I'm wanting to plant a blueberry bush now so hopefully It'll be established by next spring. I don't know if it's good to plant them in July. I'm in zone 7 (Northern Mississippi). Does anyone know if this is a good idea?
You can plant potted blueberry plants in July, but make sure the following conditions are met for best results:
Shade. When you plant a new plant in summer, especially a shallow rooted plant like blueberry, provide a light shade on extremely hot, sunny days to prevent drying and sunburn on the new plants. This is only necessary during the first year when the plants are still becoming established. You can use a light row cover, or a small frame covered in cheesecloth (or similar), or wooden slats.
Water. Blueberry plants have shallow roots, and prefer light soil. This combination calls for regular watering in dry periods. During the first year, don't let the ground dry completely between watering. A two to four inch acidic organic mulch like sawdust or shredded leaves will greatly increase moisture retention in the soil, and improve the texture over time, as they decompose.
Soil pH. This is important for success with blueberries. They prefer a pH of between 4.1 and 5.3. Above that, you are going to start having problems. If you don't know what your soil pH is, you can send your local extension office a sample and get accurate results, or you can buy a do-it-yourself kit. You can use aluminium sulphate to lower the pH in your soil, if it is too high.
Organic matter in the soil is one thing that blueberry plants really struggle without. Most soils are less than ideal in organic matter content, so adding more will help a lot. You can use milled sphagnum peat moss (also useful because of its acidic qualities) or coir (ground coconut husk), and and mix them half and half with the native soil at least 10-12 inches down, and at least two feet out from the plant all around. You will want to test for the pH after this step. Now, if you have enough compost to do this step, that would be the best, because compost adds a lot more than just organic matter.
Fertilizer. At this time of year, you don't want the plant to put out much top growth, so it can completely harden off before winter, but you do want as much root growth as possible. This means you should find a fertilizer high in phosphorus, and low in nitrogen. Something like 5-10-5 is good. If you can find something natural, that's always a plus.
Make sure the top of the root ball is at the level with the soil line at planting. Like most shrubs, blueberries shouldn't be planted too deep. Dig the hole wider than the root ball, leaving plenty of room for back fill around the sides. This helps with air pockets. When it's planted, tamp firmly around the edges of the hole, but don't get carried away and smash the root ball.
In zone 7b, you should be able to choose almost any variety. The average range is 4-7. However, many people in Mississippi plant southern highbush and rabbiteye varieties, which are better adapted to a milder winter. All varieties of blueberries require certain amounts of chill hours in winter in order to flower the next spring, and although northern blueberry bushes can thrive in higher zones, they won't fruit well. A chill hour is any hour when the temperature stays below 45 degrees Fahrenheit. This site has some valuable information regarding blueberry chill hours.
Planting in early spring is preferable, but there's nothing wrong with late planting, if it's done properly. It just means a little more effort on your part.
In-ground vs. container cultivation
If you've already bought the plant, definitely plant it as soon as possible. The statement about spring planting being preferable assumes you want the plant in the ground.
Only small varieties are suitable for growing in pots, and the pots have to be fairly large. The plants will have to come out of their nursery pots either way.
How to properly plant/care for a container-grown blueberry bush
Blueberry bushes grown in pots require similar car to those in the ground, but ther are some big differences. Below is some information solely for container-grown blueberries.
Variety. Because a container will greatly restrict root growth on a normal blueberry variety, best results are gained when you use a dwarf variety. Top Hat and Dwarf Northsky are some good choices, with Top Hat being the most popular blueberry for growing in containers.
Growing medium. This needs to be free-draining and acidic. Here is a recipe I have used in the past with good results: 5/8 topsoil, 2/8 peat moss, 1/16 vermiculite, and 1/16 coarse sand. (See here.) If your native topsoil is extremely poor, or low in organic matter, use more peat moss and less soil. It is good to have some mineral based soil for the plants to use.
Container. Blueberries don't like cramped roots, so if you get a Top Hat blueberry bush, you shouldn't use anything smaller than 18" in diameter and deep for a mature plant. Blueberries prefer to root shallowly, so if you go by soil volume, a more shallow container is better than a deeper one. Preferably, use a container about 20-24 inches wide, by about a foot deep. The container must have plenty of drainage holes.
Watering. One big mistake many people make who try this for the first time is overwatering. Blueberries don't like wet feet. On the other hand, they like to stay moist. You should wait until the top couple inches of soil is fairly dry before watering. Some people put a small portal in the side of the pot so they can open it lower to test soil moisture. Do not water if it rains frequently enough to keep the soil moist.
Soil pH. Same as for ground planted plants. See above.
Fertilizer. Because nutrients leach constantly in a container, best results are gained by an extended-release fertilizer, so that the plants have a steady supply of nutrients. A micro-nutrient/mineral mix will also improve results.
Light. Blueberries like at least six hours of uninterrupted sunlight per day. Less than that and the fruit production will drop. If your container is black, consider putting a decorative covering on the sides to keep the pot from overheating. Blueberry roots prefer not to get too hot.
Repotting. This generally doesn't have to be done if you replace the top 1-3" layer of soil every few years. If the plant does become root-bound, carefully remove the plant from it's pot, carefully remove the dirt off the roots, and cleanly cut the longest roots back by about 1/3. Repot in new soil as soon as possible, and make sure the roots are well spread, and there are no air pockets in the soil. Water well. Prune about 1/3 of the top growth, to balance the plant. Try to take out mostly old, weak, or crossed branches.
I would recommend waiting until Fall, when the weather begins to cool. Planting in the summer is problematic because high temperatures cause a high rate of water transpiration through the leaves. Fall planting gives the plants time to establish themselves before winter, and you don't have to be quite as zealous about watering. This summer is shaping up to be quite dry here in the southeast.