Photo courtesy of Doug Cole

Doug Cole, the owner and founder of D.S. Cole Growers in Loudon, New Hampshire, strives to have a detailed understanding of what he offers to his customers. Each year, D.S. Cole Growers, which grows finished plants and young plants for its customers in the northeastern region of the U.S., uses its trials to obtain a granular understanding of the different species and varieties it sells. So, when customers visit D.S. Cole at its Louden, New Hampshire, home for an open house or a trial garden visit, they get a clear indication of which plants D.S. Cole thinks are its best at a given point in time.

Below, Cole talks about how he approaches plant trials, why less emphasis is placed on early season crops and what plants he’s interested in moving forward.

Greenhouse Management: How has your approach to plant trials changed over time? 

Doug Cole: We’ve probably done them for over 15 years now, and they aren’t like going to Ohio State’s trial garden or something like that. We are trying to focus on the new varieties we are going to offer and make comparisons with current varieties that we think are important to look at. We’re also trying to pick different species to focus on each year. We might pick all of the calibrachoas one year instead of just saying, ‘Here’s everything’ because there [are] close to 1,000 items we offer and that’s too much to do. We also try and do more than true trialing and show how things can be used, too. So, we might take succulents and make a lot of different planter shapes and sizes.

GM: Are there any plant trends you are keeping your eye on?

DC: I think there’s much less emphasis paid on the early season crops. I don’t think that’s new news. But, for example, plants like bacopa or nemesia, there’s nothing wrong with them. They’re still popular, but customers are still looking for products that are going to make it through the summer and not get hurt by the heat. With colors — red, blue, orange, yellow — I don’t see a trend there. But I do see a trend with the type of plant. 

GM: When you look at what might be popular in the next year or few years, are their certain plants you are especially focused on?

DC: Foliage — what’s old is new. I can’t tell you if I’m crazy or not, but we’ve been growing soft foliage like Swedish Ivy, Wandering Jew and spider plants. We’ve been test marketing it for the last year in hanging baskets for the local finished market and it’s amazing. We just keep planting more and planting more. It seems like we are going way back into the 1980s when that was the last big boom of soft foliage. I don’t know if it’s going to take over or anything, but there’s a niche there. There’s a market there, so we have soft foliage not only in finished plants, but those items are back in our young plant program. And we’re going to see what happens. 

As greenhouse operations expand, water can be a limiting factor as demand exceeds the yield rate of a well. Zoning the sprinkler irrigation system so that sections get watered in sequence is one solution. A second option for growers is installing drip irrigation, which lowers water needs and distributes water over a longer period of time.

A third, better option is to install an intermediate storage tank between the well and the pressurized distribution system. This reservoir can be a non-pressurized corrugated steel, polyethylene or fiberglass tank that serves as a primary source of supply for the pressure pump and tank.

The system operates such that the pump in the well supplies water to the storage tank at a rate slightly less than the well yield rate. A larger pump in the reservoir transfers the water to the pressure tank and into the distribution system. The depth of water stored in the non-pressurized reservoir is regulated either by a float switch or water level sensor that controls the on-off operation of the well pump.

Meeting peak water demands

At minimum, the intermediate storage tank should hold enough water to meet peak use on the warmest day of the year, minus the amount of water that can be supplied by the well during the watering period. Let’s use the following example:

For most crops, the peak use during the summer is about 0.4 gallons per square foot of growing space per day. In a greenhouse having 20,000 square feet of growing space, the maximum daily water requirement is 8,000 gallons (20,000 sq ft x 0.4 gal/sq ft = 8,000 gal). Next, calculate the amount of water that can be pumped from the well into the intermediate storage during the watering period. Assume that the yield from the well is 15 gallons per minute (gpm) and the irrigation system will water all the zones in four hours. This make-up water amounts to 3,600 gallons (15 gpm x 60 minutes/hour x 4 hours = 3,600 gal).

The size of the intermediate storage is the difference between these amounts (8,000 gal – 3,600 gal = 4,400 gal). The well pump supplying 15 gpm would have to operate 8.9 hours to supply the total water needs (8,000 gal ÷ (15 gpm x 60 min/hr) = 8.9 hours). The pump in the intermediate storage tank would have to supply at least 34 gpm (8,000 gal ÷ (4 hr x 60 min/hr) = 34 gpm) to meet the demand of the irrigation system if all watering was to be done in four hours.

Intermediate reservoir advantages

If the tank is located above ground in the greenhouse or headhouse, the water can be tempered before irrigating the plants. If the tank is filled the night before irrigating, by morning the water will be several degrees warmer. Supplemental heat could be supplied from a hot water heater or boiler to raise water temperature.

The tank could also serve to chemically treat the water. For example, chlorine could be added to precipitate iron from the water and allow it to settle to the bottom of the tank.

The tank might also serve to collect rainwater from the roof gutters, saving well water. A one-inch rainfall on a 20,000-square-foot roof will supply about 13,000 gallons. Besides the collection gutter, the system consists of PVC piping to direct the water to a roof washer where trash, leaves and other debris re removed by a screen mesh. All rainwater storages need an overflow to handle excess water once the tank is full.

Polyethylene and fiberglass tanks up to 20,000 gallons are available. These are the largest that can be transported over the highway. Corrugated steel tanks can be larger, as they are assembled on site.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com

With the increasing popularity in perennial garden plants, producers are growing more perennials, both in number of containers and varieties. Perennials are also one of the newest categories of greenhouse crops that are being programmed and forced, when compared to cut flowers, potted flowering plants and annuals. This article will focus on the fundamental requirements for forcing perennials into flower.

Juvenility

The first factor controlling perennial flowering is juvenility. Just like animals, plants cannot reproduce (i.e. flower in order to produce seeds) until they are physiologically mature. For some plant species with long juvenile periods, like numerous tree species, the juvenile period can be measured in years. However, for plants like herbaceous perennials, the juvenile period is much shorter. It is so much shorter that years are not used (or needed) to measure the length of the juvenile period; rather, it is measured by the number of leaves or nodes on the plant. The duration of the juvenile period varies by species; some species can flower after forming six or seven leaves, while other species require more than 12 to 15 leaves to flower.

The plant material you use to grow your perennials can influence whether or not your crop is juvenile. For example, a seedling grown in a 50-cell plug tray may be mature and able to respond to flowering cues, whereas a seedling of the same species grown in a 288-cell plug tray may not be. The reason is not that the 50-cell plug has a larger soil volume per se. Rather, it is still due to the leaf number. A plug is finished when it is pullable, meaning it has achieved sufficient growth so when it is pulled out of the plug tray, all the substrate comes along with the root ball. It takes longer for a 50-cell plug to reach a pullable stage compared to a 288-cell plug. Therefore, the larger plugs will have more leaves from the longer production time and be more mature than the smaller seedling.

Juvenility is only an issue for seed-propagated perennials. If you are using cuttings (root or stem-tip) for perennial production, they are already mature, as the cuttings are being taken off mature plants. Additionally, if you are using products like bare-root rhizomes or crowns as starting material, they are also mature and ready to flower.

Vernalization

Once a perennial is mature and can flower, many species require a vernalization or period of cold temperature to flower. Although the requirements for dormancy and vernalization can be met by the same cold treatment, they are different processes (Fig. 1). Dormancy refers to a cessation of development by plant tissue otherwise disposed to develop. On the other hand, vernalization is a cold treatment that results in plants being able to form flowers. Regardless, the cold treatment of perennials usually satisfies requirements for both vernalization and dormancy.

A successful vernalization requires sufficiently cold temperatures for a sufficient period of time. While the specific temperature and duration for vernalization varies with species, there are some good guidelines that can be followed. Temperatures around 40° F are effective for vernalizing many crops. Similarly, six to eight weeks of cold temperatures satisfies most vernalization requirements; some species can require longer periods of cooling. If plants are insufficiently vernalized (too warm or too cold, not enough time at cold temperatures, etc.) or if they are vernalized but held too long in cold conditions, this can increase forcing time and reduce the quality of flowering plants.

Perennials can be cooled in a variety of environments, including outdoors and in cool greenhouses and coolers (Fig. 2). Outdoor space can be the most economical approach if available, but there is no control over the temperatures during vernalization. Greenhouses have more temperature control than outdoors. But bench space can be valuable and it may be hard to dedicate it to cooling perennials. Coolers give you the best temperature control, but this is also the most expensive option. If you don’t have the facilities to vernalize your perennials or are not interested in doing so, there are some alternative approaches. You can purchase crowns and rhizomes of field-grown plants that have been cooled prior to shipment, much like purchasing case-cooled Easter lily bulbs. Additionally, some young plant producers specialize in growing perennials and offer vernalized plugs and liners that can be planted and forced.

Other flowering options

Not all perennials need to be vernalized in order to flower. For some species, even though a cold treatment is not required to produce flowering plants, a cold treatment can increase the quality of finished flowering plants and should be provided if space is available and economical. Alternatively, some species do not require a vernalization and there is no improvement in finished crops when provided with one. For these species, a cold treatment should not be provided, unless they need to be cooled and grown with other species requiring a vernalization for logistical reasons.

Many perennials also have a photoperiodic flowering response. While there are some day-neutral and short-day species, many perennials flower in response to long days. For those species that flower in response to long days, their responses can be further categorized as obligate (long days required) or facultative (long days not required, but beneficial).

Depending on the time of year, you may need to manipulate day length to control flowering. From mid-fall to mid-spring, the days are naturally short for many species. If you are trying to force species with a long day requirement during these times, you will need to use either night interruption or day-extension lighting to create long day conditions. Whatever your light source (incandescent, light-emitting diode, etc.) is for night interruption or day extension lighting, be sure there is enough far-red light to induce the long day species to flower. Alternatively, if you are forcing short day plants during naturally short days, you may need to provide some long days after cooling to avoid premature flowering and bulk plants to get sufficient vegetative growth. Short days may need to be provided for short day crops beginning at the end of spring through the summer to force flowering during the naturally long days. Pulling blackout cloth over your crop is the only way to create artificial short days when the days are long.

Perennials can be just as programmable and predictable as many of our other greenhouse crops are. However, there are a few more steps you need to take to ensure you can force perennials in to flower on time, every time.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu

Aphids are major insect pests of greenhouse-grown horticultural crops including ornamentals and vegetables. One of the most common aphid species encountered in greenhouses is the green peach aphid, Myzus persicae (Figs. 1 & 2), which feeds on >400 plant species in 40 different plant families. Green peach aphid causes direct plant damage by withdrawing plant fluids from the phloem sieve tubes with their piercing-sucking mouthparts, resulting in leaf distortion, plant stunting and plant wilting. Indirect damage is associated with the production of honeydew (clear sticky liquid) and transmission of plant viruses during feeding. In addition, a short generation time and high female reproductive capacity allows green peach aphid populations to quickly reach damaging levels.

Insecticide resistance

Therefore, insecticides are routinely applied to suppress green peach aphid populations (Fig. 3). However, the short generation time and high female reproductive capacity of the green peach aphid also contribute to their propensity to develop resistance to insecticides. As such, the intensive use of insecticides to manage green peach aphid populations has resulted in populations that are currently resistant to a number of insecticide classes. In fact, the continual reliance on insecticides and subsequent intensive selection pressure has led to green peach aphid populations developing multiple resistance mechanisms that have enhanced resistance to most chemical classes including: organophosphate, carbamate, pyrethroid and neonicotinoid. Currently, the green peach aphid has developed resistance to >70 different compounds. Furthermore, the green peach aphid is able to overcome the toxic effects of insecticides based on six different resistance mechanisms.

These resistance mechanisms confer resistance to different chemical classes (indicated in parentheses) and are affiliated with: 1) overproduction or enhanced production of detoxification enzymes that break down the active ingredient before reaching the target site in the central nervous system (organophosphates, carbamates and pyrethroids); 2) changes or mutations in specific enzymes that decrease sensitivity to certain insecticides by modifying the target site (carbamates); 3) changes in the voltage-gated sodium channels (pyrethroids); 4) over-expression of detoxification enzymes (neonicotinoids); 5) reduced penetration through the cuticle (neonicotinoids); and 6) changes or alterations in the target receptor sites (neonicotinoids).

Table: Raymond CLoyd

Most resistance mechanisms are associated with increased or enhanced production of metabolic detoxification enzymes or modification of target receptor sites in the central nervous system, resulting in reduced sensitivity. Moreover, several resistance mechanisms may be present, thus resulting in green peach aphid populations that are resistant to multiple insecticides with different modes of action. Some of these mechanisms evolved early in response to plant chemical defenses or plant-derived toxins and have been manifested after exposure to insecticides. Furthermore, the green peach aphid is a generalist herbivore and has a greater propensity to develop resistance to insecticides due to the presence of a variety of resistance mechanisms that confer resistance to plant chemical defenses or plant-derived toxins. In fact, some insecticides have similar properties to plant-derived toxins. For instance, the pyrethroids are synthetic derivatives of pyrethrins, which are natural components of the chrysanthemum, Tanacetum cinerariifolium, flower. As such, similar resistance mechanisms can confer resistance to pyrethrins and pyrethroids.

Another type of resistance mechanism is referred to as “behavioral avoidance,” which confers resistance to neonicotinoids. Behavioral avoidance is associated with green peach aphids spending more time on untreated leaves than treated leaves. Aphids will disperse away from insecticide-treated leaves. For instance, the green peach aphid tends to avoid residing and feeding on leaves treated with imidacloprid and will move to untreated leaves. However, it should be noted that imidacloprid does have repellent activity, so “behavioral avoidance” may not solely be responsible for resistance.

Mode of action types

There are two mode of action types associated with insecticides: narrow and broad-spectrum. Narrow-spectrum (site-specific) modes of action insecticides are active on specific target sites such as the central nervous system or enzymes associated with metabolism. Broad-spectrum mode of action insecticides are active on a variety of target sites or possess multiple modes of action. For example, some of the more “selective” insecticides such as pymetrozine (Endeavor), flonicamid (Aria) and cyantraniliprole (Mainspring) may be less prone to resistance due to their broad-spectrum modes of action compared to insecticides with narrow-spectrum modes of action. In fact, pymetrozine exhibits no cross resistance (based on a single resistance mechanism conferring resistance to pesticides in the same chemical class and/or having similar modes of action) to either pyrethroid or carbamate insecticides.

The best way to mitigate resistance is to rotate insecticides with different modes of action throughout the growing season when green peach aphid populations are present. Table 1 presents a list of some insecticides that are labeled for use against aphids and their designated mode of action.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits. rcloyd@ksu.edu or 785-532-4750