Phormium tenax, also known as New Zealand Flax or Harakeke, is a striking evergreen perennial native to New Zealand. It is valued for its long, sword-like leaves that provide an architectural and exotic touch to the garden. Phormium tenax can be grown successfully in the UK by following these guidelines:

1. Location: Choose a sunny or partially shaded spot in your garden for your Phormium tenax. The plant will tolerate some shade but will produce the most vibrant leaf colors in a position with plenty of sunlight. Ensure the location is sheltered from strong winds to prevent damage to the leaves.

2. Soil preparation: Phormium tenax prefers well-draining soil and can tolerate a range of soil types, including loam, sand, and clay. However, it's essential to ensure good drainage to prevent waterlogging. If your soil is heavy clay or poorly drained, consider adding organic matter, such as compost or well-rotted manure, to improve drainage.

3. Planting: Dig a hole that is twice the width and depth of the root ball of your Phormium tenax. Place the plant in the hole, ensuring that the top of the root ball is level with the surrounding soil. Fill the hole with the soil mixture, gently firming it around the roots.

4. Watering: Water your Phormium tenax deeply and regularly during its first growing season to help it establish a healthy root system. After this initial period, you can reduce watering, but be sure to keep the soil consistently moist, especially during hot, dry spells.

5. Fertilizing: Apply a balanced, slow-release fertilizer in the spring, following the manufacturer's instructions. This will provide your Phormium tenax with the necessary nutrients for healthy growth. However, be cautious not to over-fertilize, as excessive nutrients can cause the leaves to lose their color.

6. Pruning: Phormium tenax requires minimal pruning. Remove any dead, damaged, or lower leaves as needed to maintain a tidy appearance. You can also remove any flower stalks after they have finished blooming to conserve the plant's energy.

7. Winter protection: Phormium tenax is relatively cold hardy (down to around -5 to -10°C), but younger plants or those in colder areas may benefit from some winter protection. Add a layer of mulch around the base of the plant to protect the roots from freezing temperatures. In particularly cold regions or during severe winters, you may also consider wrapping the plant in horticultural fleece to protect it from frost.

By following these guidelines, you can successfully grow Phormium tenax in the UK and enjoy its exotic and architectural appearance in your garden.