The cultivation of Irises 
15 
The cultivation of Irises. 
Although hints as to the cultivation of the Irises of the various groups will be found in the 
introductory remarks on each group, and special directions as to individual species among the 
observations with which the account of each closes, it may be not inadvisable to make here some 
suggestions as to the cultivation of Irises in general. 
Perhaps the most important point of all is that Irises are sun-loving plants. There are some 
few, such as /. foetidissima and certain Evansia species (see pp. 51, 98), that will thrive in half 
shady positions and others such as the common so-called German Irises which will continue to live 
where they are overshadowed and get little sun. But these latter will produce few flowers and 
probably dwindle gradually away. It should be remembered that, even though a species may thrive 
in partial shade in its native home, yet our summers in England are seldom sufficiently hot to 
ripen rhizomes growing in such a position here. 
With regard to soil, the general rule seems to hold good that the Apogon species thrive best 
in a moist alluvial soil that is rich in humus, while the Pogoniris group do best in heavy limestone 
soil, provided that the drainage is good and sunshine abundant. 
Few Irises enjoy wet conditions in winter, for growth is then inactive, but all appreciate 
moisture in spring when the flower-stems and foliage are developing rapidly. When the flowers 
are over, most Irises appreciate a period of rest, which is perhaps best provided by withholding 
the water supply. This does not apply to some Apogon Irises and the sibirica group, especially, 
is an exception to the rule. Other Irises that do not require a dry resting period are Kaempferi , 
laevigata , setosa, versicolor , pseudacorus, and although the large Spuria group is able to dispense 
with it there is little doubt that the marshy ground, in which they grow in the wild state, is 
baked dry by the sun long before the summer is over. 
In several cases Irises, which in their native homes are bog plants, will not flower under 
these conditions in England and must be given comparatively dry positions. Thus /. fnlva and 
I. hexagona come from the swamps of the South-Eastern United States, but they will only flower 
here when grown in warm and comparatively dry positions. 
Nature has provided us with one infallible sign, which will show us whether an Iris is a native 
of a dry or a wet soil. This will be seen if leaves of /. pseudacorus or /. versicolor are held 
up to the light side by side with a leaf of a Pogoniris, for instance, of I. germanica. The latter 
will appear of a uniform green but the former will show a number of minute blackish spots, which 
on microscopical examination prove to be due to the fact that at these points the vertical channels 
in the tissue of the leaves are blocked by growths of apparently the same structure as that which 
surrounds the passages. The increased thickness of the structure at these points produces the 
appearance of the black spots. In dealing with new plants this character is often of great use 
and gives us a guide as to the choice of soil and position, for the presence of these minute black 
spots always denotes a plant that is a native of marshy ground. 
Two mistakes are very commonly made in the cultivation of Irises; they are transplanted in 
late autumn and they are allowed to become overgrown from the height of summer onwards by 
carpeting or straggling plants. When either of these mistakes is made, the flowers will be few 
in the following year. 
In the first place, autumnal transplantation is harmful to the plant because by that time the 
main roots have sent out their lateral branchlets which are broken and maimed when torn up and 
never afterwards take any firm hold of the ground. A little observation will show that the main 
root-fibres grow down into the ground to their full length unbranched*. If therefore a rhizome is 
lifted when the primary roots are not more than an inch or two in length, no harm will be done 
if it is planted again carefully. It is obviously, however, even better to transplant before the new 
roots emerge at all from the rhizome and, in order to find them in this stage, the operation must 
be carried out immediately the flowers are over. The new growths will then develop and mature 
for the next year without the fatal disturbance of autumnal transplantation. 
In the second place, the fact that Iris rhizomes grow on the surface of the ground* seems to show 
that sunshine is necessary to their welfare. Whatever the precise effect of the sun may be, the fact 
remains that rhizomes that are smothered under the growth of other plants are apt to remain flowerless 
in the following year. 
Another common garden practice that is detrimental to Irises is that of trimming off the leaves 
in late summer and autumn. No plant can assimilate the sap that rises through its roots until it has 
passed through the leaves, and it is therefore obvious that robust, sound rhizomes will not be formed, 
if the leaves are trimmed off while they are still green. It is only when they turn brown and come 
away easily at the base that their removal is a benefit to the plant, for they then only serve to 
harbour moisture and slugs. 
1 See Plate XLVII. 
a I have known instances where rhizomes of large Pogoniris that have been accidentally buried under some inches of earth 
have extended by means of a long narrow stolon-like growth until the surface was reached, when the stolon has expanded again 
into a thick rhizome. 
