The three most important ingredients for growing healthy and vigorous tall bearded irises are sun, good drainage, and soil. Providing favorable conditions for your irises will make them much more resistant to diseases and pests. Now is a good time to start selecting the site for your iris beds or seedling beds and to begin preparing those beds to allow for a healthy population of beneficial soil microorganisms to become established because the microorganisms provide most of the nutrition for your plants.
Select a sunny location with good air circulation.
Heavy soils that consist of a large proportion of clay particles retain a good deal more moisture between rainfalls than sandy or silty soils. To improve drainage of clay soils, dig in coarse sand, grit, or gravel. If it’s not possible to amend the soil for drainage or avoid low spots, make raised beds about 8 inches above the natural soil level.
The health of your soil will determine the health of your plants. Healthy soils support established populations of beneficial soil organisms. Two important factors determine if your soil will sustain soil organisms. (1) Soil organisms need oxygen, so your soil must have a loose enough texture to provide aeration. (2) Soil organisms need food in the form of naturally occurring organic matter or compost. Compost helps to loosen the soil texture and provides food for soil organisms. The major plant nutrients are nitrogen, phosphate, and potash. When a healthy soil consists of well-established populations of well-fed microorganisms, they extract nitrogen from the air and slowly and evenly release phosphate and potash from decaying organic matter. Phosphate is the nutrient that irises quickly deplete from soil, and it is the most important nutrient for bearded iris culture. Colloidal phosphate may be added to soils deficient in phosphate. Soils containing well-established populations of both microorganisms and earthworms will have sufficient nitrogen for irises. Most soils provide sufficient potash for irises.
If using animal manure, sawdust, bark, or leaf mold to amend soil, they must be well-rotted for at least a year. Spread 4 to 6 inches of compost on the bed and dig it in deep, even deep double digging, because iris roots will go very deep. Continue the mixing process by occasionally tilling and raking the bed for the next few months. To maintain soil improvements, organic materials must be added regularly.