Answer: I will assume that your plants are healthy with lots of new leafy green growth and good roots. If you don't have a healthy plant, don't expect it to flower. Sometimes weakened plants, such as moth orchids will bloom prolifically, but this will be their swan song and they will die soon after. Grow plants right well and blooms will follow. If you're not getting blooms AND your plants are healthy, read on..
Here are the main reasons orchid will not bloom and what you can do about it. One or more reasons may apply to you:
GIVE YOUR ORCHIDS ENOUGH LIGHT Most flowering houseplants, orchids included, will not bloom if they are not getting enough light. This is generally the main reason that orchids do not bloom. Northern exposure is usually not sufficient. A bright western, slightly shaded in the hottest months, or eastern exposure work well. A southern exposure gives you the greatest flexibility. If your window is heavily shaded by trees or adjacent buildings, this will reduce light to your plants and you will have to make adjustments accordingly. If you have bright indirect light, your phals and paphs should do fine. Oncidiums generally need slightly brighter light. Dendrobiums and Cattleyas need the brightest light of alll, and do best in a Southern exposure. Skylights, unless you live in Arizona, or unless they are about five feet above the plants generally do not provide sufficient light, for all but the most shade-loving of orchids.
How do you know if your plants are getting enough light? As a general rule look at the foliage. It should be a light grassy green. Lush dark green foliage is nice to look at, but it means your plants are not getting enough light. Leaves should also be turgid and firm on phals, cattleyas, and dendrobiums. Mottled leaved paphs should show good color contrast on the leaves. If your plants are not getting enough light move them closer to the window. Yes, even a foot can make a huge difference as light falls off very rapidly within increasing distance from a window. During the summer months place plants outdoors in dappled shade if you can. Whenever you move plants into brighter conditions always do so very gradually as you risk burning the leaves. Lastly, remember to only grow plants suited to your conditions to avoid disappointment.
FEED YOUR PLANTS Most of the orchids we grow are epiphytes. Basically this means that they grow on other trees but are not parasites-they get all their nutrients from rain and wind-blow debris that lodges among their roots. When we grow these plants on our windowsill they tend to be grown in fir-bark or sphagnum peat moss. Neither of these mixes provide any nutrients. Without extra plant food or fertilizers, orchids will not grow or bloom well. The secret is to fertilize your plants regularly using a WEAK fertilizer or plant-food solution. You are more likely to kill your orchids by over than under feeding, hence the emphasis on weak. If you use a regular flowering house plant fertilizer, apply it at half the strength recommended. If you buy a special orchid fertilizer, follow the package directions. Try fertilizing your plants about twice a month. Increase feeding during the spring and summer months when orchids are growing rapidly, and reduce feeding to once a month during the winter. Be sure to rotate in a blossom-booster or flowering plant fertilizer as well. Every month, leach your plants thoroughly-water them with plain water several times and allow them to drain. This helps flush out the fertilizer salts that will build up over time. Most garden supply firms carry a variety of houseplant or orchid fertilizers-you may wish try one or two different brands to see what works best.
COOLER NIGHTS. Most orchids like a day and night temperature variation of about 8 to 10 degrees year 'round, especially to stimulate them to bloom. As night temperatures drop in the fall, or spring, place plants next to a window that is cracked at night to let in a little cool air. Give your plants this treatment for about 3 to 4 weeks and you should begin to see flower stalks emerging soon after. How low should you let the night temperature drop? For paphs and phals, temperatures of about 55 to 60 degrees should suffice. Make sure your plants are not in a cold draft, or protect them with a light curtain. Placing them in an unheated room at night will also do the trick. Remember that day temperatures need to be about ten degrees higher. A Wireless Min/Max Thermometer make this easy to track. If your orchid plants have been growing outside this summer, you can simply leave them out for a few more weeks. If temperatures threaten to fall below 55, all but the coolest growing of orchids, such as cymbidiums, should be brought in. This method generally works best with orchids that have their natural flowering season in the fall through spring months, including most phalaenopsis (moth orchids), paphiopedilum/paphs (slipper orchids) and cattleyas.
Hope this information helps you make the changes necessary to turn your non-blooming orchids into blooming plants!
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