Seeding Spring Bedding Plants

Diascia ‘Antique Trailing Rose’. One of my favorite early spring cool season beding plants. Looks great with dark blue pansies

Have you ever tried to start plants from seeds at home on a windowsill only to have them stretch and bend towards the light from a window? Growing your own seedlings is a great way to save money but seeds need very high light levels, much more than can be provided from even the sunniest wondow.  A fluorescent light fixture on a movable chain is needed to keep the trays of seedlings about 3-6″ away from the light source.  Here is a link to instructions for making a germination chamber out of PVC.  Propagating annuals from seed is fairly easy and straight forward.  Pay attention to the number of weeks the plant needs to germinate and grow.  Seed packets often list the number of days till maturity but this doesn’t account for seed germination which can tak eanother month or longer.  Don’t plant the seeds too early or the plants will grow too big and soft and stretched indoors and not take well to transplanting outside.  Once the young plants are ready to go outside be sure to acclimatize them or ‘harden’ them off.  Take them outside on sunny, warm days and move them in at night and/or when it gets cold. 

Perennials often have dormancy mechanisms that prevent them from germinating and may need some addtional treatments – stratification (cold, mosit) or scarification (scratching the seed coat open) or both – before they will germinate and flower

 The Wisconsin greenhouse industry has actually moved away from growing plants from seed for several reasons.  Some seeds are difficult to germinate in that they often have very specific temperature, light and moisture requirements to germinate.  If a grower has 50 different seed crops s/he has to know, and have the space to provide, all the different requirements each seed needs. Seeds require extra warmth and time in the greenhouse; a fibrous begonia seed may take as long as three weeks just to germinate and another 12 weeks to get to stage 4 and transplanting. The crop time for fibrous or wax begonias from seed is 18 weeks!  All this requires specialized knowledge, increases labor and most importantly heating costs.  Growing bedding plants from seed requires most seeds to be sown in January and February – the coldest, and therefore the most expensive, months of the year to heat a greenhouse in Wisconsin.  As a result, many Wisconsin now purchase pre-germinted plants called plugs. Plenty of greenhouse growers don’t grow anything from seed at this point. This saves several months of heat and labor in the greenhouse during the extreme cold months of winter, and money in the long run.  While seeds need to be sown in January and Feb, plugs can be purchased in March or April to finish at the same time as seeds sown months earlier. 

Germination is more than seeing the seedling emerge from the soil.   Here is the basic idea summarized:

Stage 1 – seedcoat absorbs water which causes first root to grow and emerge from the seedcoat.  We can’t see this happening

Stage 2 – the stem (hypocotyl) and ‘seed leaves’ or cotyledons appear out of the soil

Stage 3 – true leaves appear.  This is often a stage where fertilizer rates are increased, growth regulators applied, temperature and water lowered

Stage 4 – Several sets of leaves, roots are touching sides of plug cell. Ready to transplant in pot or cell pack

288 Plug trays at stages 1 and 2

 It is important to transplant the seedling as soon as possible at stage 4.  At stage 4 the roots of the seedling will be touching the sides of the plug cell. 

Seed germination  requires higher temperatures and moisture levels at stages 1 and 2.  Plug trays will be kept in the warm propagation room under bottom heat and with a plastic cover to retain moisture. Temperature and moisture are reduced in stages 3 and 4 with the seedlings being moved out of the propagation room onto benches in the cooler greenhouse.The plastic covers are slowly removed. 

Flat filling at GTC is done by hand.  Fill the flat completely to the top making sure the plug openings along the edge are filled.  Gently tamp the media with your flat palm.  Water the flat and refill low spots with media.  Make sure the plug tray is uniformly filled.  Bacuse the soil volume in each plug is so small, even small irregularities in soil volume in the cells will cause the soil to dry out at different rates.  Uniformity is our goal in the greenhouse.  It is extremely difficult to keep plug trays watered if the edges dry out faster than th emiddle .  We need to be able to water the plug trays consistently 

Seeds are sown in moist media as the tiny seeds are held better onto the soil and not as easily washed away. The flat then goes under the seeding machine.   After the seeds are in place  a layer of vermiculite is sprinkled over the surface to hold in the moisture. A plastic cover is placed over the seeds to hold in humidity

Some seeds require light and some darkness to germinate.  The tinier the seed, the more light it needs. Some seeds like begonia germinate much faster and more uniformly if given supplemental light.  Begonia seeds are like dust particles they are so small!  We have a growth chamber used for this purpose.  We also have a germination chamber or ‘sweat tent’ where seed flats are subjected to warm, moist conditions that improve and hasten uniform germination. 

Cool season seeds such as pansies, lettuce and alyssum have lower germination temperature requirements than warm season plants like tomatoes and petunias.  The  Ball Redbook and various websites give specific information on germination requirements of different seed species.

At Gateway we have a handy, dandy automatic seeder that seeds a 288 plug tray. Larter seeds such as marigold and cucumber are sown by hand in larger plug trays but the 288 is the backbone of our production  The seeder is manufactured in Milwaukee.

Seeding machine. Vacuum pressure aligns and holds the seeds onto a metal plate. The plate is turned over and vacuum released causing seeds to drop down plastic tubing into 288 cells of plug trays.

  Tiny seeds like begonia and petunia are purchased coated to make them easier to use in the automatic seeder. Marigold and zinnia seeds are de-tailed and tomato seeds de-fuzzed to make them easier to pick up and use in the seeder. We purchase a lot of ‘primed’ seed which makes the seed germinate faster and more uniformly. 

Watering the plug trays is tricky business and for the next three months Kathy Estep and her staff will need to be here every day (including the weekends!) to  water the plug trays with a fine mist.  The uneven plug tray shown above had the seeds splashed around by uneven watering

Media – we use a premixed media which has been tested for pH, nutrient levels and soluble salts by the manufacturer.  This means we son’t have to test it on a regular basis or worry about pH and salts.  Yeah! 

The media has a slight nutrient or fertilizer charge which supplies the germinating seedling with just enough until the first set of true leaves develops.  At this point we will begin fertilizing the seedlings with a very dilute liquid fertilizer through the irrigation system.  We use a fertiizer injection system or proportioner that meters out a tiny amount of fertilizer into the irrigation stream each time the plants are watered.  We will begin at about 75 ppm and move up to 150 by the time the seedling is at stage 4 and ready to transplant. We will discuss fertilizers in more detail later when the seeds are trasnplanted.

One of the hardest things about growing bedding plants is supplying the right temperature, at the right time.  We try to divide the houses into warm and cool houses. Pansies, lettuce, alyssum and all the cool loving crops in one house and the warm in another.  It doesn’t always work out that way.  Also its best to keep the greenhouse on the cool side early in the morning and let it warm up later.  This keeps plants short and compact.  We do not have temperature controlled vents so often the greenhouse gets too warm in the morning before we get here and plants stretch.  Later in the semester you will find me opening vents first thing in the morning as the spring sun warms the greenhouses

Chemical growth regulators are often used to keep plants short and compact and are usually applied soon after the first set of leaves appear.We will talk about this in more detail later.

The biggest problem with seedlings is damping off caused by a fungus.  I have not seen it in a long time in our greenhouse 

I did see quite a bit of root rot disease on vinca last season due to overwatering. Drought tolerant plants like vinca, lavender and succulents can’t take excess water.

You will also see a lot of fungus gnats in all the moisture created by watering a lot.  These don’t always feed on plant roots but can if in great numbers.

Its important to transplant the plugs before they become root bound.  If not transplanted on time the tissue stiffens and they will not recover. The plants will always have a thin, spindly appearance.  The roots won’t spread well into the soil and flowering will be delayed and reduced.

We will begin transplanting cool season plants like pansies in about a month


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