Growing up to 10 feet tall, kaffir lime trees (Citrus hystrix), also known as kieffer limes, have the same fragrant flowers and shiny, evergreen leaves as their citrus cousins, oranges, grapefruit and tangerines. The tree's dense shape makes it a good plant for a container on your deck or patio, where you can pluck the small bumpy limes and use the leaves in Thai or Cambodian cuisine. Take care where you place the container though -- kaffir lime trees have sharp thorns, so make sure the kids stay away.
Test the soil where you plan to plant the kaffir tree to ensure that the soil pH is somewhat acidic, between 5.6 and 6.0. Soil test kits are sold at nurseries and home stores.
Dig in 2 to 5 pounds of elemental sulfur per each 100 square feet of soil to lower the soil pH, making it more acidic. The best time to do this is in the fall, about three months before planting. Test the soil again to see if the pH has fallen.
Conduct a drainage test to ensure that the planting soil is well-draining by digging a 2-foot hole, filling it with water, letting it drain and filling it again. If the hole drains within a few hours after the second filling, it's well-draining. If not, add coarse sand, sphagnum peat moss, wood chips or a soil mix for cactus or succulents that is specially formulated for quick draining.
Choose a site for your tree where it can reach its full 10-foot height and width without too much pruning to keep it smaller and where the tree will get direct sun for at least six hours per day. Kaffir limes also need plenty of heat to ripen fully and are sensitive to frost, so select a protected site against a south-facing wall, if possible.
Plant your tree in the spring after all danger of frost has passed or anytime between October through February if you are able to provide frost protection, because kaffir limes are cold-sensitive. A tree planted in the fall has more time to develop roots that will help it get enough water during hot summer months if you live in a very warm climate.
Dig a hole three to five times wider than the container your tree comes in and deep enough so the crown of the tree, the spot where the trunk meets the roots, is at surface level, not below. You don't need to add any fertilizer when planting.
Place the tree in the hole and backfill it with dirt, pressing the soil firmly every few inches, but not packing the soil down tightly. Water thoroughly with at least 2 inches of water.
Lay 2 to 3 inches of organic mulch, such as compost, over the soil to help it retain moisture and to protect the tree's shallow root system. If you live in a cool climate, use pebbles or gravel as mulch to increase the heat around the tree.
Cover your tree with a frost blanket whenever frosts are predicted. Move the tree's container indoors if you planted in the fall or if early spring frosts are predicted. Make sure the frost blanket reaches all the way to the ground and is secure so cold air is not able to seep in. Your kaffir tree needs protection anytime the temperature drops below 32 degrees Fahrenheit.
Water your newly planted tree every two to three days for the first month and every three to five days for the next three months. Young trees from four months to two years old need water weekly. Water only when the top 4 to 6 inches of the soil dries between waterings.
Fertilize the tree after it begins to show new growth by applying 2 ounces of nitrogen during the growing season, divided into several feedings. Apply 4 ounces when the tree is 1 to 3 years old, also dividing it during the growing season. If your tree is planted in a container, apply about one-third of these amounts.