Starting Seedlings in Hydroponics
By: Peggy Bradley
Some textbooks and manuals on hydroponics
mention germination of seeds and recommend starting the seeds
in soil. Nothing could be further from the truth.
Hydroponics is an excellent technology for
germinating most types of seeds. Because there is no soil,
many of the common diseases for seedlings are avoided in
hydroponic culture. This includes a terrible disease called
damping off, where the stem of the seedling dries up brown and
then dies. This is usually caused by fungus in the soil,
fungus that is not in hydroponic growing media.
A second disease of seedlings is root rot,
caused by poor drainage in the seedling growing container.
This can also be avoided in hydroponic culture.
While ordinary soil based germination in
nursery businesses in the US expects a 70% germination rate,
hydroponics can reach 90% to 95%.
Hydroponic culture can also modify the
nutrient formula for the plant, creating a better growing
environment for the young seedling. Specifically, a higher
phosphorous compound encourages more root growth, at the
expense of green growth above ground. The seedlings with
excellent root growth are far more likely to transplant
It is for these reasons that many of the
top growing companies have switched to specialized hydroponic
systems to germinate their plants. One of the worlds largest
timber companies reportedly grows almost all of its seedlings
in hydroponic culture. They get better results, better plants,
and they remove potential for many diseases. These tree
seedlings are then planted in soil with excellent survival
rates. So hydroponic plants can be transplanted into soil.
The advise to start seeds in soil is not
correct in a hydroponic growing facility. It makes more sense
to keep everything in sterile hydroponic culture and avoid the
diseases and problems associated with soil.
The Germination Process
A seed is alive, breathing in oxygen and
respiring carbon dioxide. While the seed is dormant, or not
germinating, this rate of respiration is very slow.
When the seed is placed in an appropriate
environmental conditions it will begin the process of
germination. In this process with enough water and the correct
temperature, (between 65 and 85 degrees F), the seed will
start to germinate.
Most seeds have inhibitors within them that
prevent germination. In many cases these inhibitors are water
soluble so the inhibitors will break down in water allowing
the seeds germinate.
A germinating seed breathes more rapidly
than the dormant seed. It requires oxygen and respires carbon
dioxide in order to grow. During this stage the seed needs to
be in an environment which provides both oxygen and water to
This means an ideal environment for seed
germination is one that allows the seed to have adequate
oxygen and water such as a fluffy or air filled media
appropriate for hydroponics. Mediums used for germinating seed
have a high water-holding capacity, good drainage and
aeration. A mixture of peat moss and builder’s sand in a 1:1
volume is commonly used in nurseries. Other growing media such
as shredded sphagnum moss, vermiculite and perlite are also
used for germinating seeds.
Setting Up Hydroponic Seed Germination
Grodan, Oasis and other rockwool companies
manufacture 1" or 1.5" rockwool cubes especially designed for
germinating seeds. These cubes come in sheets of 100 or more
and can be placed in a bath of water and then a single seed
placed inside the rockwool cube. This system is a tried and
true method of starting seeds for the tomato industry. And
this is important where some of the hydroponic tomato plants
from Israel and Holland cost up to 75 cents for a single seed.
Germination rates of 90% or higher are recorded using this
Most seeds of about 100 cubes are about 10
by 20 inches. The sheet is placed in a 10 by 20 inch growing
tray, and the entire sheet is watered with a hydroponic
nutrient. The water level should be only half the height of
the rockwool cube or even a little lower. If the water level
is too high the seeds in the cubes will drown in the excess
water. The seed should be at a height where the water wicks up
to the seed so air is available to the seed.
The rockwool method works very well when
transplanting the small cube into a larger rockwool growing
cube for the plant’s adult life. This is a commonly used
method in commercial hydroponic culture for tomatoes and other
Perlite Seeding Grower
A simple homemade grower can be made from a
plastic tub or a wood carton lined in black plastic. The
container should be about one foot by two foot surface area
and at least six inches in depth. A plastic tub should be
opaque or not let light through to the roots of the plants. If
a clear plastic is used it will fill with algae and rob the
seedlings of nutrients.
A wooden crate can also be adapted to be a
seedling grower. A sheet of black plastic six mil in thickness
or stronger, is placed in the container and stapled to the
edge of the carton.
The container should have a drain hole
placed about one inch from the bottom of the growing
container. This allows any excess water to drain out of the
planting area but still retains at least an inch of nutrient
water in the bottom of the grower for young plants to have
adequate water supply as they grow.
The tub or carton is then filled with
perlite, a white porous growing material. Other materials can
be used, such as coco fiber, vermiculite, or lava rock.
However, perlite makes an excellent growing media for
seedlings because it wicks water well, provides adequate air
space and does not change the pH of the system.
Perlite is a volcanic material that has
been mined and then popped in a kiln. This allows the
particles to expand so they better hold both moisture and air.
Because it is a mineral material popped in a kiln, it should
be sterile the first time it is used.
Perlite can be purchased at a hydroponics
store, or at most garden supplies. Make sure it is propagation
grade which is pea gravel sized particles. There is also a
building material available which is called masonry grade.
This fine powder floats, so it does not make a good hydroponic
Seeds are planted in the perlite media
exactly as they are planted in soil, usually from half to an
inch deep. As a general rule the seed should be planted no
deeper than one to two times their diameter. Small seeds can
be scattered over the surface or planted in rows and covered
with a thin layer of media, just enough to keep them moist.
The maximum depth for any seed is from two to three inches.
Some seeds such as lettuce can be sown on
the surface of soil, but in hydroponics we plant lettuce seeds
in rows about a half inch deep. This preserves the moisture
area around the seed so that it will germinate.
Watering With Nutrient Water
After the seeds are planted they should be
watered once a day with nutrient water. There are a variety of
opinions on the strength of the nutrient water. Many seeds
have stored nutrients in their tissues and do not need any
additional hydroponic nutrients until they are fully sprouted.
Other seeds such as orchid are very small and have almost no
As a general rule, in production of
hydroponic seed starting, we start the plants out with a
nutrient drenched media and add nutrient water every day. This
is usually a nutrient that is mixed half strength to the
nutrient that will be used in more mature plants. Thus the
hydroponic nutrient is mixed at half the strength recommended
by the manufacturer. This is kept until the seedlings are
about two to three inches tall and ready for transplant.
Once the grower is set up, the entire
growing medias should be watered down with hydroponic
nutrient, seeds are planted and then planted seeds are
rewatered every day.
Because the seeds are very small they can
wash out of the growing container if the watering is too
intense. Even when using a watering can, it is good to hold
your hand in the watering stream and soften the impact of the
water stream on the growing surface.
Some Seeds are
Hard to Germinate
Some people recommend soaking the seeds in
water prior to germinating. This allows the seed coat to
become saturated with water to break open. Because we have
such successful rates of germination in hydroponics,
presoaking is not recommended except for seeds that actually
require this before they will germinate.
Some seeds have an extra strong seed coat
that inhibits germination. This is common in larger seeds of
plant fruits that expect to be eaten by an animal or bird to
disperse their seeds.
Other species such as sea beans expect to
be pounded along a sea surface. To mimic the natural
processes, these type of seeds can be damaged in some way to
break the strong seed coat. This is called scarification.
As strange as it may seem, seeds are filed
with a metal file, rubbed with sandpaper, nicked with a knife,
or cracked with a hammer to weaken the seed coat. These rather
brutal forms of seed abuse are actually necessary to get the
seed to germinate. Scarified seeds should be planted after the
process because they do not store well.
Scarification is needed mostly by trees and
shrubs as honey locust, black locust, wisteria, broom, mimosa
and redbud, but it is also used for common plants such as the
Some Seeds Have Inhibitors
Some seeds have natural inhibiters stored
in them that delays sprouting. This is often true in seeds
from temperate climates. The reason this is so is that the
seeds have to stay dormant through a winter and then sprout in
This chemical develops in the seed through
its creation and then keeps the seed from sprouting when it
falls on the ground in the fall. During the winter the
chemical degrades in the seed through enzyme action so it can
then sprout in the spring.
To get seeds of this type of plant to
sprout early, the seeds are gathered, placed in moist soil and
refrigerated for four weeks, a process called stratification.
During this time the enzymes break down and the seeds can
sprout. This is a process most commonly associated with
temperate native plants that require a winter season.
Desert plants have compounds that keep the
seeds from sprouting until there is sufficient moisture for
growth. The inhibitors are water soluble and allow the seed to
sprout in sufficient moisture. This type of seeds only require
water to sprout.
Hydroponic Nutrient Choice
There are several hydroponic nutrients on
the market and most can be used to germinate seeds. If you can
find a nutrient which has a higher phosphorous amount compared
to nitrogen it will likely stimulate better root growth. In
commercial hydroponics, Haifa Chemical in Israel has a
specific nutrient for germinating seeds.
An important aspect of germinating seeds in
hydroponics is that the nutrient solution have all 13
nutrients required for plants. Soil based fertilizers such as
Miracle Grow do not have all 13, and so the plants will not
survive very long outside of soil.
There are a wide variety of plant additive
products designed to aid in the propagation of plants but most
are especially for cuttings. These products include root
hormone products and other specialty products for cuttings.
These are probably not of much help in seed germination unless
the label especially mentions that use.
- If the seedlings are being kept in a
greenhouse, then sunlight can raise the temperature of the
growing media beyond 90 degrees F, the seeds will fail to
germinate and even cook, or die. Check the temperature of your
growing media with a thermometer or check it with your finger.
If it feels hot to the touch it probably is too hot for the
seedlings. The seedling operation should be moved to a shadier
place or shade cloth should be placed over the seedling
Lack of moisture
- Seedlings are very frail when first
germinated so they should be in a relatively moist environment
with humidity at 60% or higher. If seeds are being germinated
in a dry environment it can be helpful to place a plastic
cover over the seedlings to retain moisture. It is important
to make sure there is still good airflow to the young
seedlings or they will be unable to breathe.