With their attractive citrus-tree shape and bright yellow fruits, lemon trees (Citrus spp.) make a great addition to southern gardens. Texas covers U.S. Department of Agriculture plant hardiness zones 6 through 9, and since lemons are among the least hardy citrus, most species cannot grow outdoors year-round except in southern Texas. However, lemons can be protected from the cold -- so gardeners in northern Texas can also enjoy these trees.
The true lemon (Citrus limon) is hardy in USDA zones 9 through 11, and grows up to 25 feet tall. Popular cultivars include "Eureka" and "Lisbon." Meyer lemon (Citrus x meyeri "Improved") is more cold-hardy, and will grow in USDA zones 8 through 10; this smaller species reaches up to 10 feet tall. Both species require a full-sun location with well-drained soil. They will grow in sandy or loamy soils, but do not grow well in clay.
Meyer lemons are hardy enough to grow outdoors in the Dallas area, which falls in USDA zone 8a. True lemons are only cold-hardy along the Gulf Coast and in the interior plains of south Texas. You can, however, grow lemons in containers that are moved indoors in the winter or create micro-climates that will allow lemons to grow in a cooler zone than they normally would.
By planting lemons in a sheltered location, you can often grow them in a zone cooler than they are normally hardy. This means that Meyer lemons can be planted in USDA zone 7b, and true lemons can be planted in zone 8b. Planting in a courtyard or on the south or southeast side of a building provides good protection. Houses, driveways, patios and other hard surfaces absorb warmth during the day and release heat at night, so planting a lemon tree near these will help keep the tree warm. You can also protect young trees by draping a blanket or tarp over them before a frost.
When planting lemons in the ground, dig a hole that is the same depth as and slightly wider than the tree's pot. Space holes about 15 feet apart for true lemons and 8 feet apart for the smaller Meyer lemons. Make a small cone of soil in the bottom of the hole, to elevate the tree slightly higher than the surrounding soil. Remove the tree from its pot, and wash the root ball with a gentle stream of water to remove the outer inch of soil, exposing the roots.
Set the lemon tree in the hole. Fill in around the tree with the soil that was dug out of the hole, breaking up any clumps of soil. Do not add any fertilizer or soil amendments. Cover the top of the root ball with 1 inch of soil.
Use extra topsoil to construct a water ring around the tree. This is a ring of soil 2 feet across, and several inches high and thick. It helps keep water above the root ball so it soaks into the soil instead of running off. Fill this ring with water right after planting, and let the water soak in. This will cause the soil to settle, so fill-in any low places that appear. Water every two to three days for the first two weeks, then over the next few months gradually decrease watering to every seven to 10 days.
If you want to grow lemon trees in USDA zones where they are not hardy, you can plant them in containers and move the containers indoors to a frost-free location during the winter. For a young tree, start with a container about 15 inches wide and 15 inches deep. The container should have several drainage holes, and be heavy enough that the tree won't tip over.
Regular potting soil works well, or you can make your own citrus potting mix by combining 1 part each clean sand, peat and pine bark. Plant the tree so that the top of the root ball is about 4 inches below the edge of the pot. Water when the top inch or two of soil is dry to the touch, then supply enough water to thoroughly soak the soil. Every few years, when the tree roots have filled the pot, move the tree to a pot 25 percent larger than the last one. If you're happy with the size of the tree, simply remove the tree from the pot, prune off about one-quarter of the root system, and then re-pot it like a new tree.
Both in-ground and container-grown lemon trees benefit from applications of a citrus fertilizer. Citrus fertilizers contain trace nutrients like boron, copper, magnesium and manganese, as well as more common nutrients like nitrogen, phosphorus and potassium.
For in-ground plants, apply a slow-release granular fertilizer with an N-P-K ratio of 13-7-13 every three months. Application rates vary depending on the size of the tree, so follow label directions closely. For example, a tree 2 to 3 feet tall needs 0.75 pounds of fertilizer in each application, while a tree 5 to 6 feet tall needs 2.25 pounds.
For potted citrus, use a slow-release citrus food formulated for indoor plants. One with an N-P-K ratio of 5-2-6 is applied at a rate of 1 teaspoon for every 4 inches of pot diameter. Double this ratio for pots over 12 inches in diameter. Mix the fertilizer into the soil three times a year, some time in January or February, May or June and September or October.