The kowhai tree, Sophora microphylla, produces the yellow national flower of New Zealand. The deciduous tree grows up to 40 feet in height. In the Southern Hemisphere, the spring months of August and September herald the abundant blooming of the kowhai tree. Yellow, trumpet-shaped blooms appear in large hanging clusters before the tree's foliage. Pollination occurs predominantly from the tui bird.
The root system of the kowhai tree has the ability to fix nitrogen in nutrient deficient soil. Bacterial nodules on the tree's root system produce gaseous nitrogen in the surrounding soil. The soil turns it into soluble nitrates.
Abundant seeds are produced after flowering. The seeds are widely spread throughout flooding, rivers and streams. The seeds have the ability to not germinate when exposed to water. Seed dispersal also occurs from birds consuming the seeds and then eliminating them in new locations. The seeds germinate quickly, with growth appearing in 14 to 21 days.
To propagate the kowhai tree, the seed needs to be nicked with a knife or needle. Rubbing the seed with light sandpaper also works. When the outer shell of the seed has been damaged, the seed requires soaking for 24 hours in warm water before planting.
The tree occurs naturally along stream beds, in forests and beside rivers throughout New Zealand but only occurs scarcely in the Northland. The trees range occurs from 1,470 feet to sea level.
The kowhai tree can be propagated through cuttings, but rooting takes up to 100 days. When rooting occurs, the tree can easily be planted in the spring.
Both the bright-green and the black caterpillar pose serious threats to the kowhai tree. They can cause complete defoliation if treatment is not sought.