What seeds to sow in August

There are seeds that can be sown in August, however most seeds do not germinate if exposed to extreme heat, instead they prefer ambient temperatures, so when you sow always place them in a cool part of the greenhouse, or in a cool spot in the garden. Use round pots, because square ones can hold too much water in the corners, and almost fill them with seed-sowing compost. Water the pots before you sow. Cover small seeds very thinly.

Biennials, such as sweet williams and wallflowers, need to be sown as soon as possible now and they should germinate within ten days. Prick them out, into seed trays or individual pots, and plant out from September onwards. 

Hardy annuals also do well sown in late-August or early September. The white cow parsley-like Ammi majus, the shorter Ammi visnaga, the lacy white Orlaya grandiflora and Ladybird Poppies (Papaver commutatum) make bigger plants sown now. They should be pricked out in early spring into small pots of John Innes 1.  

All early-flowering perennial seeds can also be sown now, such as hellebores, primroses and aquilegias etc. Many will not germinate until after winter, so add some vermiculite or perlite to your compost to aerate and warm the growing medium.

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More plant seeds to sow in August

Umbellifers

Cow parsley-like umbellifers attract smaller insects like hoverflies. Their umbels add a cottage-garden feel and mix well with brighter-coloured annuals in the flower garden, or in the vase. They are all excellent planted near vegetables too. Many umbellifers tend to be tap rooted, think carrots, so prick them out as soon as you can and put them outside as soon as the roots fill the small pots, usually this takes 4 -6 weeks. Any tap-rooted plant should go out young.

• Ammi majus is tall with dark-green fine foliage topped by domed white umbels by midsummer.
• Ammi visnaga is shorter and more wild carrot-like, with ferny bright-green foliage topped by flat-headed August flowers that age to green.
• Daucus carota ‘Black Knight’ is a real carrot that produces almost-black flower heads and purple foliage.
• Bupleurum rotundifolium ‘Griffithi’ is a lime-green umbellifer that self seeds on its own once established, this dainty hare’s ear or thoroughwax is a charmer.
• Orlaya grandiflorum, is the most difficult of the white umbellifers, but best in some shade where it lasts for longer. Prickly seeds follow.

Biennials and short-lived perennials

Biennials form rosettes in their first year and flower in their second. Sow them now and they will pop up in 21 days on average. Try to get them outside in autumn, or in early spring. Most will set copious amounts of seeds so it's worth collecting them - see below for seed-collecting tips.

Digitalis purpurea - the Foxglove

Our native foxglove comes in many forms, but it isn't a woodlander. It is a disturbance plant prompted to germinate by light  - so leave the seeds on the surface. Plant in sunnier places if you want your foxgloves to self-seed, because they need light. Foxgoves will only germinate in cool temperatures - so no sunny windowsills! Avoid the carousel types - they are short and inelegant with flowers all the way round rather than the tapering one-side spires that foxgloves usually have.

Good foxgloves

• ‘Sutton’s Apricot’ a pale peach that’s good with blue flowers
• ‘Excelsior Mixed’ - traditional mixture of pinks etc
• ‘Pam’s Choice’ heavily maroon-spotted white
• Digitalis purpurea ‘Alba’ pure white with no spotting.

Wallflowers and sweet williams

These can be treated in the same way. However wallflowers flower in late-spring and make excellent partners for later tulips. Sweet Williams (Dianthus barbatus) are early summer plants and most are biennial. However the Noverna sweet William series (it’s short for no vernalisation - a term that denotes that low temperatures are needed before germination) should be treated as an annual and sown next spring to flower in the same year. 

Both make good cut flowers, both are good for bees and there are dark forms of wallflowers and dianthus. Both can be acquired as plugs.

Good varieties of Sweet William seeds

• ‘Auricula Eyed Mix’  and Perfume Mixed ( Suttons)
• ‘Sooty’ and Indian Carpet Mixed - a dark form - Thompson & Morgan

Good varieties of wallflowers

• ‘Sunset Red‘  ( Plants of Distinction)
• ‘Blood Red’, ‘Fireking’ and ‘Vulcan’ ( reds - Sarah Raven)
• Tequila Sunrise -  (a colourful mixture from Suttons)
• ‘Fair Lady’ (Chiltern Seeds sell a whole range including this mixture)

More biennials to sow in August

Hesperis matronalis (Dame’s Violet)
This flowers just after the wallflowers, producing scented flowers in pallid shades of mauve and white. Once you have it, it will self seed. Moth-pollinated and this plant glows in evening light.

Lunaria annua (Honesty)

The well-known honesty behaves like a biennial. It will also self-sow, left to its own devices it forms moon-like papery seeds, but packets of seeds can be sown in late August. There are whites, blues, purples and almost red and a lovely perennial form with scented flowers called Lunaria rediviva.

Tips for sowing seeds

• Seeds need even, ambient temperatures so keep them out of midday sun and fleece them in cold weather.
• Use seed compost that’s moist but not soggy, not a general purpose. The latter will contain too much food.
• Add pertlite or vermiculite when sowing perennials - to get better germination earlier in the year.
• Do not overwater and always label.

Tips for collecting your own seeds

• The key to saving seeds is to make sure that they are fully ripe and dry. Harvest at midday on fine days once the seeds are ready to dehiss, or detach themselves. Add a label to the paper bag or envelope as you go.
• Clean the seeds on a tray, to get rid of the cases etc, and place them in envelopes or paper bags.  
• Ripe seeds are viable seeds and they will store in a tin over winter. However perennials and all members of the Ranuculaceae (eg hellebores and clematis etc) should be sown immediately otherwise they lose viability.
• Only save seeds you need. A carrier bag full of foxglove seeds is a waste of time unless you have a million friends to give it to.
• Consider starting a seed exchange with nearby gardeners.

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