Rating The Varieties

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Rating The Varieties

Last month I talked about why I believe woody plants, in particular, colorful shrubs, will gain a greater market share among the plant buying public. The production protocols for shrubs can be challenging, but are no more difficult than forcing spring flowering bulbs or hardy perennials.

In general, all shrubs benefit from a cold treatment, and a few benefit from long days (LD). The duration of the cold treatment is far more important than manipulation of photoperiod.

Like perennials, we are overwhelmed by the number of shrubs available, and if truth be known, we are simply uncomfortable with plant material we don’t know. We selected plants for initial research to fulfill the following characteristics:

1. Plants must be colorful, either in flower or in foliage. Most shrubs don’t flower on new wood, so foliage color became very important. The foliar color should be sufficiently strong to create impulse buying.

2. Plants would have a relatively fast turnover. We wanted plants to be on the greenhouse bench no longer than 14 weeks in a one-gallon container. That would mean growing in winter for spring sales.

Here are a few shrubs we played with, trying to determine if they would be suitable for a spring program and, if so, what must be done to produce them in 1-gallon containers. All of these could be produced for fall programs if you wanted to hold on to them all summer.

If plants were rejected, it is because they would not be colorful enough for impulse sales, or too slow and/or too ugly. This month I will summarize some of the losers and winners. 

Shrub Forcing At UGA

We tried forcing many shrubs, some with great success. We tossed out others with the used soils, but included them here so you do not waste time with them.

Group 1: Would not fit into a mixed container program for the spring.
Reasons: Neither particularly outstanding foliage, nor are flowers produced on new wood: Indogofera ‘Rose Carpet,’ Leptodermis oblonga, forsythia ‘Golden Peep,’ philadelphus ‘Manteau d’Hermine’ (Mock Orange) and weigela ‘Monet.’

Indogofera and leptodermis appeared to be good candidates because of compact habit and interesting flowers, but the foliage was simply green and the plants did not flower until the second year. Forsythia and philadelphus were selected because they were well known on the retail side and would be easy sales. However, they were also too slow and had no eyeball power. Weigela was absolutely beautiful but was simply too slow.

All grew best when plugs were cooled (36 to 40ËšF) for a minimum of six weeks. All were forced under natural day length, and all grew without significant problems. If plants were grown outdoors throughout the summer to put on additional size, they would be appropriate for a fall program. However, flowers would still be sparse and never cover the plant until the next spring.

Group 2: Will work for a spring program, but with numerous problems.
Buddleia ‘Purple Emperor’ (butterfly bush), philadelphus ‘Variegata’ (variegated mock orange), helianthemum ‘Wisely Pink’ (sun rose) and buddleia ‘Purple Emperor.’

Buddleias are excellent plants for forcing. They require no cold and flower on new wood. ‘Purple Emperor’ was not a good candidate because of weak stems, which might make it a candidate for hanging baskets. Other buddleias were forced with better results (see Group 4).

Philadelphus ‘Variegata’ produces very handsome variegated foliage and is a well-known shrub. Plants required a minimum of six weeks cooling. However, even after the cooling period, plants grew slowly and the variegation was not as stable as we would have liked. However, if plants were moved up, they have excellent potential for fall sales.

Helianthemum ‘Wisely Pink’ is relatively unknown but could make an excellent container and low-growing garden plant. Plants were easy enough to force, but habit and inconsistency of flowering resulted in its elimination. Six weeks cold is recommended. This is an excellent garden plant but a poor “looker” in the container.

Group 3: Plants that work, but will require a bit more time, more marketing, etc.
Hydrangeas in general, abelia ‘Silver Anniversary’ and caryopteris ‘Sunshine Blue.’

Cultivars of Hydrangea paniculata (e.g. ‘Limelight’) are extraordinary plants flowering on new wood and can be marketed easily. People are in love with hydrangeas and a good photo is all that is necessary for robust sales. However, flowering is still slow and if started in Jan. or Feb. in a greenhouse, plants will likely not have many flowers for the spring market. However, plants will flower on the first season’s growth and they all flowered for us in mid summer. If they were started earlier (recommended), perhaps they might hit the late spring trade. They all benefit from six to eight weeks cooling for consistent flowering.

Cultivars of the garden mophead types (H. macrophylla) are most numerous and regardless if the flowers occur on old or new wood, they still have a long way to go before consistency, uniformity and flower power are on a par with what is expected in greenhouse crops, at least for the spring market. The best success in forcing mopheads still comes with the old-fashioned greenhouse forms. Regardless, all hydrangeas can easily be marketed because of the hydrangea craze in this country.

Abelia ‘Silver Anniversary’ is slow but with additional fertilizer, they filled out faster. I see this as filler, perhaps forced in a 5-inch or two plants per 6-inch rather than worry about individual container sales. Their plant habit is often to one side of the pot, but regardless, the foliage is terrific. Plants are handsome and flowers will occur the next spring. No cooling is necessary, nor are LD beneficial.

Caryopteris ‘Sunshine Blue’ is truly an eye catcher in the greenhouse and on the retail shelf. Similar to abelia, however, it is a slow grower and best suited to a smaller container (5-inch) or multiples per gallon. As filler in a container, it would be useful. They are also an excellent garden plant, but they are tough for a spring market unless started significantly earlier. However, they do catch the eye.

Group 4: Plants easily forced and catch the eye
Sambucus ‘Black Lace’ (Elderberry), physocarpus ‘Coppertina’ and ‘Summer Wine’ (Ninebark), leycesteria ‘Golden Lanterns’ (Himilayan honeysuckle), buddleia ‘Peacock’ (Butterfly bush), kolkwitzia ‘Dream Catcher’ (Beautybush) and hibiscus ‘L’il Kim.’

All of these consistently filled gallon containers for an April-May market and provided excellent foliage and/or good flowers. These are where one would start if trying out a woody program for the greenhouse.

Sambucus, physocarpus and kolkwitzia benefited from cooling (eight to 10 weeks) and LD, however, the cooling was far more important than the long-day treatments. LD was most beneficial only if insufficient cooling was provided.

Leycesteria and buddleia did not require cooling and in fact usually responded negatively to cold. This butterfly bush was far more upright and easier to control than ‘Purple Emperor’ (see Group 2).

Hibiscus ‘L’il Kim’ has been inconsistent. In 2006, the first year we tried it, a combination of six to eight weeks cold plus LD provided handsome plants with well-formed and beautiful flowers. Easily saleable. The following year, flowering and growth were so inconsistent as to put it in an unusable category.

We conducted in-depth work on the interaction between cooling and photoperiod in 2007, along with preliminary work with growth regulators on sambucus, physocarpus, kolkwitzia and buddleia, as well as fertility studies on abelia.

The hydrangeas (H. paniculata) we were sent all forced well and filled the container in 10 to 14 weeks but, as mentioned above, were slow to flower. ‘Limelight’ is significantly faster than the others. However, at this time, less than 20 percent of the hydrangeas have flowered.

The main problems besetting forcing of woody material is consistency of cutting material. The need for plants within a bench of the same cultivar to be uniform is critical and basic to greenhouse production. If the cuttings are inconsistent, it is likely the final product will also be. The cutting material for woodies is improving, and I am convinced there is a place in the greenhouse and on the retail shelf for woody material.

Next month: A look at production protocols for Group 4 plants in detail. Many thanks to Proven Winners and Spring Meadow Nursery for supporting this work.

Allan Armitage (allan@greenhouse grower.com) is a professor in the Department of Horticulture at the University of Georgia, Athens, Ga.

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