Cultivation

Unlike bearded irises, Siberian irises can be treated like any other perennial, so when a clump becomes shy flowering you should lift, divide and replant. Palm-sized clumps are best for transplanting; smaller divisions can take up to two years to reach a size where they will flower. Carefully dig up the rhizomes with a garden fork and cut down the leaves to a short fan (the roots must be kept moist while the plants are out of the ground). Plant the rhizomes at least 5cm deep (slightly deeper in sandy soils) and at 30-45cm spacing depending on desired effect. Spring is the best time to divide, with August as an alternative. If growing in well-drained soil, then water until they are established. Once their roots are down, they usually look after themselves. It’s best not to replant when frosts are likely; you can expect losses if you lift and divide during late autumn or winter.

Although cultivars can only be vegetatively propagated, it is possible to raise plants from seed, and you will get a range of interesting, colourful, good garden plants within just a few years. To start your seeds indoors, you will need to cold stratify them. First soak the seeds for three to five days, then place them in a plastic bag with moist vermiculite and pop the bag in the fridge for up to 60 days ( check the moisture level of the vermiculite periodically). After 60 days, some of your seeds may already have sprouted. The rest will germinate within days of being removed from the refrigerator, prick out when large enough to handle into rigid plug trays and on into 9cm pots. The plants will flower within three years after sowing. Alternatively, you can sow seeds in December to February and place in a cold greenhouse or polytunnel and germination should be straightforward. The flowers are hermaphroditic (meaning they have both male and female organs) and are generally pollinated by insects, although self fertilisation is possible.

Out of flowering the plant still has much to offer from its foliage and attractive seedpods. At The Courts Garden in Wiltshire I grew a whole bed of an unknown Siberian iris flanking a formal pool. I cut the foliage of these, much like one might with geraniums or aquilegias, in late June all the way to ground. Within days new leaves emerged, which remained green and fresh looking until the first frosts in October. By doing this however, I sacrificed the seedpods, now I tend to leave both the foliage and spent flower stalks until they turn brown in autumn or even the following spring when they have collapsed. From August the elongated green seedpods will begin to harden and turn first tan then brown as they ripen before slowly rupturing and ‘unzipping’ themselves from the tip down allowing the seed to disperse or be collected.