Greenhouse Plant Containers
Container Types
In greenhouses, large individual containers are called “pots” or “cans,” but they are simply called “containers”. An inexpensive alternative to the plastic pot is the “bag” or polybag, which has the same general purpose and function. Plants grown in small-volume containers are often referred to as “plugs.” Plug seedlings are usually grown in individual containers called “cells” or “cavities” that are aggregated into “blocks,” “trays,” or “racks.” Often, many plants will be germinated in a tray, transferred to a small pot, and later transplanted to a larger container. Making the right container choices at the right time is an important task for a greenhouse manager. This section describes some of the types of containers used in greenhouses (See Table 13.1).
One-Time-Use Containers
One-time-use containers are of various sizes, round or square, are made of fiber, such as coir or compressed paper. Some include a natural or synthetic binding material such as resins, glue, wax, latex, and even cow manure. Dry, they will keep indefinitely. Since these pots are biodegradable, they are set in the soil along with the plants. Peat pots find their best use where plants are to be held for a relatively short time and then put in a larger container or in the field. Fiber containers are semi porous and promote water and air exchange between the rooting substrate and surroundings. Other containers rely on the material itself to provide structural stability and extended life span for long term use.
Jiffy 7® Peat Pellets
The “Jiffy 7” peat pellet is made completely of peat, with fertilizer added, enclosed in a plastic net and compressed for easier handling, shipping, and storage (See Figure 13.1). Soil mixing, sterilization, or soil filling are not required: the pellets are simply soaked in water and then seeds, small seedlings, or stem cuttings are placed in the tops.
Jiffy® Super Pellets
Jiffy® super pellets Jiffy® products consist of dry, compressed peat growing media inside a soft-walled, meshed bag and come in a variety of sizes (See Figure 13.2). When sown and irrigated, the pellet expands into a cylindrical plug surrounded by mesh that encourages air pruning all around the plug. Pellets are supported in hard plastic trays, so individual pellets can be consolidated to ensure full occupancy.
Jiffy® Pots
Jiffy® peat pots are square or round pots made out of compressed peat (See Figure 13.3). They are typically two to three inches across at the top. The pot, when filled with a root medium, is ready to use for rooting cuttings or germinating seeds. All these organic-material containers allow plant roots to grow through them. Container and plant are planted together when the plants are ready.
Rockwool Cubes and Slabs
Rockwool, a lightweight hydroponic substrate is made from spinning molten basaltic rock into fine fibers which are then formed into a range of cubes, blocks, growing slabs and granular products. Because rockwool and other stone or mineral wools used in horticulture originate from rock, they are considered by many to be a natural product. Rockwool is a widely utilized substrate in commercial horticulture for the production of crops as diverse as tomatoes, melons, cucumbers, peppers, strawberries, herbs, and cut flowers; however, it can also easily be used by smaller hydroponic growers wanting to take advantage of improved root zone technology.
Zipset™ Propagation Trays
Zipset™ propagation trays are square, one-use containers composed of bleached cardboard containers that are assembled in a hard-plastic tray (See Figure 13.5). Zipset™ Plant Bands maintain their integrity in the greenhouse but biodegrade after 9 to 18 months.
Paper Pots
Paper pots or paper tube pots are more popular with seed plug and cutting propagation of ornamentals, vegetable, and forestry species (See Figure 13.4). They allow for greater mechanization with pot-filling machines, automatic seeders, and wire benches that allow air pruning of the root system. Typically, paper pots consist of a series of interconnected paper cells arranged in a honeycomb pattern that can be separated before outplanting.
Single, Free-Standing Containers
Several types of single-cell containers are being used to grow native plants for specific conditions.
Treepots™
Treepots™ are excellent for budding, grafting and starting larger seedlings. These unique, deep, narrow containers are constructed of flexible hard plastic and are available in many different sizes (See Figure 13.7). Treepots™ feature vertical ribs on the inside wall to prevent root spiraling, are reusable, and store easily because they can be nested when empty.
RootMaker® Containers
RootMaker containers have staggered walls and a staggered bottom that prevent root circling and direct roots toward the holes in the walls and the bottom of the container. Each step complements the next, building upon the previous fibrous root system.
Polybags
Polybags made of black polyethylene (poly) plastic sheeting are the most commonly used greenhouse containers because they are inexpensive and easy to ship and store (Ser Figure 13.8). They are considerably less expensive than rigid plastic containers and seem to be satisfactory, but some types deteriorate rapidly. They are usually black, but some are black on the inside and light-colored on the outside.
Hanging Baskets
Hanging baskets are also made from plastic, but of a heavier grade than that used for flats (See Figure 13.12). Hanging baskets come in a variety of colors and styles. The most commonly used sizes are 8- and 10-inch diameters, with and without saucers on the bottom to collect runoff water.
Clay Pots
Root media in clay pots are well aerated and are less likely to develop root rot diseases (See Figure 13.11). Clay pots often last a long time and can be used and reused many times, provided they have been steam-sterilized. However, the use of clay pots has declined over the years due to a number of factors.
Plastic Pots
Plastic pots are available in many sizes, shapes, and colors (See Figure 13.9). Plastic pots are lightweight, a characteristic which means lower production costs and easier handling of crops. Some types are fragile, however, and require careful handling, although other types, made from polyethylene, are flexible and quite sturdy. Since plastic pots are not porous, fertilizer salts can not pass through the walls of the pot as in the case with clay pots. Any residues that do accumulate on the plastic pot are easily removed. Also, the root media in plastic pots require less frequent watering, since air can not pass through the walls of the pot.
Exchangeable Cells Held in a Tray or Rack Containers
The major advantage of growing plants in individual cell containers supported in a hard plastic rack or tray is that the individual cells are interchangeable allowing for consolidation and spacing as described earlier. Racks are designed to create enough air space underneath to promote good air pruning. Plastic cells can be reused for several growing seasons.
Ray Leach Cone-tainer™ Cells
One of the oldest container designs on the market, the Ray Leach Cone-tainer™ cells are still popular with native plant growers. In this system, individual soft, flexible plastic cells are supported in a durable hard plastic tray (See Figure 13.13).
Deepots™
Deepots™ These are single cells, constructed of thick plastic and held in hard plastic racks (See Figure 13.14). Available in several sizes, they have internal vertical ribs for root control and supports on the bottom of each container provide stability.
Book or Sleeve Containers
Spencer-Lemaire Rootrainers™
These unique “book” containers are composed of flexible plastic cells that are hinged at the bottom, allowing the growing media and root system to be examined during the entire growing season when the books are open (See Figure 13.15).
Rectangular Block Containers Made Up of Many Cavities or Cells
Styroblock™ and Copperblock™
Styroblock™ containers are the most popular type of container used in forest nurseries and are available in a wide range of cavity sizes and densities (See Figure 13.16), although outside block dimensions are standard to conform to equipment handling. This container has also been used for growing native grasses, woody shrubs, and trees.
Plug Trays
Plug trays are very common containers in the greenhouse (See Figure 13.17). Plug trays are useful for germinating seeds or rooting cuttings, since they permit young plants to be moved easily. When the bedding plants are ready for transplanting, the plants are easily pushed out of the cell pack and transplanted without any damage to the root system. The bedding plants thus get established quickly without any losing quality. The size of plug trays that has been most commonly used in the industry for years is 11 inches wide by 22 inches long. However, with pressure from the mass market to hold prices, some wholesale growers are using slightly smaller trays-10 inches by 20 inches.
Propagation Containers
Plug Trays
Plug trays are very common containers in the greenhouse (Figure 14.17). Plug trays are useful for germinating seeds or rooting cuttings, since they permit young plants to be moved easily. The size of plug trays that has been most commonly used in the industry for years is 11 inches wide by 22 inches long (28 x 56cm). However, with pressure from the mass market to hold prices, some wholesale growers are using slightly smaller trays—10 inches by 20 inches (25 x 50cm).
Cell Plug Try Inserts
Cell plug tray inserts are placed inside the plug trays, which allow the grower to easily separate species in one tray, reducing potential disease spread, and easing transplanting (Figure 14.18). When the bedding plants are ready for transplanting, the plants are easily pushed out of the cell pack and transplanted without any damage to the root system. The bedding plants thus get established quickly without any losing quality. Cell plug inserts can be used without the support plug tray; however, this is not recommended.
Flats
Currently, container trays are almost exclusively plastic (polyethylene or polystyrene), 11 by 22 inches by 2½ inches deep, and are referred to as “flats” or as a “1020 flat” (See Figure 13.18). Flats are easily cleaned, are reusable, and can be nested and stacked for space-efficiency storage. Characteristics of flats include raised ridges on the bottom surface for enhanced water drainage and gas exchange, a curved top edge to assist in carrying flats and separating nested flats, and a reinforced rib to extend the lifetime and facilitate reuse.
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