Irises will grow in any good garden soil. Choose a sunny, well-drained location, bearded irises do not like wet feet! At least 6 to 8 hours of full sun is needed for maximum bloom. Prepare the soil before planting by adding compost or aged manure and a low nitrogen fertilizer. A high-nitrogen fertilizer or young manure can lead to too much leaf growth, fewer flowers and possibly rot. The soil pH should be 6.1 to 7.2 but irises are very tolerant in this regard.
The rhizome should be planted with the top just showing, the roots spread out and firmed in well. It may be necessary to adjust the soil level after a month or two as the rhizome may sink too low or the soil may sink leaving the rhizome too exposed. Plant tall bearded iris between 20 and 35cms apart. Water well once a week until established, this may take a month or more if the weather is hot and there is no rain. The autumn is when most new root growth is made and is the best time to plant. In Mediterranean areas, planting can take place from September until January or even February if the soil is warm, but autumn planting is best. In colder northern areas or Central Spain, September planting is best so the plants can establish before the winter sets in.
Apply fertilizer about 6 weeks before bloom time (a major growth time) and again in October. Choose a fertilizer with a ratio of 5 -10 -10, for example 5 per cent nitrogen, 10 per cent phosphorus and 10 per cent potash. Do not allow the fertilizers to rest on the rhizome and water in well.
Bearded irises multiply by vegetative increase with new plants growing from the mother rhizome. There will usually be 2 or 3 increases each year from each mother rhizome. After 3 or 4 years the clumps will need to be lifted and divided. When they are too crowded flowering will be poor. This is best done in the autumn. Cut the new growth from the mother rhizome, each increase will look like the original plant. Discard the old mother plant, cut the foliage back to about 10 to 15cms (to avoid wind-rock on new plants) and re plant and/or share with your friends! In early winter irises will go dormant and the dead leaves should be gently pulled away and tidied up and the clumps kept clean to avoid any winter rot or aphids.
A sunny, well-drained location, moderate feeding and the removal of weeds and competing plant growth will provide you with a healthy growth of bearded irises that will delight you with the quality and quantity of bloom produced.
You should maintain a clean garden as problems can hide and over-winter in garden debris, however, irises are usually healthy and suffer very little from pests and diseases. Modern hybrids have very little fungal leaf spot and we do not spray against this, it is hardly noticeable in our Mediterranean climate.
We sometimes have a problem with APHIDS which can live in the leaf axils and will cause weak flowering. If you see any sign of this, spray in early spring with a good systemic product.
Occasionally we have a little bit of SOFT ROT, this can occur after heavy rain followed by hot weather. The bacteria which causes this is present in most soils and enters the iris through an injury such as snail, earwig or borer damage. Should you find this, dig the rhizome, scrape away the infected tissue and allow the rhizome to dry in the sun for a few days. Replant in another location.
We do not use week-killer anywhere near our irises as the slightest whiff of spray on a rhizome can affect the flower colours in our experience. Best practice is to hand weed!