Garden & Greenhouse


Germinating Seeds Indoors & Seedling Care

Posted February 13th, 2014 by Kyle L. Ladenburger in

Sunflower Seedling

A great way to get a jump start on the gardening season while at the same time making the gardening experience a more personal and fulfilling endeavor is by starting your own plants from seed. A seed may appear to be a simple little thing but contained within that hard exterior shell is all the genetic information needed for that plant to grow and mature, creating fruits, grains, flowers and vegetables for us to enjoy. But before that seed can grow into a complete and beautiful plant it will require the right environmental conditions and a little bit of TLC.

The seeds of flowering plants or angiosperms are placed into two distinctive categories Monocotyledons (monocots) and Dicotyledons (dicots). This classification is based on one specific characteristic of a seedling. Cotyledons, commonly called “seed leafs”, are the first leaves that an infant plant will grow and they will act as a food source for the seedling until the emergence of the first “true leaves”, when the plant begins the process of photosynthesis to provide itself with carbohydrates for energy. Monocots are classified as such because they only have one cotyledon (seed leaf) while Dicots have two. Plant seeds classified as monocots include wheat, corn, sugar cane and bamboo. Seeds classified as dicots include such plants as beans, tomatoes, peppers and cucumbers.

A seed is essentially made up of three parts: The seed embryo, which is basically a tiny baby plant that will grow and mature under favorable conditions, the endosperm that will supply the initial food and energy source for the growing embryo and the seed coat, the hard outer shell that protects the seed until it is ready to grow.

Germination is the process in which a seed and its embryo go from a dormant state to an active growing state. Successful seed germination is dependent on certain conditions being met. These conditions include both internal and external and can vary from plant to plant or, shall I say, seed to seed. The external conditions that are most crucial are water, temperature, oxygen and with certain seeds, even light or darkness can be a factor. Water is essential because a mature seed is often quite dry. Seeds take in water through a process called imbibitions. As the water accumulates in the seed it causes the seed coat to swell and break apart. Water also activates the breaking down of the endosperm, chemically converting it into a useable food source.

Temperature has an effect on the metabolism and growth rate of cells within the seeds embryo. Seeds usually have a temperature range in which they will germinate and germination won’t likely occur above or below this range. Most seeds will germinate in the temperature range of 60-80 degrees Fahrenheit but some can do so in soil temperatures as low as 40 degrees. Oxygen is required for proper seed germination. Oxygen in gas form will reside in soil pore spaces and assist in the development of a much needed root system. If the seed is planted to deep or if the soil becomes too waterlogged the seed may not germinate.

When germinating seeds at home or in a greenhouse the first thing to think about is whether or not a certain seed should be started indoors and transplanted to another location or if it should be directly sown into the soil or growing medium were it will find its permanent home.  Plants like radishes and carrots should not be started indoors to be transplanted outdoors at a later date. Doing so may result in disrupted growth that can lead to unfavorable results. However, starting tomatoes, peppers and cucumber seeds, just to name a few, indoors is a great way to get an early jump on the outdoor growing season or for an indoor garden. When choosing a medium in which to germinate the seeds, look for one that says something along the lines of, “seed starting mix”. This type of growing medium will likely have a moderate elemental fertilizer charge which will benefit the newly sprouted seedlings. Seeds can be germinated in many different styles of trays and containers so choose the type that best fits the project at hand. If you are starting just a few seeds, a simple, flat starting tray will work great. When you are planting many seeds at once it may be wise to use trays that are divided into separate growing chambers. This will cut down on the amount of transplanting needed as the plants grow. Remember, most seeds will germinate at average room temperature, but some growers do use heat pads underneath the starting trays.

Most seeds germinate at temperatures between 60-80 degrees Fahrenheit and the added warmth in the growing medium can speed up the germination process but, for most seeds, it is not a necessity. Using supplemental lighting, like a T5 fluorescent bulb, can also help provide extra heat. Though seeds may not need light in order to germinate, the coming seedling will need light, so having a light source procured and ready is a good idea. I would advise against starting seeds in a bright window sill because the glass can alter the intensity of the sunlight and the plants may stretch and become ‘leggy’.

When preparing to germinate seeds indoors I would suggest lightly moistening the growing medium before planting any seeds. This will help to ensure that the medium is not over saturated or water logged and that the moisture is spread evenly throughout. Using the eraser side of a pencil or the tip of the pinky finger, carefully make small divots in the medium at the desired planting depth, many plants require a depth of around a ¼ inch. To find the correct planting depth for the type of seed being grown consult the back of the seed package. I have found that the suggested planting depths on the package are very accurate and not following them can result in lower germination rates. If planting in a flat starting tray space seeds at least an inch apart either in rows or in a grid pattern. Gently place 1 or 2 seeds in each divot, cover lightly with growing medium (remember oxygen is important during germination, so don’t pack the medium down to much) and spray the entire tray lightly with a hand held mister. A pre-moistened soil should stay wet long enough for the seeds to germinate, but it may need to be sprayed with the mister occasionally to maintain even distribution of moisture. Some growers use starting trays that have plastic hood type lids. This will keep the humidity around the seeds at higher than average room levels and may help increase the chance of successful germination. Be sure to check the seeds nearly daily to maintain an optimal environment.

As the seedlings begin to pop up through the soil there are a few environmental aspects that should be given proper attention right away, they are light intensity, humidity and air flow. Remember the seeds of different plants will germinate in different lengths of time so; once again, check the seed package for estimated germination times to know when to be ready. I mentioned before that many seeds can sprout in total darkness, with that said, once that plant breaches the soil a sufficient light source is imperative. Those first ‘true’ leafs will need a light source to perform photosynthesis and create carbohydrates that will help sustain both normal plant growth and, most importantly, root growth. Without proper lighting the early vegetative growth of a plant can be negatively affected and could cause long lasting problems that may result in a lower yield.

Humidity can be helpful during the initial germination process but as the little seedlings begin to grow high levels of humidity can spell disaster. As internal process burn up the seedlings energy sources, the plants will need to release oxygen as a gas through their stomata’s in a process that is called transpiration. As the oxygen leaves the plant, water and elemental nutrients are pulled up through the roots. In a humid environment the stomata will remain closed and the roots will not take in water. If the growing medium is moist, as it most likely will be, the water will have nowhere to go and the roots will likely suffocate and die. Air flow and humidity almost go hand in hand. A nice flow of air through the plants canopy will encourage the flow of carbon dioxide to the leaves and, subsequently, oxygen away from them, this is not just true for seedlings but for plants in all stages of growth. A small fan on medium or low can help keep humidity levels low and the heat from any supplemental lighting to a minimum. Be sure to keep the rooting medium moist but not too wet. Seedlings need water and going too long without it can result in serious damage but if the medium remains too wet for too long it may impair root growth. As the seedlings grow they will eventually exhaust any nutrient charge that the growing medium had to offer, so light fertilization with a nitrogen based fertilizer may be needed while waiting to transplant into a different or permanent container.

Day by day as the seedlings grow, with proper care and attention, they inch closer and closer to fulfilling their own unique destiny. And as we stand by, eagerly awaiting the literal fruits of our labor, it is important to remember that every plant we grow has entered into this life as a small, almost insignificant looking thing that so many refer to as simply, just a seed.

Kyle Ladenburger is an avid indoor and outdoor gardener and Garden & Greenhouse contributing editor. You can contact him via e-mail at

Want more information? Read these articles:

Creative Seed Starting with Newspapers to Plastic

Hydroponic Seed Starting is Easier Than it Sounds

Propagating Plants from Seeds

Seed Starting for the Home Gardener

Starting From Seed

Subscribe to Garden & Greenhouse Magazine

Subscribe to Garden & Greenhouse Email Newsletter

Leave a Reply

Your email address will not be published. Required fields are marked *