Blooming Plectranthus -- A Study on Flowering

I know, I know, but I love this plant. I think there is still a lot of room for new cultivars and new forms. Cut me a little slack, and I promise I won't be back to this crop for at least. . . six months.

We were doing some work for Ball Floraplant on 'Mona Lavender' in response to grower trouble getting this incredible plant into flower quickly without growing too tall. Once the plant is in bloom, no one has any problems, except that they can't get enough cuttings. Everyone I know who produced this plant sold out immediately, and I think it will hold its place as a top novelty crop for quite a few more years.

Mona Lavender Photoperiod

Methods. Temperatures were maintained at 65° F night and 75-84° F day. Rooted liners from Ball Floraplant were pinched at planting and placed immediately under photoperiod treatments. Plants were grown in 4-inch pots using Fafard number 52 mix with 150 ppm 20-20-20 constant feed. We evaluated time from plant to first visible bud and full flower. Height, width and number of stems formed per plant measurements were taken, along with observations on plant development and quality. (Figure 1, below).

Treatments. Ten- (standard chrysanthemum scheduling), 12-, 14- and 16-hour day treatments were evaluated. The experiment was repeated at three planting dates: Sept. 03, 2002; Dec. 04, 2002; and March 05, 2003.

What We Found

We could shut off flowering entirely by lighting to a 16-hour day. These plants never budded in the experiment. So lesson number one: The longer the day length, the longer the plants grow vegetative. The quickest treatments to flower were 12-hour days; 14-hour plants bloomed about two weeks later and were 2-3 inches taller.

Ten-hour day plants flowered approximately the same time as 12-hour treatments but were delayed by heat (both at initiation and during development) under blackcloth and were of a lower quality because there was not enough light in a given day to develop strong plants.

What it all Means

You can flower Mona under 14-hours or less; 14 hours of light will flower later and be larger than a 12-hour crop. A word of caution: Light pollution from streetlights, etc., can easily hamper flower initiation. During the vegetative stages of growth, plants seem to be sensitive to this. Once flowers are open and the plant is in full bloom, it is slower to revert to non-flowering. So, I've had Mona Lavender blooming all summer in the shade and 2-3 hours of full sun in Florida. The shade plants always look better. Last note: Cool nights help lock in flower initiation, so if you can drop temperatures in the night cycle, you'll speed initiation and deepen flower color.

Planting time did not really affect the time to flower, so under similar temperatures, plants will flower in roughly the same amount of time. This makes it easy to schedule crops for most seasons.

The cuttings all formed the same number of main branches after pinch, but it is flower initiation that gets all the secondary branching going and gives the plant its rounded habit. Without flower initiation you get long, straight stems and a tall, irregular habit.

Plectranthus Photoperiod

We ran the exact same study using 26 species and hybrids of plectranthus, as well as 'Cat's Whiskers' (Orthosiphon stamineus) and 'Red Queen' trailing coleus (Solenostemon rheneltianus or S. scutellarioides). We also drenched all plants with 1/4 ppm paclobutrazol two weeks after pinch. The information in Figure 2, page 16, tells you what we found.

There were several cultivars that never bloomed. Some of them were heavily growth regulated by the application of paclobutrazol, which kept them from growing very much or flowering. Others are not reliable bloomers under the best of conditions; these include most of the P. amboinicus types, including 'Athens Gem'. Plectranthus ciliatus 'Gold Coin' has beautiful, golden green foliage but could not handle the higher light levels, and warm temperatures in this experiment, and all plants were lost, so grow these chartreuse foliage types shaded and cool for best growth.

There are long- and short-day forms in nature, so breeders may be able to bring more of the great qualities plectranthus offer without the photoperiod issues sometime in the future.

Best flowering forms. My picks for best flowering forms include Mona Lavender, of course, but also P. ecklonii and P. ecklonii 'Erma' which are shrub-like reaching 5-6 feet in the landscape, and flowering purple and pink, respectively. These plants make an incredible late summer/fall display in the Southern landscape, but landscape performance farther North is unknown.

Plectranthus 'Nico' also has a very nice flowering of small spikes of white-pink flowers. Both forms of P. zuluensis that we had in the trial were stunning, but it took them over 90 days to get into flower, I'm hoping that is just heat delay because they have really come on strong since we started to get cooler nights. The last plant on the list was a surprise; it was 'Zulu Wonder' from EuroAmerican Propagators; this foliage form came out with 12- to 14-inch spikes of pale lavender pink and is incredible when it is in bloom. It has great potential but will require short days to flower. That's okay, it means Northern growers can get flowers for spring sales.

Best foliage forms. P. argentatus and P. argentatus 'Variegatus' are definitely tops on my list; they are horsy and gangly without PGRs but extremely sensitive to them, both cultivars were overdosed at 1/4 ppm. However, in the landscape they excel in hot, dry locations, flower freely all season (flowers are not very impressive) and add a strong gray tone to the landscape.

'Nicoletta' and P. tomentosus seem like the same plant to me but give excellent gray color in a prostrate habit. Funny thing is that these are also monsters when they are not growth regulated and very sensitive to treatment as well.

'Athens Gem' and 'Nicodemos' are all strong foliage interest in the green to white variegation area; all are somewhat upright and best in 4-inch or gallons. Don't forget P. madagascariensis, the variegated trailing type, with green and white leaves; it is a must in most mixed containers. Last of all Plectranthus ciliatus 'Gold Coin' (and other similar types) have the lime green foliage that really works great in shady mixed containers.

According to the International Plant Names Index (http://www.ipni.org), there may be as many as many as 700 different kinds of plectranthus out there. Hmmm. . . that would be almost a third of the number there are of petunia cultivars!

Author's Note: After last year's article on plectranthus I received cuttings from a generous soul out there somewhere but lost your address. Please contact me so I can tell you how the plants are doing!