Mistakes to Avoid When Growing Perennials

November 13, 2007 - 15:30

With increased perennial production and a more challenging business climate, there are several areas of production that growers should consider when producing perennials. Some of the most common mistakes I see growers make and how to avoid these common pitfalls are discussed below.

 

Avoiding Generic Production of Perennials

For years, growers have grouped and produced perennials as if they were one crop with a single set of cultural requirements. I am amazed at the number of growers who still practice this today. It is not uncommon for growers to produce up to several hundred different cultivars consisting of dozens of genera. Yet in many instances, they are loosely grouped into a single production environment just because they are all classified as perennials. This type of generic production often leads to problems with irrigation management, increased cultural problems, unpredictable flowering and poor turning of the production space.

As with most annuals and potted crops, each perennial has specific needs that must be met to maximize production and produce the highest-quality plant. If flowering perennials is the objective, greenhouse production becomes even more challenging for growers because there are often additional factors, such as juvenility, vernalization and photoperiod, to consider.

The really good growers understand and deliver the specific requirements of each specific perennial crop. These growers group perennials with similar requirements (such as temperature, irrigation and photoperiod) into manageable zones were they can optimize production by efficiently satisfying the needs of similar plant varieties.

Before even beginning to produce perennials, I encourage growers to collect the cultural and forcing requirements of their crops and develop production plans for each specific perennial. After assembling production plans with cultural requirements, growers can begin to group plants with similar needs in their appropriate environments.

 

Understanding the Mechanisms of Flowering

Many growers are familiar with forcing terminology but don’t fully understand how to assemble forcing programs to achieve season-long flowering. Some growers perceive forcing as an elaborate process that requires a great deal of management and increased production costs but doesn’t receive a premium price to offset the costs.

Forcing does require some additional management — it may involve additional costs associated with lighting — but in most cases, growers don’t have to force all of the perennials being produced for any given sales period.

In many instances, growers can supplement their current programs with a couple of varieties that are forced into flower, allowing them to increase the amount of color at the retail location. For example, early spring programs can consist of mostly day-neutral varieties, such as aquilegia or dicentra, supplemented with long-day or long-day-beneficial plants such as delphinium or leucanthemum. For early spring programs, growers can control production costs by avoiding varieties such as rudbeckia, which take more than 12 weeks to produce flowers, and grow long-day plants that can produce bloom in eight weeks or less.

To produce flowering plants, all perennial growers should understand the cold requirements of the plants they are producing. If cold is required, take steps to ensure a cold treatment is provided either to a large plug or in the final container. Where the plants are cold-beneficial, consider providing cold only to the plants required for the earliest sale dates; this will allow growers to maximize the benefits (earlier flowering, more uniform flowering, and/or more flowers per plant) of cold early in the season.

When forcing long-day plants, be sure to provide lighting until the natural photoperiod is longer than 14 hours. The critical photoperiod for flowering of most perennials is around 14 hours. Removing the lighting before this time may result in plants that flower only sporadically or produce a few flowers on the terminal shoots, with many of the lateral branches remaining vegetative and covering or hiding the flowers as they grow.

 

Grouping Plants by Temperature Requirement

Many growers produce relatively small quantities of numerous varieties of perennials within a single greenhouse environment. However, there are often numerous temperature requirements for the plants being produced. When plants are grouped within a single zone in this manner, there are often a number of varieties that develop too fast and may be ready several weeks before they are needed. Conversely, some perennials may develop too slowly and reach flowering several weeks after the anticipated sales date.

I generally recommend that growers group perennials into up to four temperature zones based on 24-hour average temperatures: 53° F, 58° F, 63° F and 68° F. The number of temperature zones and the actual 24-hour averages will depend on the requirements of the crops being produced and the facilities available to each individual grower. In many cases, growers may need to compromise if there are not enough zones available or if the quantity of perennials in a given grouping cannot justify giving them a separate environment. If compromises are made, be sure to adjust the schedules accordingly to accommodate the different production temperatures. For example, Rudbeckia fulgida ‘Goldsturm’ requires 12-13 weeks to reach flowering when grown at 68° F, but the production time increases to 16-18 weeks if the production temperature is reduced to 60° F.

 

Grouping Plants by Irrigation Requirement

Consistently good irrigation management is challenging enough for most growers, but in many cases, the way they place their perennials in the greenhouse further complicates water management. As noted previously, many growers group all of their perennials together with no regard to crop culture. In most cases, perennials are simply placed in the greenhouse in the order they were planted. As many growers have learned, placing crops with differing water requirements within the same zone often leads to difficult irrigation management and may result in the occurrence of root-rot pathogens.

I recommend growers group their plants into three primary groupings based on their water requirement: Above average (perennials such as astilbe and hibiscus), average (the majority of commercially grown perennials), and average to dry (perennials such as lavandula or sedum). When plants with an average-to-dry water requirement are irrigated the same as plants with an above-average moisture requirement, they often succumb to root-rot pathogens, and crop losses are likely to occur. Some growers have a fourth category that consists of perennials (such as Rudbeckia fulgida) that initially have an average irrigation requirement, but as the crop grows and matures it has an above-average water requirement. It is acceptable to have overlap from one category to another and manage these crops appropriately, but absolutely avoid overlapping two water group categories into the same grouping (average-to-dry plants next to above-average plants).

The other aspect to consider is to always group plants of similar container sizes and planting dates (provided the growth rates are similar) together. One-quart containers will obviously require a different irrigation regime than 2-gal. containers. Grouping plants by date usually becomes more prevalent as the greenhouse space becomes available for a second crop, or in situations where vernalized containers are placed in the same production space as freshly potted containers. In both cases, the older plant materials will require more irrigation than the newly planted ones. Whenever possible, consolidate the older plants or move them to a different location before filling the greenhouses with new plants.

 

Planting Perennials Outside the Recipe

To optimize plant performance and quality, some perennials perform best when planted at certain times of the year. As mentioned previously, each perennial has its own cultural and forcing requirements and requires a production plan, or recipe, specific to it. These recipes help ensure that everything required for optimum production is provided, allowing for the on-time production of high-quality perennials.

These recipes should be developed long before a new crop is started. Production plans lay down the foundation for a good program and provide the parameters necessary for production. These parameters should specify when the plants for each sales period can be potted as well as when potting a new crop should not occur. This is important because there are numerous times when growers plant outside of their recipes and experience inadequate growth and bulking, insufficient rooting, reduced quality characteristics, and increased crop losses and shrink. Two examples of this are provided below.

Example 1. Growers often receive and plant hostas from bareroot divisions in large containers during the summer before they are to be sold. This is a necessary step, as it allows for rooting and bulking of the plant this year, which leads to the production of a high-quality plant next year. Many growers do not realize that hostas require long days for shoot and root development. The later in the year the hostas are planted, the less root development will occur. By mid-August, many hosta cultivars will root, but they will root relatively slowly and may not become established before the overwintering period. When planting bareroot hostas in early September, growers can expect to observe no rooting. Hostas with no or few roots before going into the winter will usually survive, but the quality and the number of shoots produced will be less compared to plants planted earlier. I recommend planting hostas in June and July when using bareroot divisions; production plans should avoid late summer/early fall plantings.

Example 2. When producing 1-gal. or larger containers of aquilegia for early spring sales, it is always best to plant them in the late summer or early fall and vernalize them in the final container. Many growers plant aquilegias in the spring using vernalized plugs (such as 72-cell) only to discover that the plants produce flowers before the foliage fills out the container. In many cases, the plant remains quite small when it is in bloom. Without the foliage to provide “fullness” to the container, these crops are not marketable and usually do not amount to much during the intended sales period. The crop recipes should be written to not allow spring plantings for spring sales.

These are just two examples that demonstrate the effect that planting outside the intended production plans can have on plant quality and performance. When it comes to the production of the highest-quality perennials, always respect the production plans set in place, and take time to understand the consequences that may occur when making exceptions to your crop recipes.

About The Author

Paul Pilon is a horticultural consultant, owner of Perennial Solutions Consulting (www.perennial-solutions.com) and author of Perennial Solutions: A Grower’s Guide to Perennial Production. He can be reached at (616) 366-8588 or paul@perennial-solutions.com.

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