Forcing Flowering: Outdoors

Indoor growers benefit from the level of photoperiod control they are able to maintain with timed lighting and dark cycles. Artificial lighting is one of the most influential and intensive inputs cultivators must manage and supply to grow crops successfully. Often, significant heat is generated through artificial lighting components such as lamps and ballasts. During warmer months, growing indoors under high intensity lighting is energy intensive; not only to supply power for the lighting system but often water consumption is increased significantly for both the crop and water-cooled equipment. In fact, indoor cultivators operating in warmer regions may opt to cease operations during the peak of the summer heat. Some continue their gardening endeavours outdoors, harvesting in late summer or during autumn.

Seasonal greenhouses may be constructed relatively inexpensively to extend the growing season and provide a level of protection from the elements to the crop. They also allow growers to select varieties that would otherwise not be possible to cultivate outdoors.

Adapting greenhouse structures or creating temporary framing for blackout systems allows outdoor growers to manipulate photoperiods to force-flower short day plants under natural light, regardless of the seasonality of the photoperiod. The construction and materials for such structures should be considered carefully. Remember, you are planning to house your valued crop within the frame. Snow loads in late fall can cause the structure to collapse onto the crop if there is insufficient structural integrity. Of concern year-round are wind loads, which should not be underestimated. In either case, remember that the framework will be covered with opaque material during the “dark” portion of the cycle. This will collect heavy snow or act as resistance to wind. The force of this is transferred into the framework. If there is not sufficient strength or engineering, the force will collapse the frame and covering down onto your crop.

Most force-flowering structures are relatively simple in construction and modular. There are limitations to recommended widths and lengths per single “bay” or structure, but more bays can be installed to achieve the desired size available for cropping. Also, by keeping the structures relatively lower in height, there is much less resistance to winds. However, there is a trade-off with lower heights. With the opaque light blocking material there is little air exchange available for the crop compared to greater internal volumes more associated with taller structures. For a simple row-type covering blackout system, an average overall width might be 6 ft. (~2 m) with an average height to the peak of about six feet. The length is determined by what is practical; remember that a retractable covering must be operated daily to provide the “dark” period. If there will just be one person to do the job, it is better to keep the runs shorter. If there will be multiple sets of hands available for covering the structure during the blackout period, runs can be maintained longer. An average length for one-person operations might be 16 feet (~5 m), whereas for two people, lengths of 48 ft. (~15 m) or greater may be achievable. Larger structures can be constructed with automatic mechanical blackout systems driven by solar power. These may be in widths of 24 ft. (~7 m) or greater with an average height of 10 feet (~3 m) to the peak and lengths up to 98 ft. (~30 m) or greater. Essentially, these structures are walk-in greenhouse frames equipped with an automated blackout system. Overall lengths are incremental, typically in 4-, 5-, or 6-ft. (~1-, 1.5-, or 2-m) arch spacing. In greenhouse construction, the measurements are almost always, given in centre-to-centre measurement; this differs from conventional construction measurements.

There are many choices of coverings available for blackout systems. There are specialty plastics and materials available, or the grower may choose more local materials. For seasonal blackout systems, particularly where manual labour (as opposed to mechanized systems) is used, a durable fabric (instead of plastic) is an excellent choice for several reasons. A fabric that completely blocks light is the essential criterion. However, a material that will also breathe is very advantageous. While blocking light and retaining some heat, it will allow some transfer of air. Solid materials such as polyethylene water vapour escaping from the crop will more easily condense on the covering and drip back onto the crop, increasing the potential for foliar diseases and other problems. Also remember that the covered crop will retain more heat once it is covered with a solid material. Fabrics will allow more of this heat to escape, if desirable. A heavier fabric blackout cloth is also more resistant to ripping and tearing from frequency of use with hand-operated blackout systems.

The final arches or “hoops” of the structure can be covered with solid materials. This helps to improve the structure’s ability to block all light and helps to add strength to the overall construction once the material is secured to the ends. Remember that once covered with blackout material, it will need to be light tight inside the structure.

A gothic arch-style profile is preferred (pictured in the photos) over more traditional Quonset-type structures. The Gothic arch design lends itself very well to deflecting wind and snow loads. Quonset structures of the same material and arch spacing will almost always come crashing in with heavy wind or snow loads before gothic style will.

Besides the blackout structure itself, there are other important considerations when planning to force flower varieties out of season.

Interior drip from condensation when the blackout covering is over the crop is a key area that must be addressed. Gothic arch profile structures help interior drip run down the surface of the covering, ideally to the ground without touching the foliage. Quonset style structures tend to drip straight down from the top because it has a slightly flatter profile on the inside. The steeper the pitch of the roof/sidewalls on the structure, the greater the ability for the structure to shed moisture both on the inside and outside of the covering surface (versus dripping down).

Regardless of the design profile, there is likely to be some interior drip as condensation occurs. The varieties you choose should be resistant to foliar diseases such as powdery mildew and grey mold. Growing in blackout structures is similar to growing in a greenhouse with regard to moisture considerations. Varieties stated by the breeder to do well in greenhouse conditions should also do well in the blackout structure.

Strengthening the plants’ immunity by application of products containing salicic acid, or other beneficial compounds proven to benefit the plant’s immunity response, will help improve the crop’s resistance to potential foliar diseases. A diverse and rich diet of organic nutrients should also help improve the plants’ immunity and defense response, especially if it contains plant-available silicate. Silicate can also be applied as a foliar spray and may help the plants’ ability to prevent dehydration during hot and dry periods.

To further strengthen the plants natural flowering response to the shortened photoperiod created by the blackout system, the crop can be sprayed with flower inducing hormones/stimulators.

The variety chosen can be forced to flower as soon as it is transplanted within the blackout system. As a result, the grower may choose a variety that other growers may not be able to harvest following the natural rhythms of the season, as the variety may otherwise not mature before the season is done. This provides the grower an excellent competitive edge in the local marketplace. Also, plants that typically finish large and inconsistent in flower/fruit quality due to their size and extensive branching will finish compact and can even be harvested as single-stemmed plants.

When growing with natural light it is always advantageous to select varieties that exhibit a strong and definite flowering response with shortened photoperiods. With faster flowering varieties in warmer climate zones, it may even be possible for some growers to realize two outdoor crops during the outdoor season when using blackout systems.

The blackout system is highly adaptable and can be constructed in a variety of settings from urban to remote gardens, allowing gardeners to take advantage of available natural light without sacrificing the benefits of photoperiod manipulation more commonly associated with gardens under artificial lighting.