Trialing for Heat and Humidity Hotshots By Jennifer Boldt, Jessica Boldt and Jim Barrett

This industry is constantly seeking new crops and cultivars that have heat tolerance so consumers can be successful even during the hottest days. With day temperatures regularly more than 90¡ F and night temperatures seldom below 70¡ F, the University of Florida trial garden is a proving ground for summer landscape performance and heat tolerance.

This past year, more than 650 cultivars were planted in the trial garden at the end of March and evaluated every two weeks from mid-April until the end of July. All cultivars were grown in full sun (except for impatiens) and had to contend with the sun, heat, humidity and regular afternoon rains. The top-performing cultivars during June and July were selected as our summer winners. In order to recognize cultivars that can be used in a wide range of situations, winners were split into two categories: flowering and non-flowering. Plants don't always need to have flower power to be "good" if they can provide another benefit, such as adding unique texture or foliage color to a mixed container or landscape setting.

Many of the trial's top performers are tried and true summer annuals: coleus, coreopsis, lantana, salvia and vinca. Since most of us are familiar with the use of these crops, this article focuses on new crops that aren't normally ç thought of for heat and humidity as well as exciting new cultivar releases from stand-by crops. Complete trial results are available at

Alternanthera 'Brazilian Red Hots'

This alternanthera from GroLink was a standout in many ways. Its average summer rating was 4.8 out of a possible 5, and its hot-pink foliage could be seen from across the garden. While it had a slow start, it began to shine once the temperatures warmed up. It had consistent growth all summer and excellent insect and disease tolerance.

Begonia 'Babywing White'

'Babywing White' is a new introduction for 2007 from PanAmerican Seed and has the same glossy, dark-green foliage and semi-vigorous growth habit as 'Babywing Pink'. It performed well in full sun, with only a few fungal spots on the lower leaves toward the end of July. As the temperatures consistently stayed above 90¡ F during the day, the flowers exhibited some pinking, especially on the bracts. However, it still bloomed prolifically during the entire trial period.

Coleus 'Royal Glissade'

'Royal Glissade' is a new coleus developed by Penny Nguyen at the University of Florida and available from Proven Winners. One of her research objectives was to select for later-flowering cultivars, and 'Royal Glissade' passed with flying colors. It was one of the few that did not flower at any point during the trial. With reddish-green leaves that appear to sparkle or glisten when the sun hits them, 'Royal Glissade' is a great addition to a mixed container or landscape. In the garden, it branched readily, reached a height of 21Ú2-3 ft. and did not lodge.

Crysocephalum 'Flambe Orange'

'Flambe Orange' is one of two cryso-cephalum cultivars available from Proven Winners. 'Flambe Orange' has yellow-orange flowers and silvery green foliage. The inflorescence is indeterminate and self-cleaning, which eliminates the need for deadheading. It has a prostrate growth habit. 'Flambe Orange' took 6-8 weeks to fill in the trial plot but maintained a full plant canopy all summer. In addition to thriving in the summer heat, this cultivar also can survive a light frost and has the ability to be used through the fall as a season extender.

Euphorbia 'Burgundy Wine'

Proven Winners has a new heat-loving euphorbia: 'Burgundy Wine'. The foliage and stems have brilliant, dark-red coloration. A few inconspicuous, small, white flowers developed during June and July. It started slowly in the garden but quickly grew into its own as temperatures increased.

'Burgundy Wine' is a tall cultivar, reaching a height of 4 ft. by summer's end. It is an excellent foliage plant with high foliar disease resistance and a strong branching structure.

Euphorbia 'Diamond Frost'

'Diamond Frost', another euphorbia available from Proven Winners, was a summer winner in both the 2005 and 2006 trials. Its mass of small, white flowers could be spotted in the garden all summer. It quickly filled in and reached a mature height of 2 ft. This plant is self-branching and requires little-to-no maintenance during the season. It does have a tendency to get a slightly weedy look by the end of the season, as some of the new growth extends above the rest of the plant canopy, but this is only noticeable when viewed closely. This appearance is enhanced under stressful conditions but recovers nicely when trimmed back.

Geranium 'Caliente Coral'

Geraniums are not typically known for heat and humidity tolerance due to high disease pressure. However, a series of interspecific geraniums from Fischer USA is trying to break this stereotype. Ivy geranium-like leaves are paired with a mounding growth habit. We trialed three colors in the Caliente series: Coral, Red and Rose. While all did very well, 'Caliente Coral' was the best performer in the series. The inflorescences do not contain as many flowers per head as other vegetative zonal geraniums, but the variety makes up for that short-coming by having a larger number of inflorescences. In addition, this geranium is mostly self-cleaning and performed very well under our garden maintenance policy of no pinching or deadheading after transplanting.

Helianthus 'Dune Supreme'

Dune sunflower is a familiar groundcover plant in coastal Florida landscapes. It traditionally has had very long internodes and been used out of necessity rather than aesthetics because of its known salt tolerance. 'Dune Sup-reme', from GroLink, has shorter internodes than the common dune sunflower, but it has the same vigorous growth habit, resulting in a low-growing plant with a very dense canopy. We had to trim plants in the plot every couple of weeks so they would not take over the surrounding plots. It did not flower prolifically early in the season most likely due to high nutrient availability from the initial fertilizer application, and it would benefit from a lower fertility level.

Salvia 'Mystic Spires'

This salvia, from Ball Flora-Plant, performed extremely well from planting in the garden until the end of the trial season. It was named a best of trials award winner for both the spring and summer evaluation periods. 'Mystic Spires' is a more compact version of 'Indigo Spires'. It is a much stronger plant and requires less aesthetic pruning and shaping than its predecessor. The inflorescences reached a height of 3-4 ft. in the garden. It began flowering in April, flowered continuously through the end of July and was never deadheaded.

Scaevola 'Toppot White'

This cultivar got a late start in the garden due to a delayed liner arrival. However, once planted in the garden, it quickly made up for lost time. 'Toppot White' is a compact scaevola bred by Westhoff. Scaevola tend to flower at the lateral ends, and growers try to counteract this by using a lot of PGRs to shorten the internodes, bring the flowers closer to the plant center and avoid a flowerless plant center. This cultivar, however, readily provides uniform color across the entire plant, allowing it to be better suited for hanging baskets and mixed containers. In addition to 'Toppot White', Westhoff also has another color in the series: 'Toppot Blue'.

Vinca Nirvana series

Although vinca is a known summer performer, we recognize the Nirvana series, from Fischer USA, for its wide range of colors, range of growth habits, good display of color all summer, uniform performance and resistance to aerial Phytophthora. The Nirvana series has two distinct growth habits: upright and trailing ('Nirvana Cascade'). We trialed six upright varieties and three trailing varieties, including 'Nirvana Cascade Pink Splash', a new introduction for 2007. All cultivars within the series were extremely uniform in growth and flowering habit. Even with the incidence of afternoon rainstorms on a regular basis, this series did not develop any serious fungal outbreaks.

2007 Floriculture Field Days

We are once again preparing for another trial season and are excited to see what cultivars will stand out as superstar performers. The gardens will be on display May 16-17, 2007, during the 2007 Floriculture Field Days, co-hosted by the University of Florida and the Florida Nursery, Growers and Landscape Association ( The top-performing cultivars from the spring trial will be highlighted in a future issue of GPN.

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Jennifer Boldt, Jessica Boldt and Jim Barrett

Jennifer Boldt is trial coordinator, Jessica Boldt is a graduate student and Jim Barrett is professor of floriculture at the University of Florida, Gainesville, Fla. Jennifer can be reached at [email protected]

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