FREQUENTLY ASKED QUESTIONS
Here we answer some of the most commonly asked questions about growing irises. Just click on the question below to go to the answer.
Wondering where you can buy this iris?
You can't. Unless you are talking about buying a photoshopped image of an iris. This iris does not exist.
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 fertiliser can cause lush plants with little or no flowers. Bearded iris like good supplies of phosphorous and potassium, and less nitrogen. We recommend pelletised organic fertilisers, alternatively use slow release products.
4 - Needs Dividing. Irises can exhaust the soil after a number of years, or grow too dense. Depending on the cultivar they will need dividing every 2-5 years. They should be moved to another spot or else have their soil refreshed. (See Dividing your irises for more information)
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.
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.
MY IRISES HAVEN'T BLOOMED, WHY?
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, but it won't damage the plant and they should flower the next spring. Moving during summer is possible, 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.
For more information go to 'Dividing your irises'
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. If planted too deep 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 they are being 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, which is important to reduce the risk of rot. The remaining leaves are feeding the rhizomes for next spring.
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 fertiliser 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 IRISES?
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 (not recommended), 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. For irises in pots you may water them more often in very dry weather. (Note: It is risky watering in heat wave conditions as rot can set in quickly).
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.
Pacific Coast irises, should never dry out completely. Siberian and Japanese irises will survive moderate dryness once dormant, but must kept moist in spring and during flowering.
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 be speckled. Damage may persist for several weeks after the frost. Some cultivars will be damaged more than others and damage will be worse in the more exposed, lower parts of the garden.
Weed Killer Damage - Glyphosate type sprays (e.g. 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.
TB iris 'Holey Cheeses' showing a glyphosate damaged flower
TB iris 'Holey Cheeses' undamaged flower
WHAT SHOULD I FERTILISE WITH, AND WHEN?
It is often recommended to fertilise with a good balanced organic fertiliser. Generally a nitrogen: phosphorous: potassium (NPK) ratio of 6:10:10 is recommended, but many have success with a ratio of 10:10:10. If phosphorous levels are low a fertiliser with high levels of phosphorous can be used.
Irises that like very acid soil (Louisiana, PCIs and Japanese) are best fertilised with aged cow manure (the most acid animal manure) or azalea and camellia fertiliser.
In addition to considering NPK fertiliser, trace elements may be required. If in doubt, obtain a soil test.
It is best time to fertilise is when transplanting and then at the beginning of the growth period, a couple of months before flowering. Many iris growers fertilise in April (a good time to replant) and August.
Remember to always read the labels when using chemicals or fertilisers.
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.
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.
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.
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. This is usually caused by dryness and/or salt accumulation from salty water (e.g. bore water). You can trim the tips off if they bother 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.
WHAT SORT OF SOIL AND CLIMATE DO IRISES LIKE?
Bearded Iris and Spuria - neutral to alkaline soil is best and it needs to be well drained. If extremely acid, it may benefit from a little lime, but generally these iris do well in almost any soil. 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. Lousianas are one of the few irises that do well in tropical areas. They will also grow in cold areas, however if exposed to cold winds during spring the flower stems may be shorter than usual.
Siberian Iris - need damp boggy soil, preferably fairly neutral. They need frosty winters to flower properly.
Pacific Coast Iris - need acid soil. They grow better in areas with relatively dry summers and cool damp winters. They do well in large pots of acidic soil.
Japanese Iris - need acid soil. They should not to be immersed in water through winter and need good summer moisture. They 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 dry spot. Soak for a couple of hours or over night before re-planting. Many people recommend soaking in a seaweed solution.
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. Pacific Coast Iris are particularly fussy and can not be left out of the ground.
MY IRISES HAVE ALL REVERTED TO THE ONE COLOUR. WHY?
Your clump may have had purple and white (for example) but you now only have purple blooming. This does not mean that they have changed colour. Iris do not change colour. Your apparent "colour change" is likely to be due to one variety dominating, meaning the other is not getting enough nutrient (or light) to bloom.
See also below "An iris has appeared in a colour that I didn't plant".
AN IRIS HAS APPEARED IN A COLOUR THAT I DIDN'T PLANT, WHY?
Sometimes a colour pops up that you didn't plant - or rather, that you didn't intentionally plant! This could be due to:
- incomplete removal of a previous clump from that area - even a small amount of rhizome left in the soil can multiply and bloom;
- a mislabelled iris was purchased - you asked for all purple but received something different. It may be that the person you purchased from had a small amount of another variety in that area (see above, a small piece of rhizome left in the ground accidentally can become a clump!); the grower had one variety growing close to another, or perhaps the seller just got the rhizomes muddled!
CAN I GROW IRISES FROM SEED?
Iris can be grown from seed, but the seed will not be true to the parent plant. You will often see iris seeds advertised as being a specific colour. They will not be black irises. The only way to be sure of what you are getting is when it is a rhizome from the parent plant. Each rhizome of a particular cultivar (e.g. a named variety) that you see has come from just one plant which has produced offsets, then those offsets have produced offsets, and so on. If you want to learn more about growing iris from seed, please see the hybridising section of this site.