How to grow a gorgeous garden (without spending any money)

Harvesting seeds in late summer is the most cost-effective way to fill your garden with flowers you love

Why bother to harvest seeds from the flowers in your garden? Because you will save money, the seeds are fresh and it is relatively simple to do once you know how, says RHS chief horticulturalist Guy Barter.
The fact that the seed is local to your garden is another bonus, says Helena Dove, head of the Kitchen Garden at Royal Botanic Gardens, Kew. “You’re collecting seed that’s doing really well in your garden, whereas some bought seed is grown in Spain, for example, where it is much hotter and drier, so it might not be as acclimatised to our gardens.”
Collecting seed is a good way to increase for free the number of flowers you particularly like, or to ensure that you can reproduce from half-hardy perennials such as salvia, dahlia or agapanthus that may not make it through the winter. Swapping seeds is also a thrifty way to grow that flower you covet in a friend or neighbour’s garden – just ensure that the seed you are harvesting and receiving is from a healthy, disease-free plant to avoid any risk of transmitting infection, advises Dove. 
“Swap with people you trust and make sure they know a bit about gardening,” she says.
Legally, you are allowed to collect seed from some wildflowers in the countryside or public parks. Dove would prefer such flowers to be left alone, however: “There’s nothing you can harvest from the wild that you can’t get from a good seed supplier who will also ensure the seed is disease-free. All those seeds in the wild are needed by animals to eat, and by plants to propagate. Hands off – you’re just ruining it for everyone else.”    
However, if you do collect seeds from the wild, first make sure that you know what the flower is, that it is healthy and non-invasive, and take seed sparingly. Be careful though, because it is illegal to pick any part of a plant on land designated a SSSI (Site of Special Scientific Interest) or a National Nature Reserve. Local bylaws often apply to other nature reserves or National Trust land which forbid plant collection. There are also certain species for which wild collection is forbidden (a full list is available on plantheritage.org.uk).
Seed is generally ripe enough to collect about six to eight weeks after a plant has stopped blooming. So, flowers from spring and early summer ought to be harvestable from August. This includes marigold (Calendula officinalis); sweet pea (Lathyrus odoratus); poppy; cornflower (Centaurea cyanus); lavender (Lavandula); love-in-a-mist (Nigella damascena); aquilegia (columbine); astrantia; cosmos; scabious; zinnia, and pimpinella. 
There are various types of seed head. For example, poppy seeds are found inside pods, while others such as scabious, zinnia and astrantia, are carried on the head. You stand the best chance of propagating successfully if you collect seed from healthy and vigorous plants. Wait until the seed is ripe – so when it has turned darker, for example from green or white to brown or black. 
If you collect or cut off a flower head before the seed is ripe it will not develop. However, if you leave it too late, the seeds will have freed themselves. Calculating the right moment can be tricky for beginners. A quick test is to gently feel the faded flower head: ripe seeds will start to fall away, while a ready pod will rattle when you shake it. Barter suggests tying a piece of muslin round the flower head, then leaving it to catch the seeds when they ripen. Avoid using paper bags, which will turn to mush if it rains.
Otherwise, choose a dry day to gently rub the seed off the flower head or from the pod into a paper bag. An even easier way is to cut off the whole head or pod and extract the contents later.
Sowing seed fresh offers the greatest possibility of successful germination. However, some types of seed have specific growing requirements: for example, needing to undergo stratification, a type of pre-treatment to simulate their natural conditions of warmth and cold; requiring light; or having their coats softened before germination can occur. This can be checked online on a free RHS germination guide.
Whether you sow in the autumn or wait until the spring, Barter advocates sowing in pots rather than straight into the ground. In pots, you can use a medium of 30 per cent grit with 70 per cent compost to aid drainage and air flow. Sowing directly into the ground risks losing seeds or seedlings to adverse weather, weeds or rodents such as mice, especially if the seed needs extra time to germinate.
Leave the seed heads/pods or seeds to dry on newspaper for a few days in a warm place such as a window sill. Do not put them in an airing cupboard because it is too hot and will damage them. Crush whole seed heads and pods into a glass jar, then shake it to remove chaff from the seed and minimise the risk of disease. If storing, put your seeds in a paper or cotton bag in a dated and labelled airtight container, ideally made from glass. 
Avoid using a plastic container, which will make the seeds sweat and rot. Adding a silica gel packet helps to keep seeds dry. Most seed can be kept for a year if dried out well then stored in a cool place such as a fridge. Check on it now and again. If you have stored your seed, a good way of testing if it is still viable is to sow a few on a damp paper towel to see how many germinate.
Some plants, such as double flowers, are sterile, and so unable to produce seed. Seed from a hybrid – a mix of two cultivars or species – will not come true. For example, says Barter, a favourite pink lily cultivar might breed a white one instead. This should not prevent you from propagating, he says: “All lilies are lovely, so it doesn’t matter. I’d always encourage people to experiment and see what comes up.”
Cultivating your own seed increases a healthy plant diversity in your garden, he adds, “so the more people who do it, the better”.
RHS The Garden Almanac 2025 by Guy Barter is out now
Seeds: Collecting, Saving & Sowing workshop, September 6 at RHS Hyde Hall: for tickets, visit rhs.org.uk