- Properly size your HVAC system. For indoor grow rooms, sizing the HVAC system is critical to handle the heating and cooling loads you will get once you start growing plants in those rooms. Controlling air temperature with enough air movement and dehumidification can be hard to do when you are also trying to get the most plant density out of every room.
- Have enough heating and cooling capacity for your greenhouses. For greenhouse production, make sure you have enough heating capacity to handle winter conditions. Heating under the crop is more efficient than heating the air above. Having cooling capacity is also critical during summer weather. Whether you have natural ventilation, fan-assisted natural ventilation or pad and fan cooling, you need to make sure you can provide enough cooling during the hottest and sunniest weather to maintain those desired growing temperatures.
- Install horizontal air flow (HAF) fans. Especially in indoor grow rooms, these fans are essential to help with air flow that helps distribute temperature more evenly throughout the crop and breaks up microclimates around all leaves for better gas exchanges, in addition to helping control powdery mildew. Try to avoid “dead air” zones (without airflow) within your rooms or greenhouse zones. Fans located too high above the crop do not provide enough air movement within the crop, but fans located too close to crops will adversely affect your ability to manage moisture levels.
- Use shade curtains during high-light and high-temperature months. Shade curtains typically are designed for 30% to 50% light reduction and should be closed during the brightest and hottest hours of the day. If there is no retractable shade curtain inside the structure, whitewash can be sprayed over the greenhouse roof to act as a shade curtain until you need to wash it off when fall weather approaches.
- Measure temperature correctly. Try to locate environmental control sensors closer to the crop and in the center of the zone. Measuring temperature over a 24-hour period is what we call average daily temperature and is the most important temperature measurement for growth. You can also measure leaf temperature with a digital infrared thermometer to determine when to close the shade curtain.
Garden mums are a popular floriculture crop, and they provide opportunity for summer production. It is common for garden mums to be grown outdoors, but since that environment can’t be controlled like the greenhouse, premature buds can form (Fig. 1), which can stop a crop from reaching its target height, resulting in unmarketable plants.
Causes of premature budding
Let’s review what controls flowering in chrysanthemums. Potted or florist chrysanthemum flowering is controlled by day length, induced by short days. However, garden mum flower induction can be influenced by both day length and temperature. Garden mums form flowers in response to short days, but temperature also influences flowers. Specifically, cool air temperatures (in the mid-60s or lower) can induce premature flowering in garden mums grown outdoors.
The effect of cool temperature on flowering poses challenges for producers growing their garden mums outdoors. Premature budding is most problematic for the earliest planted garden mums, such as large container sizes that require a longer production period, or for the earliest mum crops of the season. Whether it’s due to a large container size or an early sales date, the odds of experiencing cool night temperatures can move up the timeframe the crop is planted and placed outside. Cool temperatures can occur throughout the spring, and into June for northern growers.
Stress throughout the rest of the production cycle can also induce premature flower bud formation.
The ability to mitigate temperature stress on garden mums varies widely depending on what environment the crop is grown in. As long as the target air temperature is correct and heating equipment is working properly, it is easy to avoid cold temperature stress in a controlled environment like a greenhouse. However, a large proportion of garden mums are grown outdoors, not in a greenhouse. There is little that can be done to protect mums from cold outdoor air temperatures. Short of building specialized structures for growing mums outdoors, think creatively about how you can use your existing space. If cold temperature-induce premature buds happen more often than not with your garden mum crops, see if there is any greenhouse space that may be available. Greenhouse space may be used to start the mums until the threat of cold temperatures has passed. Then they can be moved outside and finished. Alternatively, some garden mum crops are grown entirely in the greenhouse — not outdoors — mitigating risks of early-season cold stress.
Water and nutrition
Stressed garden mums are more prone to form premature buds, especially when exposed to cool temperatures. Keeping garden mums well-irrigated and -fertilized will help combat premature budding. Water and nutrition can be used to push growth that will cover the premature buds.
While growing on the dry side can help check growth and reduce disease pressure, it can also promote premature flower bud formation. Stressed plants can induce more easily than non-stressed plants. Therefore, keeping garden mums well-watered can help keep them vegetative. In addition to water, fertilizer can also help combat premature budding. Under-fed garden mums can be more prone to premature budding than plants that are well-fed. It is easier than you think to under-feed garden mums because they are heavy-feeding plants. Recommended nitrogen concentrations range from 250 to 300 ppm for constant liquid fertilization. One strategy that can be particularly useful for garden mums grown outdoors is to provide controlled-release fertilizer (CRF) at half the recommended rate, then combine with water-soluble fertilizer (WSF) at half the recommended rate. Additionally, since CRFs release more with warm temperatures, the unwanted spikes in EC from excessive nutrient release is avoided by using the half-rate of CRF.
These same recommendations for managing water and nutrition can be used even if premature buds form. By promoting vegetative growth through ample water and nutrition, vegetative growth can resume and grow over the premature buds. This will help “mask” the flowers and allow you to get your plants to their target finished size.
The basic control system for vents includes a sensor, environment control, contactor panel, vent controller and gearmotor that moves the vent arms.
Another useful tool to help avoid premature budding on garden mums is to apply ethephon, commonly available as Collate or Florel. These are ethylene-generating plant growth regulators, where the ethephon converts into ethylene after being applied to plants. One effect ethylene has on plants is that it, causes flower abortion prevents flower bud formation. So, by applying foliar sprays of ethephon to garden mums, premature buds are physiologically incapable of forming.
Ethephon applications can begin one-and-a-half to two weeks after cuttings are stuck for propagation. Applying up to 500 ppm ethephon work well for garden mums.. For pinched crops, wait a week or two after the pinch occurs. Be mindful of how much time is left to finish mums, as flowering can be delayed past the target flowering date if ethephon sprays continue too long. Adequate time needs to be allowed for plants to form flowers, and ethephon should not be applied with less than 6 to 8 weeks of production time left.
Garden mums are an important crop for many growers, but growing outdoors presents challenges not experienced in conventional greenhouse environments. However, even though cool temperatures and stress pose the threat of inducing buds prematurely on mums, there are opportunities to modify the growing environment or culture to prevent premature flower formation.
Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University. firstname.lastname@example.org
Investing in energy curtains for the walls of a greenhouse can provide a rate of return on investment of 30% or more. With new greenhouses being built with a gutter height of 16 feet high or greater, the ratio of wall area to roof area is resulting in greater heat loss. For example, a 20,000-square-foot greenhouse with 16-foot height to the gutter will have about 30% more wall surface area than a standard house with 12-foot high walls.
Most of these new houses have one or more energy curtains to insulate the roof area, resulting in the heat loss through the walls being greater than through the roof. If the above house was glazed with double polycarbonate it would require about 428,000 BTU/hr for the uninsulated wall area and only 309,000 BTU/hr for the roof area with a double screen on a night with a 60° F temperature difference between inside and outside. If the walls were glass, the savings is considerably more.
The systems for insulating the walls have improved over the last few years. A system using a center motorized curtain roller is common, faster and more efficient, and stays straighter than rolling from the top only. The gearmotor unit with a large ratio rotates the shaft. It can be located on one end of the shaft for short greenhouses or in the center for long ones. Depending on weight and design, curtains as long a 450 feet can be rolled by one gearmotor. Safety torque and limit switches protect the motor.
There are two common types of curtain materials that will provide good energy savings. Both are polyolefins that have good strength properties needed for rolling and are fire retardant to meet the safety standards in the horticultural industry.
One is a clear fabric, L.S. Svensson — (Luxous 2845), that has good light transmission so that it can remain in place both day and night. It allows about 78% light transmission and has good light diffusing properties to allow sunlight to penetrate into the plant canopy. Tests show that it will have about 45% energy savings.
The second is a blackout material (L.S Svensson — Obscura 10070) that provides greater energy savings and also daylength control for crops such as poinsettias, mums and cannabis. It is laminated with white surfaces on both sides to reflect summer heat away from the greenhouse and reflect supplemental light back to the crop area. It blocks 99.9% of the light and provides 70% energy savings.
The typical cost of a motorized screen system is $2 to $5 per square foot, depending on the size of the installation, screen material used, number of obstructions that have to be worked around and ease of maneuvering man-lifts in the greenhouse. For a wall curtain with a transparent material that is not opened and closed fequently, a manual winch system may be all that is needed, saving about $1,000 for the gear motor and control.
Investing in energy curtains for the walls of a greenhouse can provide a rate of return on investment of 30% or more. With new greenhouses being built with a gutter height of 16 feet high or greater, the ratio of wall area to roof area is resulting in greater heat loss.
Based on the calculations from my energy audits, the payback for wall screens is between two and three-quarters and five years depending on the fuel price, location of the greenhouse, type of wall glazing and temperature at which the greenhouse is operated.
Wall curtains need to be installed to provide a tight seal all the way around the edges to prevent the cold air next to the wall from draining down and into the growing area. A fixed seal of polycarbonate sheets or fire resistant screen material is frequently installed to create the seal.
Incentive payments from the USDA’s Natural Resources Conservation Service or Environmental Quality Incentives Program, or a State Farm Energy program can help defray the installation costs. Low-interest Federal loans are also available for some of the remaining cost.
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. email@example.com
Ventilation is the process of exchanging indoor air with outside air. In a conventional greenhouse, ventilation is often used as the first stage of cooling, followed by evaporative cooling and shading. There are several benefits of ventilating the greenhouse, including the removal of heat, moisture and oxygen; providing a fresh source of carbon dioxide (CO2) for plants; moving air across the crop canopy; and doing all this with relatively little energy consumption.
Greenhouse ventilation can be accomplished by simply opening vents in the structure (natural ventilation, NV) or by operating high-volume fans (mechanical ventilation, MV). Most commercial greenhouses are equipped with the ability to do both NV and MV, whereas some are only able to do one or the other. It is generally better to have both NV and MV available, so that adjustments can be made in response to changes in the crop and the climate.
Natural Ventilation (NV)
As it implies, NV relies on the forces of nature to do most of the work in exchanging indoor air with outdoor air. These forces include wind and buoyancy. Because NV depends on natural forces, the quantity of air exchange and direction of airflow can be unpredictable. However, it is also the most energy-efficient option for managing the greenhouse climate.
Wind is an “external force” that literally pushes air into the greenhouse through openings in the cover. The positive pressure created inside the greenhouse will push the interior air out. Because wind-driven NV is most effective when it is “windy,” when planning the site, it is important to consider both the orientation of the greenhouse and the locations of the vent openings to take advantage of predominant wind directions for a given location. Additionally, academic studies (including “Effects of Buoyancy and Wind Direction on Airflow and Temperature Distribution in a Naturally Ventilated, Single-Span Greenhouse Using a Wind Tunnel,” a paper for the American Society of Agricultural Engineers, which I co-authored, and “Comparison of Finite Element and Finite Volume Methods for Simulation of Natural Ventilation in Greenhouses) have demonstrated that NV is improved when the greenhouse has multiple vent locations, which allow air to enter at one location and leave from another. Examples include openings in both the sidewalls and the roof, and openings on opposite sides of an A-frame gable roof.
The other type of NV is produced through buoyancy, or the “chimney effect.” Buoyancy is achieved when hot air generated by solar radiation rises above the plant canopy and into the ridge space. If vents are opened inside the roof, the hot air will escape to the outside, as long as outside air can re-enter the greenhouse from another vent location. If there are no vents in the roof, this hot air will be trapped in the ridge space with nowhere to go. NV via buoyancy is limited in a greenhouse that is filled with plants that are actively transpiring and evaporatively cooling themselves. It is also limited to when the outside air is hotter than the inside air because heat likes to move from areas of higher temperature to areas of lower temperature. Therefore, NV by buoyancy is really most effective in cold weather, when the outside is cooler than the inside.
Mechanical Ventilation (MV)
Mechanical ventilation uses fans to exchange air with the outside. These fans are typically sized based on maximum cooling requirements and minimum ventilation requirements (for CO2 replenishment and/or moisture removal). Unlike NV, fans used for MV provide predictable quantities of air exchange and consistent directions of air flow. Fans can either push air into the greenhouse via positive displacement, or they can pull air out of the greenhouse via negative displacement.
Positive displacement MV is most commonly achieved when a fan at one end of the greenhouse pushes outside air through an overhead plastic duct running down the center of the greenhouse. This duct has holes punched on either side to blow air (somewhat) evenly over the tops of the plants. This method of MV was most popular in the ’60s and ’70s and in northern climates, where the duct would be connected to a furnace that heated the incoming outside air. This method of MV has mostly fallen out of favor for several reasons, including shadows cast on the plants by the overhead duct; poor distribution of heated air; higher energy costs for fan operation; and poor quality of the duct material that required regular replacement or maintenance.
Recently, there has been a resurgence of positive displacement MV, with the ducts mounted below benches and the implementation of recirculating air-conditioning systems in sealed, or closed, greenhouses.
When positive displacement MV relies on introducing outside air to condition the greenhouse without recirculation back to an air conditioner or climate control chamber, it is important to consider how the air will leave the greenhouse. If relief vents or other openings in the structure are not provided, the greenhouse may become over-pressurized, which can cause the structure to bow outward, doors to swing wildly open or stay stubbornly shut, and ultimately reduce the grower’s ability to control the environment.
Negative displacement is the more traditional method of MV used in greenhouses. This method uses large-volume axial fans located on the wall at one end of the greenhouse to pull air outside through vents at the opposite end. These exhaust fans have many benefits, including the ability to be used with an evaporative cooling pad; control over the volume of air exchanged based on need; and relatively low energy use compared to positive displacement fans and air conditioning.
When direct drive fans (those with fan blades connected to an axle and turned at the speed of the motor) are used, rather than belt-driven fans (where one motor drives a belt stretched between fans), the fan speed can be adjusted incrementally based on need and will also respond to changes in pressure upstream caused by failed louver openings, dirty screens and evaporative cooling pads, or other restrictions to airflow. Therefore, energy will be saved by not running the fans at full speed all the time, and motors will have a longer life due to less cycling and less overloading.
One of the biggest advantages to growing in a greenhouse is using climate management strategies and technologies.
The author is founder of Dr. Greenhouse, an engineering consulting firm that specializes in the design, specification and analysis of environmental control systems. She is a licensed Mechanical Engineer in California and received her Ph.D. in Agriculture and Biosystems Engineering from the University of Arizona’s Controlled Environment Agriculture Center (CEAC). This article first appeared in sister publication Cannabis Business Times.
While Topeka, Kansas, isn’t usually seen as a growth market, Dave Jackson says Jackson’s Greenhouse & Garden Center has found success. The operation grows everything it sells from annuals to perennials to fruit trees and shrubs for customers in about a 50-mile radius.
“This population hasn’t changed since I started in 1970,” says Jackson, who owns and operates the business with his wife The only way we’ve grown in everybody else has gone out of business. The business model I’ve chosen has not been proven to be all that successful over here for many. But that’s the deal.”
Jackson’s priority is updating his 1922 ridge and furrow Lord & Burnham greenhouse with sash bars and glass. The greenhouse still has the original ventilator system — 21-inch frames on either side that raise and lower.
A 50- by 40- foot portion of the greenhouse is going to be converted to become part of the garden store as the IGC has expanded to offer more products. The plan is to remove the Lexan polycarbonate and replace it with a steel roof with skylights. They’ll also be adding air conditioning for shoppers’ comfort.
For the rest of the 40-foot greenhouses, he’s planning to install vents that open straight up from mid-purlin. “They’re going to open like a bar room door,” Jackson says.
Jackson wants to begin construction as soon as possible once sales have slowed down a bit. “This has been an amazing spring,” Jackson says, noting that sales are up 60% from last year. “With this COVID virus, everyone is working around the house and doing things that they’ve been putting off for years.”
Luckily, Jackson’s Greenhouse grows their own tomato and pepper plants. So when vegetable plants were flying off the shelves, they were able to just keep growing more. “When we got low on something, we put in another crop. So consequently, we’re the only ones with tomatoes and peppers.”
Jackson specializes in various kinds of perennials as well and says his favorite thing to do is grow as many varieties as he can. “There’s short-run productions and lots of different perennials. We’ve got stuff that nobody even dreams of,” Jackson says.
If possible, he’s planning to build a 140 by 150 structure to make room for expanding segments of the greenhouse. First, Jackson says the company needs more space for planting hanging baskets and pots. Beginning in February and March, customers will bring inn their planters, tell employees what they want and pay in advance. The week after Mother’s Day, customers will come back for their finished containers. “That saves them 30 to 40% over what they would pay if they we rebuying new pots every year,” Jackson says. “And then they don’t have to wait until after frost and wait for another two months for it to be beautiful.”
He estimates the company plants more than 1,000 annually.
The company also needs more room for fundraisers. Each year, they’ll grow several thousand 12-inch patio pots, geraniums and hanging baskets. “It’s a great fundraiser because they go out in February or March and take the orders and collect the money, and then we grow the product.”
Including all of the updates and upgrades, Jackson is looking at spending between $75,000 and $125,000, but Jackson says that’s a small price to pay.
“For the update in the existing greenhouse, it’s going to certainly make it cooler in the summer and more habitable, which will improve our summer quality of annuals. They won’t be so baked,” he says. “Since we’re growing and people seem to plant annuals all the way through July.”
The upgrade should help with efficiency of production as well with more space to grow and less need to shift plants around.
“And I guess the real reason we are having to do this is we’re growing our own fruit trees now, and we’re growing our Proven Winners shrubs. You can’t get the new varieties from wholesale nurseries. I love having different things and all of the new stuff, so we have to grow own own.”