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Seed Starting Basics

Updated: Mar 25, 2023

The following information covers the key basic elements in starting most seeds indoors. It addresses the information needed for the large majority of seedlings but there are some plants whose seeds will require special conditions to germinate such as cold stratification and specific germination temperatures.

In starting seedlings, it is important to provide the proper conditions for them to sprout and develop.

First condition is the seeds.

Germination rates vary among seeds. Some like Rosemary, gomphrena and others have low germination rates where a smaller percentage of seeds planted will germinate as compared to seeds like corn, beans, and zinnias.

The other influence on germination rates is the age of the seeds. As seeds age they gradual loose the ability to properly germinate and their germination rates will drop.

The condition that the seeds are kept in prior to planting can also affect the germination rate. Storage in a cool, dry, dark area will maintain the highest germination rate.

The seed packs you purchase may actually state the germination rate but many do not.

Commercial seed sources typically provide seed with germination rates of 80% or better which means that for every 10 seeds planted 8 will germinate. If the grower knows that a particular seed batch may have low germination rate they will include additional seed in the pack to offset the lower number of seed that you would expect to germinate.

With fresh viable seeds on hand the next item to address is what the seeds will be germinated in. Seeds need constant moisture to germinate. As the seeds start to germinate if they are suddenly exposed to dry conditions after the seed germinates the young root that is developing in germination can be killed.

Without this root the seedling will never grow into a seedling.

The best success is had by using seed starting mix which is a material that will hold moisture and is free from pathogens.

This is different than potting mix or garden soil

You want to pay attention to the wording on the package.

Seed starting mix typically lack the nutriments needed for seedlings to develop into plants. The young seedings when first germinated have a built-in reserve of food for the young plant to use. Some seeds have more like beans than other that have little like poppy seeds.

The lack of nutriments in potting mix can easily be addressed by applying fertilizer or potting the seedlings up into a mix that does contain nutriments to support developing seedlings.

You will need to pre moisten the seed starting mix. Apply water to the mix and then squeeze any excess out.

You do not want to be using soggy seed starting mix.

With fresh seed and a good seed starting mix the next step is to properly plant the seed.

Some seeds need light to germinate. You will be told if the seed does need light or not in order to germinate. For these seeds you will only place the seeds on the surface of the seed starting mix. All you will do once the seeds are on the seed starting mix is mist the seeds on the surface of the seed starting mix to ensure that the seeds have good contact with the moist seed starting mix. This will allow them to properly absorb the moisture then need to germinate.

Most seeds that do not state the need for light can be germinates with or without light. When in doubt treat the seeds as if they need light. For those that do not need light you will mist them into the surface of the seed starting mix and then lightly cover with pre moisten seed starting mix. The depth of cover is equal to 1 to 2 times the thickness of the seed.

Now that you have the seeds in their seed starting mix you want to prevent them from drying out while they germinate. You can accomplish this by misting the surface as you see it begin to dry out

But it is easier to use a plastic cover to trap the moisture. This can be accomplished by placing the potted seeds in a plastic bag or just covering the potted seeds with a plastic sheet.

The formation of condensation on the plastic covering it is an indication that you have an adequate level of moisture.

The next item to address to get seeds to germinate is the temperature that they are germinated at. Most seeds germinate quicker at higher temperatures. The general optimal temperature is 70 to 80 deg F.

You can use a seed starting heat mat to provide warmth to the germinating seeds

The other option is just keep them in a warm area of the room.

In waiting for seeds to germinate keep in mind that some seeds will just take longer. Peppers can take 3 weeks and sunflowers can germinate in 3 days. The seed package will typically let you know how long it will take for the seed to germinate.

Once the seeds germinate the first things seen on each side of the stem are the cotyledon and are the source of the first food for the seedling.

These are not true leaves that will develop next.

Once the seed has germinated it is important to get them under the proper amount of light.

The light should have a spectrum of 5000 to 6000 K

The intensity is a bit harder to define for the home gardening. Lumens is not a measurement of what the plants are getting that they can actually use it is a measurement of what we perceive as brightness. Plants make use of light in the infrared spectrum which is one we cannot see.The correct way to determine what light the seedling need is defined by the term PAR which stands for photosynthetic active radiation.

Since very few of the lights that the home gardener will be looking at identify their PAR you want to find a light that will provide 7000 to 7500 lumens per square foot.

Without this type and amount of light the seedlings will develop elongated cells and “stretch” to try and get the amount of light they require. This condition is referred to as becoming “leggy”.

With the viability of LED light one of the drawbacks of the previous halogen lights is eliminates. LED light do not produce heat that can burn the leaves of young seedlings.

In using the LED lights as soon as your seeds germinate you want to remove the plastic cover and position the lights with in 2 to 3 inches of the surface of the seedlings.

Keep in mind that as you have seedlings to the side of the lights that as this distance of the light to the leaves of the plant the light diminishes at a greatly accelerated rate.

If you are looking to cover an area then you want the spacing between the light bars used to be close enough to maintain the required light level over the entire growing area. The separation between light bars should be 3 to 4 inches. A good indicator that you do not have your light bars is if your seedlings are not growing straight up and are instead they are growing off at an angle towards a light bar.

As the seedlings grow you will move the light up to maintain the distance from the light to the tops of the seedlings. You can do this by having pulleys attached to the light that allows you to raise and lower them as needed or you can just put something under the seedling trays to move the tops of the seedling closer to the lights.

Many times, especial with smaller seeds you will have more than one seedling in each pot since it was difficult to ensure that you only planted one seed per pot. You have two options.

You can try and divide the seedlings growing in the same pot. You can try this once they are showing their first true leaves. You will ease the clump of seedlings out from the pot they are growing in and then you will try to separate the roots apart such that each seedling has roots.

You typically will not get the root amount even between plant the key is to try and maintain as much roots for each plant. These separated plants will be repotted with the seedling’s stems buried up to just below the cotyledons

The other option is to just cut all but the strongest seedling in each pot off at ground level. This removes the competition from the seedlings and allows the remaining one to grow quicker and stronger.

As your seedlings grow you can use a fan to blow across the plants.

What this will do is cause the stems of the plants to stiffen and become stronger. The reason the fan accomplishes this is that the motion it causes in the plants drives the cell walls to thicken and become stronger. This is the same as if the seedlings were outside in the wind.

If seedlings out grow their pot, you can put them into a larger pot.

The rule of thumb is when moving seedlings up to a larger pit the new pot should be twice as large as the pot it is coming from. The pot used can be larger but if too large it can be harder to maintain the proper moisture level in the soil around the roots of the young plant.

In watering your seedlings, it is best to water from the bottom by standing the pot in a tray of water.

The purpose of watering from the bottom is it forces the roots to reach down into the soil and form a proper root system

Once the soil will not absorb any more water you remove the excess water from the tray. You do not want to leave the pot in the standing water because it will water log the soil and cause root rot to occur.

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