Growing quality transplants requires good seed, a sterile and well-draining growing medium, proper temperature and moisture conditions, adequate light, and a structure that has built-in capacity to adjust these factors, especially temperature and humidity. However, many home gardeners and small growers in West Virginia start seedlings inside their houses using artificial light or often relying only on the window light. Commercial seedling growers generally use heated high tunnels or greenhouses for the same purpose. But, these indoor growing conditions in many cases are less than ideal due to the lack of options to manipulate temperature, humidity and light intensity. Planting medium used for growing seedlings also may not contain optimum fertility or be free from infectious plant pathogens. In addition, various gases can cause injury to seedlings due to their sensitivity at the early stage of development.
Temperature
Temperature can affect germination, seedling vigor and even cause mortality. Not all plants germinate well in the same temperature. For example, warm weather crops, like tomatoes, will germinate better with temperatures in the 70 F range, while cool weather crops, such as cabbage, kale or broccoli, have a much lower germination temperature. During the early spring, plants in non-heated structures, like high tunnels, can encounter near freezing temperatures and show dead areas between the veins of older leaves and on the tips of the very youngest leaves. Tomatoes, specifically, are very susceptible (Figure 1), but in most cases, plants recover with the onset of warmer temperatures. Greenhouses that do not have good heat and air recirculation systems can have high temperatures underneath heaters, causing seedlings in that area to become heat stressed and subsequently attacked by opportunistic fungi.
Figure 1. Cold injury on tomato leaves (dead areas between the veins) in a high tunnel without supplemental heating.
Soluble Salt and pH of Growing Medium
Most people use a potting mix, such as Sun Gro, Miracle-Gro or Pro-Mix, to grow seedlings in plastic plug trays. Some growers use locally available composts or topsoil. Normally, these media contain enough nutrients to support early growth of seedlings. However, some growers mix extra fertilizers, such as Osmocote or some other pre-plant fertilizer, that can increase soluble salt content to such an extent that it can cause injury to seedlings. The symptoms first appear as marginal leaf scorch, stunting and may eventually kill the whole seedling (Figure 2). If you notice any injury or plant stunting, you should measure pH in addition to soluble salt to make sure pH is between 6 and 7 and EC or electrical conductivity, which is a measure of soluble salt, is not more than 1.7 millimhos per centimeter from a 1 to 2 (substrate to water) dilution ratio. Too high or too low pH also will obstruct nutrient uptake by seedlings. During seedling production, it is better not t o use any pre-plant fertilizer. As plants start growing actively, fertilizer can be supplemented based on the need.
Figure 2. Tomato (left) and crucifer (right) seedling mortality due to high soluble salt in the growing media.
Humidity
Relative humidity in indoor conditions for vegetative growth of plants should be between 60% to 70%. Humidity below or above that range can affect plant growth. Under very high humidity, indoor-grown tomatoes, crucifers and some other plants may show a physiological disorder called edema (Figure 3) that develops when roots take up water faster than it can be used by the plant or transpired through the leaves. Water pressure builds up in the internal cells of the leaves, causing them to burst and leave dead cells that are visible as blisters, which are primarily on the undersides of leaves. Plants usually recover when transferred to low humidity growing conditions or when humidity is controlled by some other means, such as an exhaust fan or opening the sides and windows during warmer temperatures.
Figure 3. Tomato leaves in an indoor growing condition showing a blister-like appearance due to edema.
Toxic Gas Injury
Injury due to a faulty heating system and the generation of toxic gases also may occur in some cases when starting seedling production indoors. If exhaust from a methane or propane heating system is obstructed or an incomplete combustion of gas occurs, ethylene gas can be formed, which can cause injury to tomatoes, even at a very low concentration (greater than or equal to 0.1 parts per million). Symptoms include stunted growth, pronounced downward curling or twisting of the leaves and shoots (epinasty), and a thickened stem (Figure 4). Ornamentals, such as baby’s breath euphorbia ( Euphorbia hypericifolia) and Mexican heather ( Cuphea hyssopifolia ), also are very sensitive to ethylene.
Figure 4. Ethylene injury on a tomato seedling inside a greenhouse caused by a faulty heating system.
Light and Moisture
Light is generally not required for seed germination, except for a few species of plants with fine seeds that need to be planted shallow. However, a fluorescent or LED lamp with photosynthetically active radiation will usually be the best choice to ensure that post-germination seedlings get the quality, intensity and duration of light they need. A timer can be attached to the light source to ensure the light stays on for an appropriate number of hours. Without adequate light, seedlings will be leggy, or etiolated, and will not establish well in the field. Watering should be done only when needed – growers can check moisture by feeling the medium by hand or by observing when the soil has pulled away from the wall of the pot. Excess watering and a lack of adequate drainage will deprive the plant’s root system from air and also enhance fungal growth, which may cause damping off or early mortality. Damping off can be prevented by using clean containers and a sterile, well-draining potting mix, and by following good cultural practices. If containers are reused, those should be surface sterilized by dipping them in a 10% bleach solution for 30 minutes, followed by thorough drying.
Acclimation and Hardening Off
Indoor-grown plants are not ready to withstand harsh conditions in the field. Some plants are more sensitive than others. Cucumber transplants grown indoors can get extensive sunburn if taken directly to the field. It is a good idea to put plants outside for five to seven days prior to planting, gradually increasing the sunlight exposure hours each day.
Broad Mite Injury on Pepper
Greenhouse-grown pepper seedlings can have substantial injury from broad mites. The primary symptom is curling and distortion (Figure 5) of leaves that can easily be confused with growth regulator-type herbicide injury. These mites are very tiny and may require a very high magnifying lens to detect their presence. Most of them can be found on the lower side of the leaves or on the stems.
Figure 5. Broad mite injury on a pepper seedling (left), plant in the field at pre-bloom stage (center), and a broad mite under 50-times magnification (right).
Author: Mahfuz Rahman, WVU Extension Specialist
Last Updated April 2022
