Summary and discussion– Morgan 2003


“Hydroponic Tomatoes: The Complete Guide to Soilless Success

Part Two: Production Systems and Crop Management”

L. Morgan

Published 2003, The Growing Edge 15(1), pp 60-73.

Summarized by Anita (Teena) Hayden, PhD (UTK, June 2007)


This is the second in a series published in the popular magazine The Growing Edge (Corvallis, OR). The author, Dr. Lynette Morgan, is a regular contributor to the magazine and is a horticultural consultant who holds a PhD in vegetable production from Massey University in New Zealand. Although her perspectives on greenhouse culture, markets, and cultivar selection are slightly different from what we might experience in Tennessee, she writes for an American audience and the basic science and production issues she addresses are universal.


Types of Tomatoes Recommended for Hydroponic Production - Dr Morgan begins this lengthy article with a review of the different types of tomatoes developed specifically for hydroponics. The variety ‘Trust’ is the commercial standard favored in the US for its fruit size (200-300g or approximately one-half pound). Varieties with smaller fruits (averaging 140-150g, or 0.3 pound) are suited for either single or multi-truss harvesting. Heirloom varieties are sometimes tried by hydroponic growers, but high rates of culled fruit due to cracking or poor color as well as deformed fruit that do not pack well have discouraged most growers. However, two varieties, called ‘Moskvich’ and ‘Thessaloniki’ did perform better in greenhouse trials. 


Specialty varieties such as Plum or Italian tomatoes are becoming more popular for hydroponic producers. Unfortunately, blossom end rot (BER) is a problem in these elongated fruits, especially under warmer conditions. Cocktail, cherry and miniature tomato fruits average 20g each (less than an ounce), produce much lower yields and hive higher labor requirements.


Soilless Systems – Media-based Systems

Selection of media should be based on what is locally available and cost effective. Rockwool slabs are used world-wide, are pre-sterilized and have excellent water and air holding capacities. Rockwool does not interfere with the nutrient formulation, however on a commercial scale, disposal of old rockwool is an environmental concern. Media from natural sources, such as sawdust, bark, gravel, coir, vermiculite, sand, perlite, pumice, expanded clay, rice hulls, and others may be preferable. These substrates are often used in re-usable containers (Dutch pots or PVC channels), or disposable bags. Organic substrates, such as compost or vermiculture (worm castings) are gaining popularity in smaller systems, however the nutrient management of these systems is quite challenging compared to traditional hydroponic systems.


Substrate Selection Criteria

Economics – compared other greenhouse operation costs (heating, cooling, labor), growing media costs are not high, however media can be bulky and growers should consider using locally available media when possible.


Pests, Diseases, and Contaminates – Media contaminated with disease spores, pests, or high levels of toxic salts (such as sodium in coir) can be a serious problem. Compost, in particular, can contain very high levels of heavy metals, such as copper, that can retard growth. Sawdust from timber that has phytotoxic compounds or treated lumber should also be of concern. Pests, such as fungus gnat larvae, can damage roots of young seedlings.


Physical Structure and Consistency – The two most important characteristics of substrates are its water-holding capacity and aeration. Both of these can change over time as the root systems grow and the media breaks down. Root-zone problems can show up in the crop as wilting during the middle of the day, brown roots, flower or fruit drop, leaf yellowing, or overall lack of plant vigor. The life-span of the media is an important selection criteria, but growers should take into consideration the labor involved in maintaining the media. Rockwool has been used for as many as six to seven tomato crops before replacement, and perlite is also successfully re-used for many crops. Organic substrates have much shorter life-spans in greenhouse systems.


Nutrient Interactions

Organic substrates, such as compost, sawdust, or peat-based media can interact with the nutrient solution and interfere with plant uptake of nutrients by binding with certain nutrients or affecting the pH. “Inert” media, such as rockwool or perlite, are more consistent.


Disposal & Labor

Although smaller growers do not necessarily need to worry about disposal of media, commercial growers should not discount the cost of disposal. Cost of labor to fill containers with media is another factor that some start-up operations fail to consider.



Most media-based systems use drip irrigation that run several times per day, and are non-recirculating, passing the solution through the media once with excess solution running to waste. However, more growers are realizing the need to recover and recirculated the solution, achieving zero waste. The skills needed to successfully manage these systems are very high. The environmental advantages of recirculating systems are obvious, but another advantage is the ability to over-irrigate in order to permit adequate leaching of the media and prevent salt build-up without the worry of wasting fertilizers.


The frequency and amount of irrigation to apply is a common problem for inexperienced growers. The amount of irrigation needed is dependent on the media used, stage of growth of the crop, and environmental conditions. Young seedlings may only need one or two irrigations per day, whereas a mature crop may need a dozen or more in order to supply sufficient water and nutrients to the crop. Commercial growers often rely on computer-controlled systems based on sensors placed throughout the crop, but simple collection trays under drippers and plants located throughout the house that permit the grower to monitor the volume of drainage may be sufficient. Generally, obtaining 10% drainage at each irrigation cycle should be sufficient for most conditions, but certain media or environments may require additional drainage to keep the root zone in good condition.



Non-media based systems

Nutrient Film Technique (NFT) and Deep Flow Technique (DFT) are two media-free systems that utilize a recirculating flow or stationary pool of nutrient solution. NFT systems involve channels of PVC or plastic sheets where a small amount of fertilizer solution (the “film”) flows across the roots of the plants. NFT systems for tomatoes need to be made larger than those for lettuce or strawberries. DFT is less common for tomato production, and requires careful attention to the oxygen levels in the root zone. Aeroponics is a system where the roots are sprayed with the nutrient solution inside a dark chamber. Aeroponics is not widely used for tomato production due to the high capital expense and maintenance required. Ebb-and-flow, continual flow gravel beds, and aquaponic systems are other non-media based systems that can be used for tomato production, but usually on a small scale.


Nutrient Management

Huge yields are possible from modern varieties grown under ideal nutrient management, however, there is no one optimal nutrient formulation for all hydroponic tomato crops, so the grower must continually monitor the status of the plants and understand the chemistry of the fertilizers. Recirculating systems change over time as crops pull needed nutrients from the solution. Although the solution may be adjusted daily with fresh stock fertilizer or water, certain compounds, often potassium, can become depleted without the grower’s knowledge. Non-recirculating systems permit less chance of a fertilizer becoming depleted, however even these systems may fail to deliver the needed nutrition to the crop during certain crop stages.


Cropping Systems

Bush-type tomato plants (“determinate” varieties) are usually grown in the field, whereas vine-type varieties (“indeterminate”) are most common in the greenhouse. Commercial greenhouse operations generally grow vine-type crops for at least 10 months by layering, or leaning and lowering, the vines as they grow to keep the level of the harvested fruit at a convenient height. As the plants are lowered, fruit is harvested and older leaves are removed. The advantage of the long vine system is that the majority of the time the plants are in the house, they are producing marketable fruit. Disadvantages include the difficulty in maintaining high-quality fruit as plant vigor declines with age, as well as the risk of pests and diseases such whitefly or Botrytis becoming entrenched in a mature crop. Also, growers in extreme climatic conditions may choose to grow shorter-term crops to target the harvest period for the most profitable times of the year.


Single-truss cropping systems are one method to utilize the new vine-type varieties in growing systems that favor bush-type plant architecture (such as NFT or ebb-and-flow systems). In single-truss systems, plants are grown in very high-density plantings (12-16 plants per square meter, or approximately 1.5 plants per square foot), and the plant is cut at the 2nd or 3rd leaf above the first fruit cluster. The concept behind this system is to produce at least four crops per year of the highest quality fruit. Fruit size can be exceptionally large in this system, with better flavor characteristics than the same variety grown under long-term conditions. Continual harvest can be achieved by seeding every three weeks, with yields as high as 60 kg per square meter (12 pounds per square foot) per year. 


Yields and Flavor

Hydroponic tomatoes have the potential to produce exceptionally high quality fruit with excellent fruit quality and yields through the manipulation of the fertilizer mix and the environmental control. Light is the main factor that determines the amount of sugars and the size of the fruit. The soluble solids (sugar) content of fruit produced during a 16 hour day is significantly higher than that produced during a 12 hour day, while acidity is not affected by day length. Sugars are also influenced by moisture and high temperature. High air temperatures above 84F are detrimental to both yield and quality. Soft fruit and blotchy color are both linked to high temperatures. Removing too many leaves from a plant can also result in lower fruit quality, since the leaves closest to a fruit are responsible for producing the sugars that ultimately end up in the fruit. Key to fruit quality is managing the root zone. Increasing the electrical conductivity (EC) of the nutrient solution helps produce firmer fruits with higher flavor. Although this has been seen in both beefsteak and cherry tomatoes, the cherry tomato varieties respond well to very high ECs (10mS/cm compared to the usual 2.5 mS/cm). The downside of increasing fruit flavor through increased EC is a reduction in yield. Therefore, a commercial grower who is marketing fruit as a commodity and selling strictly by the pound has little incentive to improve flavor if it means less yield. However, a smaller operation with a roadside stand may be able to compete with grocery chain stores if they can emphasize the flavor attributes of their product.



(Hyperlink to definition of indeterminate -plants that have a vine-like character. Contrast to a bush-like character, called a “determinate” plant. These terms come from the botanical terminology referring to plants that cease growing larger as soon as flowers are produced, so their size and shape is “determined” by flowering. Indeterminate plants continue growing even after flowers are set)