Factors to consider when choosing a substrate for lettuce production, greens production

Whether a grower chooses to propagate in rockwool, cellular foam or an organic substrate, there are other factors that can impact crop production.

Since lettuce, greens and microgreens are relatively short term crops compared to other greenhouse vegetables like tomatoes and cucumbers, a grower has to make sure that he starts out with a consistent substrate and quality seed. Sylvain Helie, an agronomist and phytotechnologist at Jiffy products of America, said one of the key factors in growing lettuce, greens and microgreens is having a good seed supplier.

“Before a grower chooses a substrate he has to know his market,” Helie said. “Is he going to sell to retailers like grocery stores or to restaurants? market dictates what and how the crops will be grown. If a grower is working with chefs, they are likely going to provide the grower with feedback as to the type of crops and packaging they need. e proactive grower is not only going to provide his customers with what they want, but is also going to supply them with new plants to sample.” Helie said growers should pay particular attention to new varieties coming from the United Kingdom and Europe.

“It’s very similar to what is happening with new flower varieties from proven winners,” he said. “A grower of greens and microgreens won’t survive if he doesn’t have new varieties. e grower has to know what’s new as well as what novelties are available. To be able to do this the grower has to have a reliable and innovative seed supplier. If a grower is supplying chefs with greens, he has to be able to o er them different, attractive colors—greens, yellows, reds.”

Choosing an irrigation system

For the grower with limited production area and limited resources, Vijay Rapaka, manager of grower research at Smithers-Oasis Co., said nutrient lm technique (NFT) is the ideal system for lettuce and leafy greens, especially for a small size operation.

“The NFT system is easy to install,” Rapaka said. “ e grower recirculates the nutrient solution and maintains the nutrient balance in a stock tank or nutrient mixing tank. Using NFT, the seedlings can be grown in gutters or channels. ere are a lot of systems that are commercially available.

“With NFT the roots are being bathed in a shallow lm of nutrient solution that is constantly recirculated . The root system grows in and above the shallow stream of nutrient solution.”

Phil Johnson, crop consultant propagation at Grodan, said the grower who is operating a NFT system can choose between two different types of gutter irrigation systems.

“Growers have different options when choosing a gutter system,” Johnson said. “For the most part the choice comes down to the initial setup costs.

“There are rigid gutters that are more permanent, which Johnson said are basically extruded plastic, similar to gutters on a house.

“These systems can be fixed rigid gutters or they can be put on rollers,” he said. “ e movable system allows the gutters to be moved back and forth so they can be pushed together or apart to maximize space. when the plants are young, the gutters are spaced close together. As the plants grow and take up more space, the gutters are moved farther apart.”

Johnson said there are also gutter systems that are made of a flimsier plastic that are disposable after a few crops.

“These gutters have a shorter lifespan,” he said. “They are relatively inexpensive so they cost less to install. A grower can use the gutters for a couple of seasons and then throw them out and then install another clean setup.”

Johnson said the disposable gutters can be a good option for growers looking to start a business.

“Essentially it can be expensive to setup a system from scratch,” he said. “Once a grower gets his business established he can put in a better, more expensive system. is disposable type of system would be ideal for a small family business looking to get started.”

Another irrigation system that Johnson said is being used more in Europe and larger operations is floating rafts. The growers use large rafts of polystyrene and rooting cubes are stuck in holes in the ra s and then floated on a big channel of water.

“The crops are usually started at one of the end of the greenhouse as a small propagated plant,” he said. “The rafts are pushed the length of the greenhouse as the plants grow larger. Eventually the ra s reach the other end of the greenhouse and the plants are harvested making room for the next crop that is moving in behind them. It’s a constant ow similar to a production line, but it’s with plants.”

Choosing a growing substrate

Rapaka said that the basic function of the substrate is to establish uniform vigorously growing seedlings. Once the seedlings have true leaves and roots growing out of the substrate, the seedlings are ready for transplant. It takes two days for the seed to germinate. The seedlings are ready to transplant in 10 to 16 days depending on the season.

The seedlings take up less space so one efficient way to use the space is it hold the seedlings longer in a nursery area. However, the seedlings should not be allowed to become overcrowded because stretching can occur. Rapaka said the higher the plant density per tray the sooner the seedlings need to be transplanted.

He said there are various substrate configurations available and growers should select the ones that best meet their production needs and specifications. Once the seedlings are transplanted into a NFT or floating raft system, the substrate simply acts as an anchoring material and the root system grows outside the substrate.

Michael Christian, CEO and president at American Hydroponics, said when a grower starts producing lettuce or greens using a NFT system, the preferred growing substrate is a root cube. “

The root cube is used basically as a seedling germination and plant support structure,” Christian said. “ The media is actually the nutrient solution in which the roots live and derive oxygen and nutrients.

“The grower usually starts with a seedling tray and moves the two-week-old seedlings into a plant site in the nursery channel. After two more weeks the plants are moved into a finishing channel. Two weeks later the plants are harvested. is production schedule allows a grower to achieve 26 crop turns per year.”

Christian said when he starts working with a new grower he usually recommends using an Oasis cube.

“For the beginning grower, we’ve found, it’s easier to sow the seed into the Oasis cube,” he said. “Growers who are just starting out are doing their planting by hand and it’s easier for most to do with Oasis. Also, we can guide the grower through the watering cycle to make sure the Oasis cubes are not too wet and are at the right moisture level for optimum seed strike, uniformity and growth.”

Christian said one of the advantages of the rockwool cube is its durability in shipping.

“When we ship long distances, for instance large containers to other countries, we ship rockwool,” he said. “It is very durable and can withstand the rigors of shipping.

“The two substrates are similar in their seedling configurations. some growers will use rockwool during the winter and Oasis during the summer.”

Rapaka said that uniformity and vigorously growing seedlings are crucial to ensure successful production. The Oasis Horticube is a cellular substrate engineered with an optimal air:water ratio for rapid germination and seedling growth. Horticubes have precise dibble holes and uniform depth which allows for either manual or automated seed sowing. Horticube sheets are etched around each cube allowing for easy separation into individual cubes at transplant.

“The Horticube material is inert,” Rapaka said. “The pH of the cube is about 5, but once water and the nutrient solution are applied it changes to the pH of the nutrient solution. The cube doesn’t have any buffering capacity or cation exchange capacity and it doesn’t contain any fertilizer. It offers growers the flexibility to change the pH and electrical conductivity to what they want.”

Propagating the starter plants

Johnson said most growers produce their starter plants in a small propagation area.

“Seed germinate in the rooting cube within 48 hours a er being sown,” Johnson said.

Rapaka said that after initial watering, the Horticube trays should be placed in a dark room at temperatures between 66 F-68 F. Darkness is not required for germination, but it can help the seedlings start uniformly. e seed doesn’t need to be covered with vermiculite or any other type of material.

Rapaka said the trays need to be moved from the dark room to the greenhouse a er 48 hours. Leaving the trays beyond 48 hours can lead to stretching of the seedlings.

“Depending on the climate, time of year and size of the rooting cubes, the plants are usually grown on until the stage where the leaves of the plants are touching each other,” Johnson said. “The plants are generally maintained for 14-21 days before the cubes are moved into the production area. en the cubes are separated and put out in either a NFT gutter system or ra system.

“It’s cheaper to keep the plants in the propagation area because a smaller area is being heated and cooled. It’s also easier to create a good microclimate around the small plants. e bigger the plants can be grown in the propagation area without allowing them to get leggy, the less time they will spend in the production area. The benefit of this during the summer is when larger plants go out into the production area where the climate is hotter and drier, the plants are more able to deal with and manage these conditions.”

Johnson said one of the advantages of sowing the seed into rockwool cubes is the uniformity they provide when growing a crop.

“A grower wants plants to grow at the same speed so they are the same size,” he said. “when the plants are moved out into the production area a grower doesn’t want to have to be harvesting a few plants at a time because they are growing at different rates. A lettuce grower wants to be able to bench clear a crop just like a flowering crop. e grower wants uniformity across the crop from day one.

“Rockwool is inert with no buffering capacity. is enables a grower to make a very precise fertilizer recipe so that the nutrient solution delivered is what is available to the plants. Rockwool is also a clean substrate and won’t clog filters or cause the irrigation system to become clogged over time.”

Peat pellets work with organic program

Helie is working with an organic grower of lettuce, greens and microgreens who is using Jiffy 7 pellets and QSM (quick soil mix) cubes.

“Jiffy was able to supply the grower with a pellet that was made from only peat and lime. e pellet contained no conventional fertilizer charge,” Helie said. “we sent the ingredient list to the grower’s certification organization and it was accepted. The product did not have to be OmRI listed. Jiffy has also produced a rectangle-shaped QSM cube that the grower can use in his NFT gutter system to produce lettuce and greens.”

The grower is also using the rectangle-shaped cubes to produce microgreens. A 26- by 13-inch, vacuum-formed, white germination tray holds 18 of the cubes.

“The trays are solid with no drain holes,” Helie said. “The employees know exactly how much water needs to be added to the tray to thoroughly wet the cubes. The microgreens seed is germinated in the cubes and after five to eight days the plants are ready to harvest.”

Helie said the grower is also using the round-shaped QSM to grow microgreens in clear plastic clamshells.

“One application of water and fertilizer is applied to one cube in each clamshell,” he said. “ is expands the cube and the microgreens seed is sown. In five to eight days the microgreens are up, the clamshells can be closed and shipped to market.”

 

For more: American Hydroponics, www.amhydro.com. Grodan, www.grodan. com. Jiffy products of America Inc., www.jiffygroup.com. Smithers-Oasis Co., www.oasisgrower.com.

David Kuack is a freelance technical writer in Fort worth, Texas; dkuack@gmail.com.