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Sunday, January 31, 2016

How & When To Pot Seedlings

In my last post, I discussed seed starting.  Once the seeds have germinated, the next step is to re-pot them so that the plants can continue to grow well before you place them in the garden in spring.

The time to re-pot a seedling is when it has grown its first set of true leaves and when its roots have filled most of the cell in the seed starting flat.

Cotyledons vs. True Leaves: Image from:
The first leaves that a plant produces are called cotyledons.  These are usually 2 smooth, oval-shaped leaves that are the first to appear after the seed germinates.  You do not need to re-pot the seedling when you see these.  Instead, wait for the true leaves to appear.  These are the 3rd and 4th leaves that appear after the seedling has grown for several weeks.  These leaves look like the mature plant's leaves, only smaller (because they haven't grown to full size yet.) See diagram above.

To re-pot the plant follow these simple steps:

1. Fill a small pot of diameter 2-4 inches with a 50/50 mix of well-rotted compost (not manure!) and perlite or a good light potting soil. Try not to use plain dirt from your back yard unless you have to.  This is because the soil from your back yard may contain organisms that can damage the young seedling and/or may not contain enough nutrition for the young plant. If you make your own compost,  it is ok to use that mixed 50/50 with perlite or sharp sand.

2. Water the soil so that it is about as wet as a wrung out sponge.

3. Use a pencil or stick to make a hole in the center of the pot that leaves enough space for the root system of the seedling.

4. Hold the seedling by one cotyledon (baby leaf), and use the pencil to loosen the roots from the tray that the seedling was started in.  Be careful not to tear the stem or true leaves.  Be as careful as you can to avoid damaging the roots as well.

5.  Place the roots of the plant into the hole in the new pot.

Special Cases:  
With most plants, you should only place the roots beneath the soil surface, leaving the stem and leaves clear of the soil. This is to avoid rot.  A few plants are exceptions to this rule.  For example, tomatoes and peppers may be buried more deeply.  For these plants, it is a good idea to
bury the roots AND the stem of the plant up to the level of the cotyledons.  This is because the stems of these vegetables are able to grow more roots.  Burying them deeply at this stage will actually help the plant to produce a stronger root system as well as stronger stem.

6. Remember to label your plants! 

 7. Place the newly potted plants in a bright (preferably South-facing) window.  This time of year, in the north, it is a good idea to continue using a grow light since the days are still very short and since most windows do not let in enough light. Continue using your grow light until the spring equinox (when there are at least 12 hours of sunlight a day.)  or until you place the plant in your garden.

8. Water the plant when the soil feels dry when you stick your finger into the pot up to your first knuckle.  Do not leave the plant sitting in a dish of water. Do not over-water.  This can cause root rot. Most plants fail indoors because the gardener "loves them to death." That is, they get over-watered and/or over-fertilized.  At the same time, you shouldn't under water either. Do not wait until the plant has wilted to water it. Check the soil!

9. If you have used good quality compost and/or potting soil, you should not need to fertilize the plant between now and when the plant is placed in the garden in April or May (in Michigan).  Most compost and potting mixes will contain many months worth of nutrition for a typical plant.

10. If you want a plant to be bushy or to have multiple branches (e.g. peppers, basil, most other herbs etc.), wait until the plant has produced around 6 true leaves and pinch or cut off the tip of the plant below the top two leaves at the level of the stem.  This will cause the plant to branch at this level. With some plants you may continue to pinch or even prune the plant periodically so that it will continue to bush.  Herbs especially benefit from this treatment. See image below:
When & where to pinch.  Image from:

11.  Be patient and RTFSP!  That's right! "Read The ____ Seed Packet!"  The seed packet when it is the best time to put the plant out in to the garden, how far apart to space the plants & etc.  Wait for spring and the last frost.  Wait for it..... waaaaaait.....

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