September 2009 - Changing Light in Fall Effects Blooming and Growing

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A monthly growers advice column by Courtney Hackney.

Fall in the tropics can mean many things depending on where in the tropics one looks. There may be more rain, less rain, clear skies, and even cool nights.  But there is one universal facet to fall everywhere, shortening day length.   Here, as in the tropics, many species of orchids and their hybrids are reacting to shorter days.  Fall blooming cattleyas are already showing buds deep in their sheaths or even blooming.

In my greenhouse in Florida, many fall blooming cattleyas have already bloomed even though fall does not begin until September 22.  Blc Norman’s Bay completed blooming in early August and immediately began growth on two new leads.  Late summer blooms followed by late fall blooms is not uncommon if fall blooming cattleyas are mature and well-established.  

Typically, cattleya flowers last longer and are better colored when blooms develop during cool nights.  This is especially true when yellow pigments are involved.  The rich deep purple of Blc Oconee ‘Mendenhall’ or red from Blc Owen Holmes ‘Mendenhall’ is not as intense in summer since the yellow pigments in flowers do not develop as well in the heat.

Depending on your location, September is often a good month to initiate flower spikes in standard Phalaenopsis.  A day to night temperature difference of 15 degrees F for a week to 10 days will cause your phals to initiate spikes.  It is important to initiate spikes as soon as possible since the growth of phal spikes will slow as days get shorter and available energy decreases.

Hobbyists with orchids outside can let nature take its course.  If you grow in a closed greenhouse, the temperature can be lowered to that of the outside by simply keeping exhaust fans on at night.  Some commercial growers with cooling pads run cooling pads along with exhaust fans all night to lower the temperature inside the greenhouse below the external environment.

The vast majority of orchid hobbyists grow their orchids inside in windows or under lights.  Many have difficulty getting their orchids to re-bloom and assume they are not providing enough light.  Often though, it is the lack of seasonal change in day length because of interior lights, or because of a sufficient drop in temperature, especially for phalaenopsis.  Leaving windows open next to phals or even placing orchids outside for a couple of weeks can accomplish the prerequisite temperature drop. 

Windowsill growers should watch the change in the sun’s angle carefully.  Orchids growing happily during summer may become vulnerable to burning in a south-facing window.   Similarly, leaves may be thinning in trees shading orchids outside and orchids may burn.  Greenhouse growers may consider reducing shade, since the sun’s angle and intensity have changed since the summer’s peak in late June.

The declining solar radiation and longer nights also means that your orchids need less water.  Pay more attention to over-watering now since there is less time for orchids to dry each day.

  Typically, less fertilizer is also required this time of year.  Cattleyas fertilized with Nutricote last spring will have used up what was released and no additional fertilizer will be necessary until next spring.   There will be some residual fertilizer that has been incorporated in the medium and this should suffice until spring.  Phals and other orchids that grow and flower all year long will need additional fertilizer, but not as much as they did during the peak growing season.  Phals often produce larger spikes if fertilizer is withheld for a month or so this time of year.

INDEX TO ARTICLES: For more Orchid Growing Tips