Transplant stunting can be caused by several factors during greenhouse production, during shipping and handling, during planting, and post planting.
Problems with greenhouse media
Each year there are some problems that arise with vegetable transplants related to issues with the growing medium. This is often seen as poor growth, yellow plants, or stunted plants. Greenhouse media manufacturers have good quality control measures in place but things can go wrong on occasion – inadequate mixing, critical components missing or in the wrong proportions (i.e. wetting agents, fertilizers, lime), or defective, poor quality components. Media can also be affected by poor storage and handling. Most commonly this occurs when it is stored outside and bales or bags get wet. In addition, media has a certain shelf life – old media often dries out and is hard to get rewetted.
When growers start filling trays, any media that does not handle well should be viewed as suspect and should not be used. Contact your supplier and have them inspect and run tests on the suspect media. Avoid using overly dry or caked media, media that is hard to loosen, media with a bad smell, water soaked media or media that is hard to wet.
Most media (but not all) will come with a starter lime and fertilizer charge. The fertilizer is designed to give about 2-3 week of nutrients. If the fertilizer is missing or improperly mixed or in the wrong proportion, seeds will germinate but seedlings will not grow much and will remain stunted. In this case, liquid fertilizer applications will need to be started soon after plant emergence.
Peat based media are acidic in nature and we generally can grow at lower pHs than soil. Plants will perform well from 5.4 to 6.4. Lime is added to peat based media and reacts over time after first wetting so pH will rise over time. Above 6.4 we often see iron deficiencies in transplants. This also occurs if irrigation water is alkaline (has high carbonates) causing pH to rise too high over time.
In high pH situations, to get transplant growth back to normal, use an acidifying fertilizer (high ammonium content) for liquid feeds. Use of iron products, such as chelated iron, as a foliar application on transplants can help them to green up prior to the pH drop with the acid fertilizer. In severe cases with very high media pH, use of iron sulfate solutions may be needed to more rapidly drop the pH. Acid additions to greenhouse irrigation water may also be considered for where water is alkaline.
If lime is missing or inadequate, and pH is below 5.2, plants may have calcium and magnesium deficiencies or may have iron or manganese toxicities. This also occurs in media that has been saturated for long periods of time. To correct this situation, apply a liquid lime solution to the media and water it in well. Calcium deficiencies will lead to damage to growing points and stunted and distorted plants.
Media that does not wet properly may not have enough wetting agent or the wetting agent may have deteriorated. They will be difficult to water and will not hold water well thus stressing plants. Application of additional greenhouse grade wetting agent may be needed.
If the initial media fertilizer charge is too high, or if too high of concentration of liquid fertilizer feed is used, or if incorporated slow release fertilizer “dumps” nutrients, high salt concentrations can build up and stunt or damage plants. Leaf edge burn, “plant burn”, or plant desiccation will be the symptoms. Test the media for electrical conductivity (EC) to see if salt levels are high. The acceptable EC will depend on the type of test used (saturated paste, pour through, 1:1, 1:2) so the interpretation from the lab will be important. If salts are high, then leaching the media with water will be required.
Problems with transplants in small cell sizes
More and more transplants are being grown in small tray cell sizes. These small size transplant plugs can become extremely root bound and may not put on new roots after transplanting. Another issue is when small cell transplants become waterlogged by overwatering. There will be limited oxygen to roots in this situation and plants may turn yellow and remain stunted. This is very common in peppers.
Problems related to transplant height control and greenhouse conditions
Growers use a range of techniques to manage transplant height in the greenhouse. This includes limiting phosphorus (P) fertilization, minimizing day-night temperature differentials, brushing plants, limiting water, and using plant growth regulators (limited for vegetable transplants). Each of these if not properly managed can cause long term stunting. Most growth regulators labelled for floral crops are not labelled for vegetables. Plants exposed to limited P may have a severe deficiency that will take several weeks to grow out of. Warm season transplants exposed to cold air can become yellow and be stunted because roots stop growing. This is particularly a problem near vent inlets and in hardening off areas. Plants that are overly water stressed drop leaves and take a long time to recover. Plants exposed to damage from heaters that are improperly venting exhausts into the greenhouse may suffer severe damage and show yellowing, distorted growth, and leaf drop. Diseases of roots, Pythium in particular, can be an issue, particularly when plants are placed directly on the ground (even if landscape fabric is in place). This can be a major source of plant stunting and transplant losses.
Herbicide use in and around greenhouses
Transplant deformities and stunting can also occur when herbicides have been used to kill weeds in and around greenhouses. In the enclosed environment of a greenhouse, volatilization is enhanced and severe damage can occur from many common herbicides. Greenhouse vents and fans can draw in herbicides applied nearby also causing severe damage.
Problems during shipping and handing
With the wet weather, many growers received boxed transplants from southern sources but could not plant immediately. Plants that are shipped without trays (already pulled) or that are bare rooted that are packed tightly in boxes must be planted quickly. Delays will lead to plant deterioration, leaf loss, and potential disease buildup. Once transplanted, some of these plants may now grow out.
Planting and plant stunting
Transplants that are planted in extended cloudy periods may not grow well in the field, especially if plants have come out of the greenhouse after an overcast period. In years with cold, cloudy, windy weather after transplanting, we have had large losses of transplants in the field. It is critical to have warm soil conditions after transplanting to allow roots to grow out into the bed quickly. In cold, cloudy conditions, plants shut down physiologically, little root growth occurs, and the existing roots on the transplant do not function well. If there is any wind, plants lose more water than they can take up and they die due to desiccation. This is accelerated when the sun does come out – the first sunny day after an extended cold, cloudy period will often result in extensive losses of weakened transplants. Extra caution should be taken to minimize root injury during transplanting, particularly with seedless watermelons. When transplanting, make sure that there is good root to soil contact and there are few air pockets around roots. Plant stunting can also occur with improper application of chemicals or fertilizers in the transplant water (phytotoxicity, salt damage)