Permaculture Plants – Australian Indigo, the Nitrogen Fixing Tree for Small Sites, an Alternative to Tagasaste for Urban Gardens

In permaculture, tagasaste (tree lucerne) is used as a nitrogen-fixing tree on large sites and rural properties, but it can grow around 5-7m tall and equally wide, which is way too large for smaller urban properties. Luckily for Australian permaculture gardeners, we have a much smaller nitrogen-fixing tree available which works much better in small-scale gardens.

Indigofera australis, also known as Australian Indigo or Austral Indigo, is an attractive ornamental native Australian evergreen shrub varying in size and  habit, growing 1-2m tall and equally wide, more typically closer to 2m tall and wide in size. It’s also from the Fabaceae (legume) family, just like tagasaste, and is a nitrogen-fixing plant, which can be used to increase soil fertility, and as a nurse crop tree to protect growing young trees.

The delicate looking pinnate blue-green foliage can be lightly pruned to create a showy specimen or feature plant in a garden. During spring, bright pink to purple flowers are produced in long spires, which are a good nectar source and attract bees, butterflies, nectar-eating birds and other beneficial insects.

The leaves and stems can be used for making natural dyes, giving a range of colours from soft yellows through to light blues, depending on the plant growing conditions and the dyeing technique used.

The indigenous people of Australia used the crushed leaves of Indigofera australis to throw into the water to kill or stun fish and eels, and also placed roots which were hammered in salt or fresh water to poison fish.

This shrub can be used as a low windbreak, and it will tolerate extended wet periods and saline soils.

This Australian native shrub is fairly hardy, being reasonably drought tolerant and able to handle light to moderate frosts. It will grow in full sun or light shade in most soil types, ranging from sandy soil to clay loam. It is adaptable to a wide range of climates, from sub-tropical, through to temperate and cool and even semi-arid. It’s best pruned lightly, don’t cut into old wood as it won’t regrow. It can be propagated from seed, division or by taking semi-hardwood cuttings.

Indigofera australis is capable of regenerating after fire, which breaks the dormancy of seeds in the soil, but it can also regrow by suckering from the roots that stay alive below the ground.

It’s easy to grow, and can be grown amongst fruit trees without any problem., I have mine growing in a food forest garden between two young dwarf citrus trees and an apple tree, in a full-sun location, and it flowers prolifically, attracting lots of bees. It’s not phosphorus sensitive like some Australian native plants (such as those from the Proteaceae family) so it doesn’t need any special care or native fertiliser formulations.

Propagating Indigofera australis

Propagation of Indigofera australis is done from seed, but the seeds have a thick seed coat and seed treatment is required to break the physical dormancy.

Two methods of seeds treatment are described as follows:

1. Scarifying – the seeds can be scarified by contact with an abrasive surface, such as fine sandpaper.
   
2. Hot water treatment – this involves dropping the seeds into a container, filling it with boiling water, then and allowing the seeds to soak in the water overnight as it cools. Seeds that swell larger than their original size can be sown, while seeds which stay floating can be discarded as they’re usually infertile.

After sowing the seeds, germination should take around 3-4 weeks. The season to sow seeds is in late winter to early spring.

Indigofera australis can also be propagated from cuttings, and being evergreen would be propagated using softwood cuttings in early spring.