Long-stemmed roses are prized at 24 inches long, but generally, any roses with 20-inch stems or longer are considered to be long-stemmed. It is not easy to grow this variety of roses outdoors, but it is quite possible in a greenhouse since you can control and maintain an environment most suitable for growing roses. Along with an ideal environment, proper planting, care and pruning is necessary to grow long-stemmed roses.
Select your roses. Just about any variety can be grown into long-stemmed roses. If you don't want to buy plants, take a 6-inch cutting from a healthy rose plant. Cut a 1-inch diagonal slit on the bottom of the stem and place it in a poly bag for a day. Dip it in some rooting hormone, available at your nursery, and it is ready to plant in a container.
Plant your roses. Transplant existing plants or plant your cutting a couple of inches into the soil. The soil should be rich and well draining. All-purpose potting soil or potting soil labeled for roses is ideal. The container should be large and have holes in the bottom for proper drainage.
Maintain your greenhouse with an ideal environment for growing roses. Keep it around 61 degrees Fahrenheit at night and 70 degrees during the day. Greenhouses generally have temperature controls that turn on and off ventilation fans to hold the correct temperatures. Keep the humidity level around 60 to 70 percent. Humidity control systems keep the level where you need it by adding humidity or dehumidifying as necessary. In addition, add supplemental carbon dioxide with a carbon dioxide generator. Without supplemental carbon dioxide, your roses probably will not grow long enough to be considered "long-stemmed."
Prune your roses. This is the key to long stems. As they grow, only let the one main bud bloom on each branch. Consistently prune off all the rest of the buds and offshoots. This main bud will continue to grow larger as the stem grows longer.
Water your roses 1 to 3 inches a week. Water a little each day or every other day, especially for the first month after planting. Do not saturate your roses at once, which could rot the roots. Water the bottom of the plant if you don't have an drip irrigation system in the morning, if possible.
Fertilize your roses when they first produce leaves and when they first produce blooms. Use a rose food fertilizer, available at your nursery, and follow dosing instructions since each brand might have different strengths and release rates.