How to Grow Dipladenia and Mandevilla
Dipladenia is actually a member of the mandevilla family. It is widely thought that mandevilla and dipladenia are the same plant, but while similar, dipladenia foliage is somewhat smaller and the plant is more shrub-like.
Mandevillas will require some special attention because they cannot remain outdoors all year round in most parts of the country. When planting in containers or in a garden outdoors, use a rich soil mixture of sand and humus and ensure good drainage. A container with a hole in the bottom and a trellis, frame or stake for support is the best situation for this vine. The North Carolina State University Cooperative Extension says mandevillas need 6 full hours of direct sunlight as an outdoor plant.
However, both plants have the same care and growth requirements and are very similar. These gorgeous, vining plants have soared in popularity and can be used in mixed containers, hanging baskets, or on their own in a container. They are easy to grow and should flower their heads off all season long.
Mandevilla and dipladenia look great on their own in a pot or as the centerpiece of a mixed container. I like to grow them in a large and have them climb up a trellis, obelisk, trellis. I like to surround with a contrasting annual or foliage plant. I have also planted them in pots with oregano and coleus.
Mandevilla's care requirements are similar to hibiscus and will flower best in full sun, but will tolerate part shade. However, if you live in a really hot area, mid-day shade is recommended.
Drainage and Watering - Unlike many flowering plants, dipladenia or mandevilla will tolerate some dryness and continue to flower. That said, they prefer a consistent level of moisture and you should try to keep the soil damp, not wet. When watering, make sure to water slowly to give the soil time to soak up the moisture. When watering with a hose, spray the leaves too. Also, make sure that your pot has good drainage and that you use a good quality potting mix.
Pot Size - For consistent production of flowers, don't transplant your dipladenia into too large a container. If you do, it won't hurt your plant, but it will spend more energy producing roots and top growth than flowers, so you may see fewer flowers until the roots have hit the bottom of the pot. If your plant is root bound and does need a bigger pot, look for one that is wider, but not much deeper.
Fertilizer - Most plants you buy at the nursery have a slow release fertilizer already in the soil so you probably don't have to worry about feeding your plant for the first few months. After that you will need to fertilize it regularly. You can either use a diluted, plant food every other week or add a slow release fertilizer to your soil. Always follow directions on the package.
Overwintering Dipladenia - If you live in a cold climate (anything lower than zones 9-11), it is possible to overwinter dipladenia indoors. Take in your plant before evening temperatures dip below 50 °F. Put it in a place with as much direct sun as you can, though it may even survive if you can provide lots of indirect light. Dipladenia doesn't like the cold so protect it from drafts. Don't be alarmed if your plant doesn't flower or sheds some leaves in the winter. In the fall, you may see long shoots or sprouts, which you can trim back lightly or train onto a trellis or support. Stop feeding through the winter. Though plants generally need less water in the winter, central heating can dry the air and your plant very quickly. Keep your plant on the dry side during the cold months, but make sure it doesn't dry out too much. In the spring, increase watering and resume fertilizing. Don't cut the plant back, or you will miss out on next seasons flowers. You can put your plant outside once the nighttime temperatures are consistently above 50°F.
Diseases - Red spider mites, mealy bugs, fulsarium and cercospora are the most common problems you may run into.
Information gathered partially from Ask.com