13 steps
to healthy seedlings

Causes of
weak spindly seedlings

Seed planting depth

When to sow

Cucumber 'Glacier'

Seed starting basics

At left is a Cucumber 'Glacier'.

The seedling in back is just pulling the last of its seed leaves out of the soil.

Notice that the seed leaves (cotyledon) are usually not like the true leaves. Cucumber adult or true leaves have multiple points and are usually fuzzy, not smooth.

13 Steps to Healthy Seedlings

1. Set up seed starting area with adjustable fluorescent shop lights.
2. (optional) Soak large round seeds in a bowl of warm water for a few hours. Smaller or flat seeds can be presprouted on damp paper towels in a closed plastic bag.
3. Prepare planting containers, fill with fresh potting soil (removing any large lumps), and water soil with warm water. Place these smaller containers in a flat or any shallow container to facilitate handling.
4. Plant seeds and mulch top of soil with a thin layer of perlite. Label your containers with plant variety and date!
5. Cover the entire flat filled with planted, mulched & labeled containers with a plastic dome fitted to the flat or just use a large plastic bag (close the bag). If using a bag, use wire or sticks to hold the plastic away from the surface of the soil.
6. Place the flat over a seed starting heat mat. This is actually optional, but will certainly help your seeds sprout fast and well. You will get a certain amount of residual heat from the fluorescent lights when they are on. If your seed starting area is already in a warm room, your seeds will do well enough without the heat mat.
7. Adjust your fluorescent lights so that they hang just above the dome/plastic bag. Set the timer so that the lights are on 16 to 18 hours a day (but no more). Some seeds need light to sprout. In any case, you want the seedlings to have plenty of light available as soon as they sprout.
8. Check the flats at least once a day. The MINUTE you see sprouts above the soil surface, prop up the edge of the dome (or open the end of the plastic bag). Set up a small auxilliary fan to ensure good air circulation.
9. The next day after the first sprouts appear, remove the dome or plastic bag entirely. Adjust the fluorescent lights so that they are just an inch or two above the plant leaves. Be sure that fan is still running. Brush the leaves of the plants with your hand whenever you think of it. That and the "wind" from the fan will encourage stocky growth.
10. Be sure to keep the seedlings watered, but not swamped. I use a water bottle with a pop-up top which allows me to water each cell or cup individually. If the soil seems too dry for that type of watering (or you have too many seedlings to be giving them that level of attention), set the containers in a non-draining flat with some lukewarm water for a little while until the soil has wicked up enough moisture. (If containers are too dry, watering overhead will just run off the sides.)
11. As soon as true leaves develop, start fertilizing with a half-strength solution of fish & kelp emulsion. (The plants are delicate at this point, so I usually apply the fertilizer with a spray bottle.)

As the seedlings grow, they will begin to touch their neighbors. That is the time to pot up to a larger size container.

If you started with larger containers to begin with, you will be able to delay (or even avoid) this step.

12. Read below the causes of weak spindly seedlings and avoid them!

Seed planting depth

Plant seeds at a depth
about 3 times their thickness.

As a guideline:

tiny seed
(such as basil)

just press into the soil surface (don't cover)
small seed
(such as carrot, lettuce, cabbage)
plant about 1/4 to 1/2 inch deep
medium seed
(such as beet, spinach, radish)
plant about 1/2 to 3/4 inch deep
large seed
(such as cucumber, beans, peas)
plant about 3/4 to 1 1/2 inch deep

for help with timing your seed planting.

First of all, take a look at the seeds in your hand. They are not all alike. Some are smaller or shriveled, some are damaged. Don't plant those!

A healthy seed will have more energy reserves to start growing vigorously.

Choose the larger, relatively plump & perfect seeds for best results.


just planted flats

At right is a flat just planted.

When I plant tiny seeds (such as basil), I generally use a 4" pot to start. I fill it with good potting soil, water it with warm water, rough up the soil surface a bit, then sprinkle the seeds over the entire surface.

(Don't cover small seeds with soil.)

I press the soil surface down slightly to set the seeds and then add a thin layer of perlite on top. The perlite drains very well and dries quickly, thus keeping fungus from forming at the soil surface.

Label the containers with plant variety and date. Don't assume that you will remember.

Very soon after the seeds sprout above the surface, I prick them out with a fork and plant the very tiny plants using bent-end tweezers into their own cells or pots.

Larger seeds (and tomato seeds) I usually planted separately in their own small "cell" in a flat, a 4" pot, or a small paper cup (6 to 10 oz. size, with drainage holes punched out of the sides and bottom) to begin with. They are planted at a depth that is about three times their largest dimension.

Plant 2 seeds to a cell or cup. When they sprout, cut off the weaker one with sharp scissors. (Don't pull it out, or you will damage the roots of the stronger seedling in the process.)

I plant bean and pea seeds with the "eye" down, because that is where the root emerges.

I plant cucumber, melon and tomato seeds so that the thinnest part is on top and bottom and the flattest part of the seed is on either side. That is so when the seed sprouts, it can easily push up through the soil and still have plenty of energy reserves to keep growing. (Think of a table knife--It is much easier to cut through something sideways than to lay the knife flat and try to push through.)

Larger seeds with heavy seed coats I always put in a small bowl, pour warm water over them, then let them soak overnight before planting.

Often I will presprout seeds (even tiny ones, like basil) between the folds of a damp (not wet) paper towel that I then place in a zip lock plastic bag. I keep the seeds in the kitchen and check them daily (or more often); as soon as I see a root sprouting, I plant them in potting mix mulched with perlite as above.

(If I've not checked soon enough, and the seed is rooting into the paper towel, I just carefully tear around the root and plant it, moist paper towel and all) It's amazing how long a seedling can survive in the plastic bag "greenhouse" growing in its damp paper towel "soil"! (I have forgotten to check seeds many a time.) I grew an orchid for years that way.

tweezersBent nose tweezers really help when planting sprouted seeds or seedlings too tiny for your fingers. (Handle seedlings by their leaves to avoid damaging the stem.)
flat assorted

For planting, you can use different sizes of cells in a flat, chosen to allow enough initial root space for a seedling.

With experience, you will know which seeds require more room as they begin to grow.

Using a slightly larger cell for planting means that the seedling can grow longer without the root disturbance involved in transplanting.

Another container I use for planting is paper cups. I punch holes in the bottom AND sides (see below) for drainage and to get air to the roots.

Tomatoes, cucumbers and squashes I usually plant in 10 to 12 oz. cups. They seem to need the extra room for their rapid growth.

It is important to keep your plants growing. Not potting them up to a larger container (or outdoors) soon enough will slow their growth, they will become rootbound, and they will lose momentum.

Below are some paper cups ready to be filled with potting mix. I use several different sizes (3, 4, 6, and 10 oz).

I don't usually mix different size cups in one flat, as that makes it difficult to keep the fluorescent light at the right level for all of the plants. Put just one size cup in a flat.

paper cup punched
paper cup flat
stawberry container

I also use plastic deli type container with lids for planting seeds. Just open the lid when the seeds have sprouted.

The strawberry containers at left that I saved from last year's shortcake spree already have drainage holes built in. You may have to make drainage holes in other containers.

planted flat covered with domeThe fully planted, perlite-mulched and watered flat is then covered with a plastic dome made for that purpose.

(You can also use a plastic bag instead of a dome lid. Just be sure to prop it up in some way --with bent wire or sticks-- to keep the plastic from touching the soil surface or the sprouting plants.)

The flat is placed on a wire frame suspended over a heat mat specially made for starting seeds. Notice how humid it is inside the dome. A dry soil surface would be too hard for the seeds to sprout through.

Fluorescent lights are above the flats, positioned just above the dome.

As soon as the first seeds sprout, remove the cover entirely. At this point, I keep the fluorescent lights suspended just a couple of inches above the seedlings at all times, adjusting them as the plants grow. The lights are on a timer set to keep them on 18 hours a day.

I also set a small fan nearby to ensure good air circulation. This is in addition to the larger greenhouse fan that is always running. A little "wind" helps plants to grow sturdy and stocky stems, not weak and elongated. Additionally, whenever I think of it, I brush my hand over the little plants' leaves. This is also supposed to encourage stocky growth.

My heat mat has no thermostat (I need to add one), so sometimes the soil gets pretty hot. One year that I was actually measuring the temperature, it was 100 degrees Fahrenheit, and seeds such as cucumbers and zucchini sprouted above the soil surface overnight! I think 70 - 85 degrees F. is a better temperature to aim for.


light and hooks

Above note the wire hooks and chains used to adjust the lights so that they are always close to the top of the plants. (Discard those little "S" hooks that come with the light fixture-- they always fall out.)

Also note that because the light fixture is longer than the shelf unit, it extends out beyond the sides. I consider this a good thing because the ends of the bulbs produce less light, and I don't want the seedlings under there anyway.

flat on shelf

This setup is just some ventilated plastic shelving (from Home Depot) with 4 foot shop lights suspended from the bottom of the shelves above with wire hooks and the chain that comes with the light fixture.

I use ordinary cool white inexpensive bulbs. This shelving is in the greenhouse, so there is also some full spectrum light available. As plants grow taller, they usually get moved to the top shelf to make room for more newly-planted flats on the lower shelves.


Dolichos lab lab 'Ruby Moon' seedlings

Dolichos lab lab 'Ruby Moon' seedlings

Arrow points to seed coat still clinging to cotyledons (the rounded pair of things at the top of the stems below the leaves). Seedlings get their initial growing energy from the cotyledons, which are the bulk of the seed mass. These soon start shriveling as the leaves take over the task of producing food. Note the short stout stems, a result of avoiding the causes of spindly weak seedlings. Three of these seedlings are already opening their first true leaves.


Causes of weak spindly seedlings:

Temperatures too high--either day or night or both. During seed starting times, I try to keep the greenhouse temperatures in the 70's during the day, and in the low to mid 50's at night.
Not enough light. Seedlings will stretch toward the light. I keep seedlings under fluorescent lights about 16 hours a day. The lights are just a couple of inches above the plants.
Not enough air movement. You should have a fan running gently near the plants at all times. This helps them grow stocky and also prevents damping off fungus.
Also, brushing your plant tops gently with your fingers for a couple of minutes a day will help them grow stocky.
Seeds planted too deep or soil surface was too dry. Seeds have a limited amount of energy (in the cotyledons) to push themselves up through the soil.
Not enough food. Once you see true leaves, you should be giving the plants weak solutions of fertilizer (I use fish & kelp emulsion).


Ipomoea 'Grandpa Otts' seedlings

I have raised the lamp for this photo. Normally the fluorescent bulbs are just a couple of inches away from the plant tops. As they grow, I adjust the light.

This setup is just some ventilated plastic shelving (from Home Depot) with a 4 foot shop light suspended from the bottom of the shelf above with wire hooks and the chain that comes with the light fixture.

Notice the seed leaves of the Ipomoea purpurea 'Grandpa Otts'. They are an interesting shape. The adult/true leaves of this Morning Glory plant are heart-shaped.I try to keep the seedlings growing strong, potting up to the next size container as the leaves of the seedlings begin to touch their neighbor.

Ipomoea purpurea 'Grandpa Otts'

Below is what this plant's flower looked like in my garden last year. Flowers only last one day and usually close and wither around noon. Notice the heart-shaped true leaf. Each of these adult plants often opened 30+ flowers a day!

Ipomoea purpurea 'Grandpa Otts'

Ipomoea 'Grandpa Otts'

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seed sources . when to sow

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