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FREQUENTLY ASKED QUESTIONS

 

 

IRISES

LACK OF BLOOM

DIVIDING AND PLANTING

ROT

WATERING

DEFORMED BLOOMS

FLOWERS TURNED WHITE

WITHERED STEMS

LEAF PROBLEMS

FERTILIZING

SOIL AND CLIMATE

 

MY IRISES HAVEN'T BLOOMED, WHY?

BEARDED IRIS

1 -    Summer Sun. Irises require full sun for at least half a day in summer. Insufficient summer sun does not allow proper development of flowering. Full sun all day is desirable for most, however dark colored blooms that flower later in the season will last better if given some protection from the hot afternoon sun. Full winter shade is not a problem.

Even if your irises are in full sun, the rhizomes may be shaded by mulch or creeping perennials such as violets or evening primrose. Ensure that the soil over the rhizomes  is not shaded.

2 - Winter Chill. Just as they like good summer heat, so they like to be chilled in winter. The degree of chilling required depends on the cultivar, but many Dwarf Bearded require actual frosts to bloom. Reblooming irises may need less chilling than other varieties. Some people report good success in inducing dwarf iris to bloom by putting ice on top of the plants a number of times over winter. If you live near the waterfront in Adelaide it may not be cold enough, similarly the  western suburbs of Perth and eastern suburbs of Sydney. Most of Brisbane would not receive enough winter chill.

Also be aware that if they are  against a north facing wall, the wall may retain enough heat at night to prevent sufficient chilling.

3 - Too Much Nitrogen. Excessive use of nitrogen rich fertilizer can cause lush plants with little or no flowers. Bearded irises like good supplies of phosphorous and potassium, and less nitrogen. We recommend pelletised organic fertilizers such as Rapid Raise or Dynamic Lifter. Alternatively use slow release such Osmocote.

4 - Needs Dividing. Irises can exhaust the soil after a number of years, or grow too dense. Depending on the cultivar they will dividing every 2-5 years. They should be moved to another spot or else have their soil refreshed.

5 - Settling In. Sometimes when you buy a plant it may not flower the following spring, but rather will concentrate on establishing itself and then will flower the second spring.

6 - Mild Rot. The part of the rhizome most susceptible to rot is the top of the rhizome where the flowering  fan is. Sometimes this may rot a little over winter and the rest of the rhizome survives and the newer small fans along the sides continue to grow, but the fan that would have produced bloom has died. This usually causes reduced bloom rather than total lack of bloom.

SPURIA IRIS

1 - Recently Moved. Spuria irises don't like being moved and usually will not bloom the following year. Occasionally it will take 2 years to get bloom.

2- Lack of Summer Sun. Spurias like full summer sun.

SIBERIAN IRIS and JAPANESE IRIS

1 - Winter Chill and Summer Sun required.  requirements are as for tall bearded irises, except Siberian prefer actual frosts.

 

WHEN SHOULD I DIVIDE MY IRISES?

 The best time for Bearded Iris and Louisiana Iris is generally immediately after flowering or in late autumn - early winter. If it is too close to flowering time, bloom will be small, late or not happen at all.  Moving during summer is OK, but bearded iris don't grow much over summer so don't water them a lot to get them to grow, you will only induce rot.

Irises that go dormant (Siberian, Spuria and Japanese and bulbs) are best moved during dormancy. Spuria iris can be moved soon after they begin their spring growth, but should be planted immediately and not allowed to dry out.

Pacific Coast Iris are fussy about when they should be moved. They should only be moved once they have started their winter growth, usually 4-6 weeks after the beginning of the autumn rains i.e. mid May - mid June.

Most species (iris I Ungicularis etc) should be moved in autumn with the rains.

 

HOW DEEP SHOULD I PLANT MY IRISES?

Bearded iris should be fairly shallowly planted, with about 1-2 cm  (half inch) of soil over the rhizome. Too deep and they won't get enough summer heat or winter chill. If exposed to the sun they may develop scorch in the Australian climate. (Books that talk about exposing the rhizome are generally written for cooler climates where every effort is required for sufficient summer heat).

When replanting ensure that the bottom of the rhizome makes good contact with the soil underneath it.

Other (beardless) iris should be planted a bit more deeply, 2-4cm. If your Louisiana iris rhizomes end up above the surface of the soil, it doesn't really matter.

 

SHOULD I TRIM MY IRISES ?

Irises need only to be trimmed when moved. The leaves are cut back because the roots have been cut. There is no need to trim any other time, simply remove old, spent leaves.

 

MY IRISES ROTTED, WHAT HAPPENED?

Bearded Iris are more easily killed by over-caring than anything else. Excessive moisture, especially over summer will induce rot. Make sure your bearded iris are well drained. Do not cover the rhizomes with moisture holding mulch (e.g. lawn clippings, pea straw), particularly if you use overhead watering. If rot starts to happen, remove the rotten part of the rhizome and expose the cut surface to the sun for a few days, then replant. Excessive use of nitrogen fertilizer can produce lush plants susceptible to rot.

Spuria iris don't like summer humidity, so don't cover with damp mulch in summer. Some parts of Australia e.g.  Coastal Sydney and north where there is very high summer humidity may not be suitable at all.

 

HOW MUCH SHOULD I WATER MY IRIS?

Bearded Iris - once every week or two over summer will be fine. Once a clump is established, many will survive without any summer watering. In a dry spring some watering to keep the soil slightly damp may be required. You may put mulch over the roots, but not over the rhizome. When transplanting in summer, water once or twice a week for the first few weeks then no more than once a week. If your irises go completely dormant from lack of water over summer, don't worry they will start growing again in autumn, as long as the rhizome is still fairly firm. In pots water them 2-3 times a week in very dry weather.

Spuria Iris - Should not be completely dried out until flowering has finished. Once flowering has finished, if not watered they will die down, go dormant and then come away again with the winter rains. If watered every week or two through summer they will remain green until about April when they will briefly die down , then start their winter growth.

Louisiana Iris are swamp irises so need plenty of moisture during winter-spring.  They can be mulched fairly heavily. If growing in pots immerse the pots about 2/3 in water but not completely. They can survive reasonable summer dryness but are best kept moist all year round. If they do dry out rehydrate slowly rather than suddenly soaking in water.

Siberian Iris are bog irises and like good winter and summer moisture. Once they go dormant in late summer, they can dry off somewhat. It's OK to mulch them heavily.

Pacific Coast Iris like good moisture in winter and spring but not boggy. In summer occasional watering is best, but some seem to be happy with complete summer neglect.

Japanese Iris like relative dryness during winter when dormant and plenty of moisture during the growing period over spring and summer. If growing in pots in a pond remove from the pond during winter.

 

MY FLOWERS ARE DEFORMED, WHAT HAPPENED?

Frost Damage - Frost on blooms are out will cause the flowers to go a bit transparent and mushy. Buds that are fully formed will also be damaged by heavy frost. The edges will be slightly curled, ruffling will be lost and plicata colouring will speckled. Damage may persist for several weeks after the frost. Some cultivars will be damaged more than others and it will worse in the more exposed, lower parts of the garden.

Frost Damage

Weed killer Damage - Glyphosate type sprays (eg Roundup) used within a couple of months of flowering will damage the blooms. If slight traces have drifted onto the plant the blooms will be whitish around the edges and some ruffling lost. Heavier exposure will lead to completely deformed blooms. Slight exposure to glyphosate will not normally kill your irises but newly transplanted ones will be more susceptible.

 

  

Mild Glyphosate Damage                    More Severe Glyphosate Damage - Dutch Iris

Very Severe Glyphosate Damage - Tall Bearded Iris

 

MY BEARDED IRISES HAVE TURNED WHITE

There are no proven cases of irises actually turning white, however there are several reasons why your patch may end up being all white. Firstly - glyphosate damage -see above. Secondly the patch has been taken over by a white one. Some white cultivars are extremely vigorous and will overgrow less vigorous cultivars. There may have just been a small piece left from previously or else occasionally nurseries make mistakes and a piece of white was mixed in with what you bought. It didn't flower for several years but multiplied rapidly. Then, by the time it bloomed, it had taken over most/all of the patch of irises.

 

THE FLOWER STEMS WITHER IN THE MIDDLE AND FALL OVER.

This usually happens as a result of weather damage. If the withering is mid- stem it is caused by wind and sudden weather change  drying out the stem during a period of very fast growth. Some cultivars will be more susceptible than others.

Withered Stem

If the stems falls over at a junction in the stem and is brownish and mushy, water has been trapped in the little leaf at the junction and rot has  followed. Usually happens in very wet springs, especially if warm also.

CROOKED STEMS

If there is steady wind while the flower stems are growing quickly they will tend to grow almost sideways and the straighten once the wind stops.

Louisiana stems are naturally a bit snake-like.

THE LEAVES ARE DAMAGED

Very mild silver speckling is caused by hail. Not a problem in itself but it does seem to make the plant more susceptible to fungal leaf spot. 

                  

Hail - left             Fungal Leaf Spot - right              Fungal Leaf Spot

Fungal leaf spot is brownish circles over the leaves. Some people just ignore it, if severe you can spray with fungicide. Because most irises have waxy leaves a systemic fungicide may be best as complete coverage would not be required, however they are toxic. See your garden centre for advice.

Brown Tips happen over summer. Usually caused by dryness and/or salt accumulation from salty water (e.g. bore) You can trim it off if it bothers you.

Snail damage is sometimes caused by tiny white snails, producing torn looking leaves. Some cultivars seem to have less waxy coating and are more susceptible.

Pineappling - is when the leaves grow short, tight and sort of scrunched. It is caused by climate change, often worse on new stock or when the weather is very changeable. Will come right in time.

 

WHAT SHOULD I FERTILIZE WITH, AND WHEN?

We recommend generally fertilizing with Rapid Raiser, Dynamic Lifter or Osmocote. However irises that like very acid soil (Louisiana, PCs and Japanese) are best fertilized with aged cow manure (the most acid animal manure) or azalea and camellia fertilizer.

It is best to fertilize when transplanting and then at the beginning of the growth period, a couple of months before flowering.

WHAT SORT OF SOIL AND CLIMATE  DO IRISES LIKE?

Bearded Iris and Spuria - neutral to alkaline soil is best, needs to be well drained. If extremely acid may benefit from a little lime. Best suited to climates with relatively dry summers and cool to cold winters.

Louisiana - need damp - wet acid soil. If your soil is neutral or alkaline then plant in very large pots with a mix of old cow manure and acidic potting mix. Immerse pots two thirds in water. So long as adequate summer moisture is provided, climate is not a problem. One of the few irises to do well in tropical areas.

Siberian Iris - need damp- boggy soil, preferably fairly neutral.  They need frosty winters to flower properly.

Pacific Coast Iris - need acid soil. Better in areas with relatively dry summers and cool damp winters. Do well in largish pots of acidic soil.

Japanese Iris - need acid soil. Should not to be immersed in water through winter and need good summer moisture. Need some winter cooling.

 

HOW LONG CAN MY IRISES STAY OUT OF THE GROUND?

Bearded irises can survive out of the ground for several months, if stored in a cool spot. Soak for an a couple of hours before planting.

Others are best replanted as soon as possible, although they will survive a while if kept cool and damp. Louisiana irises can be stored in shallow water for a few weeks.

Article - courtesy of Impressive Irises

 

 


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Last modified: 30/4/2010.