Iris Garden Guide

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Rhizome Planting Instructions

  • Do Not set new rhizomes too deep, as this is the number one reason for failures.
  • After digging and amending the soil, prepare a higher ridge of soil for the rhizome to lay horizontally on, which allows the roots to hang down on either side. This ensures the roots will be planted deeper than the top of the rhizome which should be slightly exposed after proper planting.
  • After planting, the fleshy rhizome should be at or just below the natural soil grade.
  • Set plants 18 to 24 inches apart for optimal performance.

TIP: For a quick color display set 3 rhizomes in a triangle about 6 inches apart, with 24 inches between each group of 3.

NOTE: Closer planting requires earlier dividing to prevent overcrowding and can inhibit optimal flowering.

  • With an initial planting with 18 to 24-inch spacing, the first division is about 3 to 4 years. Division at this interval also helps to control iris borer and diseases, which are discussed in detail below.

Iris Care - Maintenance After Establishment

  • Winter Protection
    • German Bearded Irises are quite adaptable tolerating USDA zone 3-9, however in cold climates, mulching plants the first winter with clean straw or chopped leaves are recommended to prevent heaving.

NOTE: If mulch is used over winter, be sure to remove it early in the spring to allow sunlight to reach the fleshy rhizomes.

  • Dividing a Mature Patch
    • For maximum blooming, dig and reset the bed every 3 to 4 years, or sooner if plants become overcrowded.
    • Three weeks after blooming, dig and lift all the old plants with a fork or spade. Avoid cutting into the rhizomes.
    • Shake or wash soil clean and allow to air dry.
    • Inspect all the leaves looking for any linear brown streaks, which is usually evidence of Iris borers. Iris borer larvae are plump pink worms with darker heads. Of course, if any of these are discovered, discard them and the rhizomes they were found in to limit their spread and destruction.
    • Healthy, existing foliage can then be cut cleanly to 6-8 inches long fan. Dispose of cut material into the trash, do not compost.
    • Once dry, thoroughly inspect rhizomes for damage, soft spots, or diseased areas.
    • Any questionable rhizomes should be thrown out.
    • If only part of a rhizome is soft, cut those sections out using sterilized pruners or sharp clippers.
    • Divide healthy rhizomes in the same way, ensuring that divisions are at least 3 inches long, each with healthy roots and a fan of foliage.

TIP: Try to make cuts where tubers have natural bends or forks.

NOTE: Any cut rhizome ends can be dusted with sulfur powder, which reduces the risk of potential fungal infections.

    • Allow rhizomes to cure in a warm, dry location out of the direct sun, for a few days.
    • While your divisions are drying, re-work the soil in the beds thoroughly as needed.
    • Replant as you would any new Irises. (See instructions above). This is also a good time to share any extras you don't have room for with family, friends, or neighbors.
  • Iris Borers
    • Iris borers are the most destructive pests of Bearded Iris.
    • Eggs are laid by moths in fall around the bases of plants.
    • Those eggs will hatch in early spring and the tiny larvae start eating their way into the leaves.

NOTE: This short time of larval movement from hatching to entering the leaf, is the only time that insecticides can be applied for control. Prevention by way of proper sanitation is most critical.

    • Early signs of borers are ragged leaf edges and small holes in the leaves that appear to be surrounded by moisture due to exuding leaf sap.
    • Pinching the leaves or cutting the leaf off just below the holes at this time can dispose of the borers.
    • As the borers feed moving downward in the leaves they continue to grow and mature. By August they will have usually eaten into the rhizome, where they linger and feed just long enough to infect and ruin the rhizome. After this short stint, they emerge and enter the soil around the plants to pupate. Pupating larvae are the least vulnerable to pesticide treatments. Therefore, thorough mechanical working of the soil during division is very beneficial.
    • As mentioned, controlling borers with insecticides is not very effective because the borers are feeding inside the plants. Therefore, removing all debris and dead leaves in fall and spring and burning it or tossing it into the trash will remove eggs before they hatch.
    • Keeping the soil carefully cultivated around the rhizomes can also reduce borer pressure.
    • Sanitation, monitoring, and removal of the larvae when discovered, along with periodic dividing and resetting of healthy rhizomes offer the best borer control measures.
  • Bacterial Disease
    • Soft rot is a bacterial disease characterized by rhizomes that have a soft milky ooze along with a foul odor.
    • Most commonly caused by secondary infections from wounds and borer feeding.
    • Effects of bacterial infections are wilting and dying leaves.
    • Result is the leaves rot off at the base, with the rhizomes rotting. Plants are eventually killed if infections are not addressed.
    • Remove all diseased parts as soon as they are detected.

NOTE: Soft rot is most common on Iris growing in poorly drained soils and in shady areas.

  • Leaf Spot Disease
    • Leaf spot is a fungal disease that causes brown spots with a yellow halo on the leaves.
    • This disease is most common during damp, or humid weather and is exacerbated in Iris beds with poor air circulation and in beds where leaf debris has been allowed to build up.
    • As infections spread, spots will continue to increase in size and quantity eventually congealing to cause entire leaves to wither, brown, and die. While this type of fungal disease causes unattractive plants, it is usually not fatal, unless it is ignored.
    • As with most fungal diseases, the best management of Leaf spot disease relies on proper sanitation, cultural care, and fungicide applications when necessary.
    • Remove and destroy all infected or symptomatic leaves. Never compost fungal-infected leaves. Removing dead leaves in fall and spring is usually effective for controlling their spread.
    • Avoid working in Iris beds when the foliage is wet from rain, due or irrigation.
    • Maintain proper spacing and prevent overcrowding by doing necessary divisions.
    • Ensure plants are planted and growing in well-draining soil.
    • For stubborn, reoccurring leaf spot infections use rotational applications of copper, sulfur, or bio-fungicide in combination with either Fung-onil® or Captan® by Bonide®.

German Bearded Iris

Irises are hardy and considered easy to grow, but to perform well, they must have good drainage, ample sunshine, and good air circulation. They can be planted in almost any season but are usually best planted from mid-July through September. During this time, plants are semi-dormant and the newly set rhizomes have time to establish before winter.

  • Irises will grow in partial shade but prefer full sun. Sunshine is one of the best guarantees for healthy, disease-free new growth.
  • Select a location that is well-drained. Avoid any areas with standing water.
  • If the soil is too heavy for good drainage, create a raised bed and work organic matter into the soil.
  • Bearded Iris will perform well in average but organically amended soils, with drainage being most important.
  • They prefer more alkaline soils than acidic soils, but don't stress out about adjusting the pH as most soils have a suitable average pH for Iris.
  • If fertilizer like bone meal or super phosphate is added prior to planting, be sure it is not in direct contact with the rhizomes and blended into the soil well.

NOTE: Bearded Iris is not a heavy feeder. If too much fertilizer or the wrong fertilizer is applied, it can interfere with their growth and blooming. The fertilizing adage for Irises is, "Less is best". Most naturally derived, liquid fertilizers with low nitrogen and higher phosphorus are a good choice. Consider Neptune's Harvest Fish & Seaweed with a 2-3-1 formula.

Dwarf Iris

Dwarf Iris belongs to the same family as the tall, bearded Iris varieties, and has the same basic requirements with similar pest and disease susceptibilities. The main difference is that they are smaller in size growing to a modest height of only 4-6 inches tall.

  • Like their larger bearded cousins, they prefer well-drained, organically rich soil with average pH.
  • Dwarf Irises bloom earliest, usually from late winter to early spring.
  • For best visual displays they are often planted closer together, and with other early spring blooming bulbs such as Crocus, Daffodils, and Snowdrops.
  • Borders, rock gardens, and entry beds that are viewed close-up are good places for dwarf Iris varieties.

Siberian Iris

  • Siberian Irises are more widely adaptable, with few insect worries, and they are quite a disease resistant.
  • Unlike German bearded iris, Siberian Irises have a fibrous root system, below a regular crown.
  • Unlike bearded Iris, Siberian Iris can tolerate periodic flooding or some amount of standing water.
  • They are vigorous growers that can be producing 25 to 30 blooms in just a few seasons, in optimal conditions.
  • Their strongly vertical, rich green foliage is dramatic and sturdy. They are often mistaken for ornamental grasses, when not in bloom.
  • Planting
    • Siberian Irises perform well in moist, organically rich soils in a wide-ranging pH of 5.5 to 7.0, in sun or shade.
    • New bare root crowns should be planted with the top roots no more than 1 inch deep.
    • Individual crowns should be spaced about 24 inches apart.
    • Divide mature plants every 3 to 4 years to prevent overcrowding and reduced flowering.
  • Watering
    • Water new Siberian Irises using the 1 inch of water per week rule, per plant for the first season or two until they become established. Mature Siberian Iris stands can tolerate more drought than immature plants but understand that drought-stricken plants may not bloom as well or consistently as those that have regular water.
  • Fertilizing
    • Fertilize Siberian iris in spring with a complete, flowering plant-specific food to keep them healthy and blooming.