November 2003

Blooming Plectranthus — A Study on Flowering By Rick Schoellhorn, University of Florida

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

We were doing some work for Ball Floraplant on ‘MonaLavender’ in response to grower trouble getting this incredible plant intoflower quickly without growing too tall. Once the plant is in bloom, no one hasany problems, except that they can’t get enough cuttings. Everyone I know whoproduced this plant sold out immediately, and I think it will hold its place asa 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. Rootedliners from Ball Floraplant were pinched at planting and placed immediatelyunder photoperiod treatments. Plants were grown in 4-inch pots using Fafardnumber 52 mix with 150 ppm 20-20-20 constant feed. We evaluated time from plantto first visible bud and full flower. Height, width and number of stems formedper plant measurements were taken, along with observations on plant developmentand quality. (Figure 1, below).

Treatments. Ten-(standard chrysanthemum scheduling), 12-, 14- and 16-hour day treatments wereevaluated. 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 a16-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 quickesttreatments to flower were 12-hour days; 14-hour plants bloomed about two weekslater and were 2-3 inches taller.

Ten-hour day plants flowered approximately the same time as12-hour treatments but were delayed by heat (both at initiation and duringdevelopment) under blackcloth and were of a lower quality because there was notenough light in a given day to develop strong plants.

What it all Means

You can flower Mona under 14-hours or less; 14 hours oflight 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 tonon-flowering. So, I’ve had Mona Lavender blooming all summer in the shade and2-3 hours of full sun in Florida. The shade plants always look better. Lastnote: Cool nights help lock in flower initiation, so if you can droptemperatures in the night cycle, you’ll speed initiation and deepen flowercolor.

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

The cuttings all formed the same number of main branchesafter pinch, but it is flower initiation that gets all the secondary branchinggoing and gives the plant its rounded habit. Without flower initiation you getlong, straight stems and a tall, irregular habit.

Plectranthus Photoperiod

We ran the exact same study using 26 species and hybrids ofplectranthus, as well as ‘Cat’s Whiskers’ (Orthosiphon stamineus) and ‘Red Queen’ trailing coleus (Solenostemonrheneltianus or S.scutellarioides). We also drenched allplants with 1/4 ppm paclobutrazol two weeks after pinch. The information inFigure 2, page 16, tells you what we found.

There were several cultivars that never bloomed. Some ofthem were heavily growth regulated by the application of paclobutrazol, whichkept them from growing very much or flowering. Others are not reliable bloomersunder the best of conditions; these include most of the P. amboinicus types, including ‘Athens Gem’. Plectranthusciliatus ‘Gold Coin’ has beautiful, goldengreen foliage but could not handle the higher light levels, and warmtemperatures in this experiment, and all plants were lost, so grow thesechartreuse foliage types shaded and cool for best growth.

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

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

Plectranthus ‘Nico’ also has a very nice flowering of smallspikes of white-pink flowers. Both forms of P. zuluensis that we had in the trial were stunning, but it tookthem over 90 days to get into flower, I’m hoping that is just heat delaybecause 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’ fromEuroAmerican Propagators; this foliage form came out with 12- to 14-inch spikesof pale lavender pink and is incredible when it is in bloom. It has greatpotential but will require short days to flower. That’s okay, it means Northerngrowers can get flowers for spring sales.

Best foliage forms. P.argentatus and P. argentatus ‘Variegatus’ are definitely tops on my list; they are horsy andgangly without PGRs but extremely sensitive to them, both cultivars wereoverdosed at 1/4 ppm. However, in the landscape they excel in hot, drylocations, flower freely all season (flowers are not very impressive) and add astrong gray tone to the landscape.

‘Nicoletta’ and P. tomentosus seem like the same plant to me but give excellent gray color in aprostrate habit. Funny thing is that these are also monsters when they are notgrowth regulated and very sensitive to treatment as well.

‘Athens Gem’ and ‘Nicodemos’ are all strong foliage interestin the green to white variegation area; all are somewhat upright and best in4-inch or gallons. Don’t forget P. madagascariensis, the variegated trailing type, with green and whiteleaves; it is a must in most mixed containers. Last of all Plectranthusciliatus ‘Gold Coin’ (and other similartypes) have the lime green foliage that really works great in shady mixedcontainers.

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

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