knock off with water spray; apply insecticidal soap; inspect new plants carefully; use slow- release fertilizers; avoid excess nitrogen; encourage lacewings, lady beetles/bugs, spiders
Bacterial soft rot of iris Bacteria Dieback starts at leaf tips; leaves yellow/ wilt/separate from base; rhizomes rot; foul odor; plants may die Cut out diseased rhizome tissue/air-dry cut surfaces for 1 to 2 days before replanting; for severe infections, destroy plants/surrounding
soil (do not compost); remove plant debris regularly; disinfect tools; good air circulation; prevent plant injury; control iris borers; plant rhizomes at proper depth
Blight (southern), “white mold Fungus Leaves/stems/ entire plants wilt,
brown or blacken, and may die; water-soaked lesions on lower stems; crown/rhizome
rot; fluffy, white fungal mats with mustard- seed–like balls on stems bases/nearby soil
Destroy infected parts/ plants (if severe), white fungal mats, and surrounding soil to at least 6 inches beyond plant and 8 inches deep (do not compost); remove plant debris regularly; disinfect tools; solarize soil; resistant varieties; good drainage
Blight (Botrytis), “gray mold” Fungus Yellow/ brown/gray spots with water-soaked margins on leaves/flowers; gray mold; buds remain closed; stem lesions; wilt/rot; scorched appearance (“fire”) in some plants Destroy infected parts/ severely infected plants (do not compost); remove plant debris regularly; disinfect tools; good air circulation/ sunlight; avoid overhead watering; prevent plant stress/injury; weed; rotation
Ink spot Fungus Yellow-margined, reddish brown leaf spots; brownish black powdery mass in spots; plants collapse; inky black stains on some rhizomes; rhizomes rot Destroy infected leaves/rhizomes (do not compost); remove plant debris regularly; good air circulation; 3-year rotation
Iris borers Insect Leaf tips turn brown; pinholes chewed in leaves (caterpillar/lar- vae entry); holes bored in rhizomes; slimy, stinky mess at plant base and rhizome Squash by hand or remove and discard affected foliage before pupation (and new moths, mating, eggs); inspect suspected rhizome damage, discard (burn/bury) affected ones; clean beds of plant debris after a hard frost
Iris weevils (“flag weevils”) Insect Beetle-like insects feed on flowers, seeds, pods of wild blue flag iris (Iris versicolor) and purple Siberian iris; deposit eggs in ovary, which pupate in seed- pod; visible when flower is in bloom Remove/destroy seedpods
Leaf spot (“bacterial leaf spot of iris”) Bacteria Varies; water- soaked rust/black leaf spots between veins later dry/fall out, leaving holes; leaves yellow/ distort/wilt/die; stem cankers Destroy infected parts/ severely infected plants (do not compost); remove plant debris regularly; disinfect tools; prevent plant stress/in- jury; good air circulation; avoid overhead watering
Leaf spot (fungal) Fungus Varies; leaf spots on lower leaves enlarge and turn brown/black; fuzzy growth or pustules in lesions; disease progresses upward; leaves die Destroy infected leaves/ severely infected plants (do not compost); remove plant debris regularly; disinfect tools; resistant varieties; good air circulation; avoid overhead watering
Leaf spot (iris) Fungus Yellow/brown leaf spots with water-soaked/reddish brown/yellow margins dry out; black specks in lesions; leaves curl/yellow/die back from tip; poor flowering; plants weakened Destroy infected parts (do not compost); remove debris regularly; resistant varieties; good air circulation/sun; avoid overhead watering; weed
Nematodes (foliar) Nematode Typically, starting on lower leaves, yellow/brown/purplish,
angular spots or streaks change to dark green/brown/blackish lesions between leaf veins; dead areas may drop out; distorted/curled leaves may wilt/drop; symptoms move upward on plant; stems, buds, and flowers may also be affected; poor flowering; stunted or bushy growth
Destroy infected leaves/plants (do not compost); remove plant debris regularly; choose resistant varieties, if available; good air circulation/spacing; avoid overhead watering/keep leaves as dry as possible; weed
Nematodes (root-knot) Nematode Roots “knotty” or galled; plants stunted/yellow/wilted/weakened; leaves and other parts may distort or die; poor flowering Destroy infested plant debris after flowering season, including roots (do not compost); disinfect garden tools; choose resistant varieties; solarize soil; plant French marigolds (Tagetes patula) as a trap crop; rotate plantings
Nematodes (stem and bulb) Nematode Typically, leaves turn yellow then brown/distort/blister/wilt/die; swollen stems, crowns, leaf bases; poor/distorted flowering; plants stunted/die; fluffy white masses (“nematode wool”) may be present; discolored/distorted rhizomes soften/brown/crack/die; rhizome cross-section may show brown concentric rings destroy infested plants and those within 3 feet, including soil (do not compost); disinfect tools; choose healthy, nematode-free seed/ plants; rotate plantings every 5 years, if possible; plant green manures such as mustard and radish; weed
Slugs/snails Mollusk Irregular holes in leaves/flowers; slimy secretion on plants/soil Handpick; avoid mulch; use copper plant collars; avoid overhead watering;
lay boards on soil in evening, in morning lift and dispose of pests in hot, soapy water; drown in deep container of 1/2 inch of beer or of sugar water and yeast sunk to ground level; apply 1-inch-wide strip of diatomaceous earth around plants
Thrips Insect Stunted plant growth; stippling; leaf drop Knock off with water spray; use sticky traps; beat/shake foliage/flowers; clip off unopened/infested buds and shoot tips; submerge in mixture of 7 parts water to 1 part isopropyl alcohol and discard; water adequately, avoid excess nitrogen; deadhead flowers
Verbena bud moths Insect Tunnels/bores into plant shoots/stalks, seeds, and buds; moths do no harm Handpick, or cut infestations, burn infested shoots and buds
Viruses Virus Varies; leaves with yellow/light green mottling or rings; distorted leaves/stems/flowers; flowers streaked; stunting Destroy infected plants (do not compost); remove plant debris regularly; disinfect tools; resistant varieties/certified virus-free plants; control sap-sucking insects (aphids, whiteflies); weed
Whiteflies Insect All stages suck sap on leaf undersides; leave sticky “honeydew” (excrement), sooty, black mold; yellow/silver areas on leaves; wilted/stunted plants; adults fly if disturbed; some species transmit viruses Remove infested leaves/ plants; use handheld vacuum to remove pests; knock off leaf under- sides with water spray
in morning/evening; set yellow sticky traps; apply insecticidal soap; invite beneficial insects and hummingbirds with native plants; weed; reflective mulch

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