With their showy flowers and low-maintenance needs, bottlebrush trees (Callistemon spp.) are often chosen in warm growing zones for hedges or statement plants. They grow best in full sun and soil with low alkalinity, but other than that aren't very picky about where you plant them.
Bottlebrush thrive in a wide variety of soil conditions, except for highly alkaline soil. They will tolerate slightly alkaline soils, but high alkalinity results in chlorosis, which turns the plant's leaves yellow. Soils with a pH higher than 7.5 have moderate or high alkalinity. You should test the soil pH before planting bottlebrush plants.
You can stabilize the pH of some soil types, but there is no way to permanently lower the alkalinity of soils with a high concentration of lime. Test lime content by placing about 1 tablespoon of dry soil in a cup and moistening it with white vinegar. If the soil bubbles, it contains too much lime to amend the soil sufficiently. If it does not bubble, regularly adding organic amendments can decrease the alkalinity enough to grow bottlebrushes.
Amend soils before planting with an organic amendment low in mineral salts, such as plant-based composts and sphagnum peat moss. This will help stabilize the soil pH and improve soil texture, even in soils that do not test as alkaline. Spread a layer of amendment 2 to 3 inches deep over the planting surface, and work it in to a depth of 6 to 8 inches. If you're using organic matter to lower soil pH, apply this same amount each year for the next three years, then apply 1 to 2 inches each year after that.
Dig a planting hole 1 to 2 inches shallower than the rootball and three times as wide. This prevents trees from settling too deep in the soil after planting and provides loose soil on either side for new roots to grow. Remove the container and set the tree in the hole, then fill in around the plant with the same soil you dug out of the planting hole. When you're done, you want the tree at the same depth it was previously growing or slightly higher than the surrounding soil, with the soil sloped up to meet the top of the rootball.
When digging planting holes for multiple trees, space far enough apart to allow for the tree's mature size. For instance, red bottlebrush (Callistemon citrinus, U.S. Department of Agriculture plant hardiness zones 9a through 11) grows 10 to 15 feet tall and wide, and weeping bottlebrush (Callistemon viminalis, USDA zones 9b through 11) grows 15 to 20 feet. Spacing them 4 to 9 feet apart gives them room to grow with some overlap. Dwarf bottlebrush (Callistemon citrinus 'Little John,' USDA zones 8 through 11) grows only 5 feet wide so you can plant them 2 feet apart.
In their preferred growing zones, bottlebrush can be planted year-round. You'll just have to make sure that you can supply adequate water for the new roots. In Florida, for example, it's easier to plant during the cooler fall, winter and spring than trying to supply enough water to keep the soil moist during hot summers.
Right after planting, water thoroughly to settle the soil. If the soil level drops after watering, add more soil and water again. For the first year after planting, water regularly to keep the soil moist. Bottlebrush are native to damp locations and are more likely to establish deep root systems if you give them plenty of water.
After they are well-established, most bottlebrush varieties are drought tolerant and need very little supplemental watering. The exception is weeping bottlebrush, which requires plenty of water to keep the soil moist.
To keep them growing and flowering well, fertilize bottlebrush with a low-phosphorous fertilizer. Phosphorous is represented by the middle number in a fertilizer's N-P-K ratio, so look for fertilizers with a lower number in the middle. For example, apply a dry slow-release fertilizer with an N-P-K ratio of 12-4-8 by scattering 1 tablespoon per square foot of soil surface. Do this twice a year, once in the spring and once in the fall.