After a seed has sprouted—but before it enters the preadolescence of vigorous vegetative growth—plants go through an oft-overlooked and frequently under-appreciated seedling (pre-veg) stage. While the time spent in this stage is usually limited to a week or two, it can be fraught with opportunities for mistakes by unseasoned growers and is less forgiving of errors than mature plants.
Anatomy of a Seedling
Inside a seed is a miniature, undeveloped plant, packaged in a protective shell. A seed can survive without light or additional moisture for months (sometimes years) in a state of suspended animation under conditions that would kill the same plant in any other life stage. Seeds should be kept in a cool, dark environment away from moisture or high humidity for long storage periods. Before sprouting, some seeds require a moist period of lowered temperatures to simulate winter in a process called stratification.
Seeds sprout when environmental conditions are favorable and moisture enters through small holes in the seed coat called micropyles. The moisture causes the dehydrated plant to swell, and hydraulic pressure bursts the seed coat from the inside. Food stored in the endosperm feeds the sprout until it has developed the ability to photosynthesize. The sprout sends down a root (radicle) to draw in moisture and nutrients, then sends up and unfurls a single seed leaf (cotyledon) in the case of monocots (monocotyledonous plants), or a pair of seed leaves in the case of dicots (dicotyledonous plants).
Plants with only these first seed leaves are commonly called “sprouts.” Meristem cells are among the most important to new development as they are undifferentiated and can further develop into a variety of specialized cells depending on location and need. The apical meristem at the tip of the radius develops into the root system, while the apical meristem between the seed leaves develops into the first growth tip. Once the first true leaves start to appear the plant is considered a seedling.
Once sprouted, the plant goes from one extreme to the other; from its most durable stage to its most vulnerable.
Seedlings can wither and die in a matter of hours without proper moisture. They don’t have an extensive enough root system to reach very far within the growing medium to take advantage of large containers, and as such are frequently started in smaller containers, then transplanted as they increase in size. Nursery pots sized to fit 1020 trays are a common choice, as are plastic beverage cups.
Seedlings planted directly outdoors should be well marked and protected from flooding or other traumata. Markers or garden maps can be helpful in ensuring each mature plant can be identified if multiple varieties of the same type of plant are grown. While it can be obvious to visually tell the difference between a squash sprout and a tomato sprout, telling the difference between similar tomato varieties can be difficult before fruit set, so keeping a record is handy.
(Learn more about how to deal with seedlings with Growing Strong Seedlings.)
Dos and Don’ts for Pre-Veg Seedlings
Overwatering can be as detrimental as giving too little water. Aside from the potential to drown the plants if kept in stagnant water, an abundance of moisture or humidity can lead to a fatal fungal infection known as “damping off,” which is generally considered untreatable once infected.
Seedlings require light to grow, but high-intensity discharge lighting can be detrimental. Of particular risk is heat damage, which can not only overheat the plant directly, but can cause indirect damage by drying out the grow medium.
Their small size means they are more susceptible to physical attacks and mishaps than established plants. Even a single insect bite that would go practically unnoticed on a larger plant can put an end to a tender seedling.
Errors in nutrient solutions are magnified when dealing with seedlings. Nutrient solutions for seedlings should be kept mild and simple. They are not tolerant of overfeeding and are particularly susceptible to nutrient burn and negative reactions from intense additives.
Particularly in the case of moving indoor-grown seedlings to the outdoors, hardening off can help reduce the stress from a change of environments. One approach to hardening plants is taking them outside during the mildest part of the day for an hour for the first day, and for an additional hour each consecutive day (two hours on the second day, three hours the third day, etc.) for a week to 10 days. By giving the plants time to acclimatize from their (hopefully) idyllic indoor conditions to the harsher realities of outdoor life, the shock of the experience can be mitigated and the stress reduced. Hardening off is generally not required when moving to a milder environment, so outdoor plants can be moved indoors (after a quarantine period) with relative safety.
Similar to hardening off, if the seedling will be grown under harsher high-intensity discharge indoor lighting, it is less stressful to gradually increase the intensity over a period of time than making a harsh transition to the new environmental conditions. This can be accomplished by first moving to the periphery of the lit area for a few days before placing directly under the lighting fixtures.
It behooves a gardener to take particularly good care of their seedlings, as they are the tender starts to robust mature plants. Healthy seedlings enjoy a more vigorous growth spurt from transitioning into adulthood, whereas sickly starts can take additional time to recover from their retarded development before maturing.