Fig. 1. Hundreds of lettuce and culinary herb seedlings grown in phenolic foam cubes. The cotyledons are fully expanded and the first true leaves will develop next.
Photo: Christopher J. Currey

The first phase of production for any hydroponic food crop, whether leafy greens or vine crops, is propagation. In ornamental horticulture, there are numerous propagation methods that are used to produce new plants. For hydroponic food crops, there are three primary methods of propagation: seed propagation, cutting propagation and grafting. The aim of this article is to discuss when these different propagation techniques are used, as well as key points to managing the greenhouse environment, culture and systems.

Seed propagation

Propagating young plants for hydroponic production with seed is the most common method of propagation (Fig. 1), since the clear majority of food crops can be easily grown from seed. The cost of seed for hydroponic crops varies widely across different crops. For instance, seed for some culinary herbs is extremely cheap, while seed for some greenhouse cucumbers can cost close to $1 per seed!

Seedlings are grown in either organic or inorganic substrates. Inorganic substrates such as stone wool or phenolic foam are the most commonly used substrates for seedlings. These inorganic substrates are commonly sold in sheets that have from 50 (large cubes) to more than 200 individual cells (small cubes) in the same sized sheet. Organic substrates such as ground sphagnum peat moss or coconut coir may also be used, though these loose materials need to be stabilized chemically (using glues or binders) or physically (using paper or mesh wrappings).

Fig. 2. Here we see a common propagation system used for producing lettuce and herb seedlings in greenhouses that utilizes nutrient-film technique finishing systems.
Photo: Christopher J. Currey

Closely managing the environment — moisture, temperature, light and mineral nutrients — can have a big impact on seedling production time and quality. After seeds are sown, they should be thoroughly watered in using a low-strength fertilizer solution from the beginning. While the seeds are germinating, air and substrate temperatures should be warm, ranging from ~70 to 75°F, depending on the species. After germination has occurred, air and substrate temperatures can gradually be reduced to levels similar to the grow-out environment. While some seeds need light to germinate, the intensity of light doesn’t need to be high at first. However, once the cotyledons start to expand, increase light intensity to enhance seedling growth and development. The time to produce transplantable seedlings varies with species and culture. Some lettuce can be ready to plant in as little as two weeks, and tomatoes and pepper seedlings can be ready to transplant into larger 4-inch blocks in three to five weeks.

Seedlings can be grown in a wide variety of systems and with a range of methods, all ranging widely in price, level of automation, etc. The simplest method is to produce seedlings right on the greenhouse bench or floor, like how other seedlings or transplants may be grown. Electric heat mats can be used to provide bottom heat to maintain substrate temperatures. Alternatively, seedlings can be produced in more specialized systems (Fig. 2) that are designed specifically for producing young plants.

Cutting propagation

Plants are also vegetatively propagated through cuttings for certain crops. Though seed is the most common method used for propagation, certain plants such as perennial culinary herbs are propagated by cuttings. Stem-tip cuttings are most frequently used, as opposed to root or leaf cuttings. Unrooted cuttings can be purchased from commercial suppliers or you may choose to grow stock plants and harvest your own cuttings. If you do decide to grow your own stock and propagate cuttings, be sure that you are only propagating non-protected varieties or are paying the royalties to propagate any protected varieties.

Cuttings can be rooted in the same substrates used for seedling production. While rooting hormone isn’t necessary, it can improve the uniformity of your cuttings and, therefore, your finished crop. Like any seedling production, light, temperature, nutrition and water all affect the growth and development of new, or adventitious, roots and, therefore, the final quality rooted cuttings. Since the cuttings lack roots, mist needs to be applied to keep them turgid. While the mist needs to be applied frequently in the beginning of propagation, it should be reduced as adventitious roots emerge and grow into the substrate; after seven to 14 days (depending on the species), mist may be discontinued and cuttings may be irrigated on an as-needed basis. The air and substrate temperature should be warm (~72 to 75°F) at the beginning of propagation. As roots develop and shoot growth resumes, bottom heat may be discontinued and air temperatures may be lowered. Alternatively, while some shade may be used for the first few days in propagation to help keep cuttings turgid, light intensity should increase as cuttings grow to promote root growth and reduce the time to transplanting. Finally, even though roots are not present, fertilize cuttings periodically beginning at sticking to maintain tissue nutrient concentrations, increasing concentrations as the roots develop and the cuttings start to actively grow.

To successfully propagate cuttings, the two most important components of any propagation system are: 1) a mist system; and 2) bottom heat. The mist system is vital to keep these rootless cuttings turgid; an automated system is easily the most convenient method for applying mist. Bottom heat is used much like in seed propagation — to maintain warm substrate temperatures. However, instead of maintaining substrate temperatures to promote seed germination, the warm temperatures promote adventitious root initiation and subsequent development.

Grafting

Fig. 3. This tomato plant was grafted using the tube method.
Photo: Christopher J. Currey

Another type of propagation method that is increasing in popularity among some hydroponic vine crop producers is grafting. Grafting is the process of combining a desirable root system, or rootstock, from one plant with a shoot, or scion, from another plant that produces desirable fruits (Fig. 3). While grafting is used to impart some type of disease resistance to plants when planted into field soils in high tunnels, grafting vine crops for hydroponic production is primarily to improve vine vigor.

The most widely used grafting method is the tube graft. It is the most commonly used method for the most popular greenhouse vining crop from seed in the U.S., tomatoes. Although grafting is considered a type of vegetative propagation, the rootstocks and scions come from young plants propagated from seed. To produce the rootstocks and scions for grafting, follow the procedures for seedling propagation and production.

One of the most critical aspects of grafting is healing, when the vascular tissues of the rootstock and the scion are connecting to allow water uptake by the scion from the rootstock. By providing the appropriate conditions, the healing period is between five to seven days.

As previously mentioned, the vascular tissues of the scion and rootstock are forming connections during this period, after which water will be able to be taken up by the roots and translocated to the shoot. As a result, the healing environment should be one that inhibits transpiration and water uptake while maintaining turgidity. Keeping the relative humidity high helps reduce transpiration and water loss, keeping the scion turgid. Additionally, since light promotes transpiration, recently grafted plants should be placed in darkness for one or two days, after which time the light intensity can increase up to ~100 µmol·m–2·d–1.

The take-home message

The ultimate goal of propagating hydroponic food crops is to efficiently produce high-quality young plants in a timely fashion. Whether you are using growing seedlings, rooting cuttings, or grafting plants, starting off with healthy young plants can improve uniformity and yields. Careful monitoring and management of the greenhouse environment (light and temperature) and culture (water and nutrition), coupled with the appropriate systems for the different propagation methods, will allow you to start each crop off with the best plants.

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