How to grow irises for wet or moist soil
Early summer has arrived when you see the cheery yellow, violets and blues of water irises. Some species just need moist soil – a pond isn't vital.
- Easy to grow
- Completely hardy
- Flowers in May and June
- Thrives on the edge of a pond or soil that is constantly moist
- Water irises love a rich soil
- Divide every three or four years
- Plant any time from March to mid-November
All you need to know
Choosing an iris for wet or moist soil
The main species of iris that like moist soil are Iris laevigata, Iris ensata and Iris pseudacorus. They are all sold either as bare-rooted plants or potted in compost.
- Iris ensata needs rich, acid soil, but does not grow in standing water
- Iris laevigata prefers an acid to
and can grow in pond margins neutral soil
The acidity/alkalinity of soil is measured on a pH scale, which runs from 0 to 14. Soil classed as neutral has a pH between 6.5-7.0, even though a pH below 7 is still technically acid, because the widest choice of plants grow in this range. You can easily check your soil pH with a simple testing kit, widely available in garden centres or online. Many plants thrive in neutral soil, except those than need notably acid or alkaline conditions.
- Iris pseudacorus grows best in acid soil, but will tolerate a neutral
and can grow in pond margins. It is very vigorous pH
This describes how acid or alkaline soil is, and is expressed as a number between 0 and 14. Soil with a pH of 7 is neutral, an acid soil has a lower value, and an alkaline soil has a higher one. Most plants grow best in soil that’s within a specific pH range, so it’s useful to know your soil’s pH. Testing kits are easy to use and widely available in garden centres and online.
There are variegated versions of these irises, which may add an extra colour dimension and extend the interest in your garden.
It isn’t always easy to find a good selection in garden centres, so consider visiting a specialist aquatic nursery or buy online using our Find a Plant.
When to plant irises for wet or moist soil
April is the best time to plant, as their roots will have a full growing season to get established. However, you can plant from March to mid-November as long as you water to keep their roots moist while they establish – thankfully, if planted on the margins of a pond, this won't be needed anyway.
Where to plant
Bog gardens, the edge of a pond or a damp border, are good places to plant water irises. They all need a sunny spot and planting 50cm (20in) apart. They are hardy and will be happy in all UK climates.
How to plant
If you’re planting Iris laevigata or Iris pseudacorus into shallow water, use aquatic soil and baskets. This can help prevent the irises from becoming too invasive, and the baskets certainly make it easier to lift and divide clumps compared to those planted in the mud at the base of a pond near the water's edge.
If you’re planting into moist borders, plant as you would any other herbaceous perennial.
- Plant Iris laevigata and Iris pseudacorus rhizomes level with the water surface
- Make sure that Iris ensata crowns are not submerged in water
Caring for older plants
The main job will be dividing congested clumps of irises. You can do this summer if they are taking up to much space or when flowering declines. Usually, this is done every three years or four years.
Pruning for wet or moist soil
You can cut away the spent flower stems once the flowers have faded. Remove the stem down to where it meets the foliage.
By November all the foliage and old flower stalks will die off, so all you need do is cut them off at ground level (or just above the new shoots if this task gets left to late winter).
The easiest way to share irises with your friends is by division in summer after flowering.
- Lift the rhizomes (swollen root-like structures that are actually adapted stems) and shake off the soil
- Use a clean knife to cut healthy young rhizomes into short sections of about 10-15cm (4-6in)
- Try to make sure that each section has roots attached and one ‘fan’ of leaves
- Throw away any rooting pieces with holes in them, or no roots or foliage
- Trim the roots by up to one third and trim the leaves down to 15cm (6in) (naturally giving you a fan shape)
- Plant the rhizomes 3-5cm (1¼-2in) deep, in the bog garden, moist border or basket if you are planting into a pond
- Be sure the new divisions don’t dry out
You should have some flower the following year, but the performance will be even better in the second year after division.
For a more challenging project, you could try growing irises for wet or moist soil, from seed.
- Collect the seed pods when they are ripe in autumn
- When ripe, the pods will change colour to brown and start to split open. Shake the seed into a paper bag
- Soak the seed for 48 hours and sow on the surface of John Innes Seed Compost mixed with 25% extra grit by volume. Cover the seed with a thin layer of grit
- Place the seed tray/pot in a cold frame or at the base of a wall for shelter
- The seed should germinate in spring, but the seeds may not come up all at once. Don’t give up –more seed may germinate in the second year
- Seedlings from named cultivars do not ‘come true’ and will produce a range of flower colours
- They take two years to reach flowering size
Irises for wet or moist soil don’t generally suffer from pests and diseases.
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