Germination 3 - Spotlight on Light Lovers

Some seeds can manage being buried, others will grow anyhow but quite a few need to be exposed to light in order to germinate. Often these seeds are smaller so when surface sown they don't need covering. Some will be happy with a layer of grit or vermiculite often applied after they have started germinating. Vermiculite is light and somewhat light reflecting and the added layer prevents these tiny seedlings from being washed away when watered as well as preventing moss growing on the compost surface. This is more important for those seeds that take longer to germinate or are somewhat erratic in their germination.

Some of the most frequent problems customers encounter are with the following plants Begonias, Petunias and Primulas - which all need light to germinate:


There are two types: superflorens and tuberosa types. These tender perennials, which are grown as annuals in the uk, are normally sown indoors, from January to March.


If the seed is pelleted (encased in a fine clay substance for ease of handling the very small seeds) making  it easier to sow, use individual modules for ease of growing on is preferred.

Surface sow on moist, fresh compost mixed with 50% perlite. Do not cover. These seeds do best with 10 hours of daylight so using grow lights on a timer would be beneficial. 

Keep moist at all times so cover with glass/film or propagator lid. 

They need a temperature of 22-24C (71.5-75F) both day and night. If using a sunny windowsill or room to start these off take care to ensure that the temperature is maintained during the night and the newly emerged seedlings do not get too hot during the day. It’s surprising even in a centrally heated house how low the temperature can drop overnight.

Germination usually takes place in 14 to 21 days but for some varieties can take even longer, sometimes up to 3 months!

Growth can be very slow in the early stages so keep the temperature warm for the first few weeks and then reduce slightly as the seedlings establish themselves before potting on. Grow on and plant out after the last frost in your area, usually around the end of May.


Petunias are of two types: Grandiflora or multiflora. The grandiflora types have fewer, but larger flowers and the multiflora types are more branching and therefore form more flowers.

Petunias are another tender perennial which is grown as an annual in the UK. They are normally sown indoors in February to April.


They need light to germinate so do not cover. Germination is normally from 7 to 14 days at a temperature of 20-24C (68 -75F). Keep seeds moist all the time, but not waterlogged, so use a seed compost with added vermiculite or perlite to help with drainage.


Talking to my son on the way into work I remarked I had been researching primula germination and his response was ‘to keep in the fridge at 2-4 degrees’. A cheesy reference that made me smile. Indeed I was feeling at that point that my research was going round in circles with confusing germination recommendations that were somewhat contradictory. It is understandable as there are more than 450 varieties of Primulaceae and even more hybrids as they have been bred and hybridised since mediaeval times.

The majority of primula varieties are found in Asia with others found in temperate habitats across Europe, North America and Mexico. Most Primulas need damp, well drained soil some are more bog and water feature friendly and some, such as the alpine primulas, are happier in rocky and slightly drier soil. These primulas  still need enough moisture especially over the drier summer months.

Native primulas such as Primula veris - Cowslip, Primula vulgaris - Common Primrose and Primula elatior - Oxlip are wild flowers in the UK. Polyanthus are a cross between a cowslip and a primrose.

The auricula primulas are alpine primulas and have been hybridised to select for distinctive colours and patterns. Their two tone flowers were much loved by the Victorians where cross-breeding varieties extended the variation of auricula forms which also led to competitive events and displays.. Auriculas are often grown as pot plants. They do better if protected from wet  weather especially if bred for competitions, but are hardy in the UK.


The general principle for sowing most primula seeds is that they are surface sown in good moist, well drained, compost. Adding grit or perlite can help with improving drainage. 

Some Primulas also require stratification in order to germinate, a few like a period of warmth, then a period of cold.  Sowing in late winter from February to April works for most primulas but delay this up to a month if you are in a particularly cold area. This early sowing time allows for some cold weather to help break seed dormancy.

When sowing the seed sow thinly with space between each seed, on the surface of the pre-moistened compost. Cover with a sheet of glass or plastic lid to keep the compost moist. Do not let it dry out before germination or you may not get any germination at all.

Temperatures of 15-18C (59-64F ) are ideal for the germinating seeds as temperatures over 18C (64.4F) reduces germination and over 20C (68F) can cause dormancy. 

Leave them outside in a coldframe or sheltered position away from direct sunlight or inside in a cool north facing room. If leaving them outside to germinate protect from slugs and other wildlife. Be patient as while germination can take as little as 14 days, many varieties take much longer - some up to 6 weeks or more.

Examples of Primula varieties and seed germination times:

Primula variety


Days to germination

P. veris (Cowslip)

Stratify before sowing. 2-3 weeks warm followed by 4-5 weeks at 2-4C (36-39F)

May be very slow and erratic

P. vulgaris (Common Primrose)

Cool conditions.

Erratic. May take more than a year for some seeds to germinate.

P. elatior (Oxlip)

Stratify before sowing. 2-3 weeks warm followed by 4-5 weeks at 2-4C (36-39F)

P. denticulara (Drumstick primula)

20-30 days approx.

P. vialii 

16C (61F) May need period  warm then cold stratification

21-40 days approx.

P. candelabra

16C (61F) May need period  warm then cold stratification


15-18C (59-64F)

21-30 days approx.

P. sieboldii (sow Nov-Jan)

Outside in sheltered site

6 weeks approx.

P. veris ‘Hose-in-hose’ Cowslip

10-15C (50-59F)

3-6 weeks approx.

Other seeds that need light to germinate:

The above are some examples of seeds requiring light for germination. Also check if there are any other germination tips such as the need for cold stratification before you sow, this will give them, and you, a better chance of germinating well.

Ageratum, Angelica, Browallia, Calceolaria, Chamomile, Coleus, Columbine (Aquilegia), Dill, Exacum, Ficus, Gaillardia, Gerbera, Gloxinia, Impatiens (Busy Lizzie), Lavender, Lettuce, Lobelia, Marjora, Nepeta, Nicotiana, Osteospermum, Pelargonium (Geraniums), Poppy, Rosemary, Saintpaulia, Savory, Anthirrhinum (Snapdragon), Strawflower, Streptocarpus, Sweet alyssum, Yarrow.

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