There are more than 50 varieties of Kowhai (Sophora species) trees in the world. Most varieties have fern-like foliage, bright yellow blooms, and long wavy seedpods. Kowhai trees prefer open spaces and full sun. They grow in a variety of soil types, as long as the soil is moist. Some varieties of Kowhai trees are sensitive to frost. Kowhai trees can be grown from seeds. For best results, start seeds in a greenhouse in late winter. The seeds take up to four weeks to germinate. Trees grown from seed may take up to 30 years to bloom, depending on the variety.
Buy Kowhai seeds from a reputable supplier. See Resources for an online supplier. Keep the seeds in a cool dry place until you are ready to plant them.
Use a knife or file to nick the hard outer shell of the seeds. Place the seeds in a bowl and cover with warm water. Let the seeds soak for 12 to 24 hours.
Fill small pots to within 1 inch of the top with potting soil. Place a seed on top of the soil and lightly cover with more soil. Press the soil down firmly with your fingertips. Water thoroughly with a watering can or water hose.
Transplant the seedlings to larger pots when 3 or 4 leaves appear. Fill the pot halfway to three-quarters with potting soil. Set a seedling in the soil and add more soil around the rootball. Do not mound the soil up around the stem of the plant. Press the soil down firmly with your fingertips and water well with a watering can or water hose.
Continue growing the trees in the greenhouse for 2 years. Transplant the trees to larger pots whenever the rootballs get too big for the current pots. Follow the directions in Step 4 for transplanting the trees.
Move the trees outside to a semi-shady place in the spring after the predicted date of the last frost. This will allow the trees to harden off before being planted in their permanent locations.
Plant the trees outside in late summer or fall. Choose a place in full sun with moist soil for the trees. Use a shovel to dig a hole deep and wide enough to hold the rootball of a tree. Set the tree in the hole and push the soil into the hole around the rootball with your hands. Tamp the soil firmly around the tree without mounding the soil up on the tree trunk. Spread the excess soil evenly around the tree. Water thoroughly with a watering can or water hose.