Starting flowers from seed is the cheapest way to fill a garden, and often the only way to grow the varieties you actually want — the seed catalogues carry far more choice than any garden centre. A single packet of zinnia or cosmos seed, sown at the right time, can produce more plants than most gardens have room for, at a fraction of the cost of buying them. The skill is not in the sowing itself, which is simple, but in the timing, the light, and the transition from indoors to outdoors, which is where most seedlings fail.
This guide covers starting flowers from seed indoors and direct sowing outdoors, the flowers that are easiest for beginners, and the common mistakes that kill seedlings. It covers hardening off, transplanting, and the flowers that are better bought as plants. If you are new to this, our flowers transplanting guide covers the next step, and your local extension service can tell you the last-frost date for your area, which is the single most important date in seed starting.
Know your dates: last frost and days to germination
Every seed packet carries two numbers that matter: the days to germination and the weeks to sow before the last frost. The last frost date is the anchor for the whole seed-starting calendar — tender annuals such as zinnias, marigolds, and sunflowers cannot go outside until the frost has passed, so they are sown indoors four to six weeks before that date. Hardy annuals such as sweet peas, calendula, and cornflowers tolerate light frost and can be sown outdoors earlier, or indoors six to eight weeks before the last frost. Perennials are slower: many need eight to twelve weeks indoors, and some, such as lavender, need a period of cold before they will germinate at all. If you do not know your last frost date, your local extension service can give it to you, and our flowers seasonal care guide uses it as the anchor for the whole year. Sow too early and the seedlings grow leggy indoors and outgrow their pots; sow too late and they flower late or not at all. A week either way matters less than getting the light right.
Sowing indoors: mix, containers and moisture
Start with a seed-starting mix rather than garden soil — it is fine, sterile, and free-draining, so seedlings do not damp off. Fill small pots or trays, water the mix before sowing so it is damp but not wet, and sow the seed at the depth on the packet — as a rule, twice the width of the seed, and some seeds, such as poppies and petunias, need light to germinate and should be pressed onto the surface rather than covered. Cover the tray with a clear lid or plastic wrap to hold in moisture, and remove it as soon as the seedlings emerge, since a closed, damp tray is where damping-off disease starts. Water from below where possible, by standing the tray in water for a few minutes, so the surface stays dry and the seed is not displaced. Label everything at sowing time — seedlings of different flowers look remarkably alike, and an unlabelled tray in April is a guessing game. Use clean containers every year, and never reuse old mix for seedlings.
Light: the difference between sturdy and leggy
Light is the most common reason indoor seedlings fail. A windowsill is rarely enough, even a sunny one — seedlings stretch towards the glass, grow tall and pale, and flop over at the first breath of wind outdoors. The solution is a simple grow-light setup: a shop light or a dedicated grow light suspended a few centimetres above the seedlings, raised as they grow, and left on for fourteen to sixteen hours a day. The light should be close — a light a metre above the seedlings is little better than a window. If the seedlings are leaning or stretching, the light is too far away or on for too few hours. The grow light calculator can help you work out the setup for your space. Rotate the trays every day if you are using natural light, and keep the room cool rather than warm — a seedling grown at 15–18°C is stockier and hardier than one grown at 24°C, even if it grows more slowly.
Direct sowing outdoors: the simplest method
Many flowers are best sown where they are to grow, with no indoor stage at all. Sunflowers, nasturtiums, calendula, cornflowers, poppies, and cosmos all germinate readily in warm soil and dislike transplanting, so direct sowing suits them. Prepare the bed by raking it fine and watering it, sow the seed at the depth on the packet, and mark the rows so you can tell the seedlings from the weeds. Thin the seedlings to the spacing on the packet once they have two or three true leaves — this is the hardest step for beginners, because it means pulling up healthy plants, but unthinned seedlings grow weak and flower poorly. Water gently and keep the bed moist until germination, which may take a week or two depending on the soil temperature. Direct-sown flowers often catch up with indoor-started ones by midsummer, since they never suffer transplant shock. Our flowers soil preparation guide covers getting the bed ready, and the flowers spacing guide gives the distances to thin to.
Hardening off and transplanting
Hardening off is the gradual introduction of indoor seedlings to outdoor conditions, and skipping it is the most common cause of seedling death. Over seven to ten days, put the seedlings outside for a few hours in a sheltered spot, bringing them in at night, and increase the time and the exposure a little each day until they are out all day and night. Start in shade and still air, and work up to sun and wind. A seedling moved straight from a warm windowsill to a sunny, windy bed will wilt, scorch, and often die within a day. When it is time to transplant, water the seedlings well first, plant them at the same depth they were growing, water them in, and shade them for a day or two if the weather is hot. Our flowers transplanting guide covers the technique in detail. Do not transplant in the heat of the day, and do not let the rootball dry out while you are working — keep the tray in the shade and plant quickly.
Flowers that are easy — and flowers to buy as plants
The easiest flowers to start from seed are zinnias, marigolds, cosmos, sunflowers, nasturtiums, calendula, and sweet peas — all germinate readily, grow fast, and forgive a beginner's mistakes. Our growing zinnias, growing marigolds, and growing sunflowers guides give the specifics. Medium-difficulty flowers such as snapdragons, petunias, and dahlias from seed need more care with light and moisture but are very rewarding. Some flowers are better bought as plants: lavender and many perennials are slow and erratic from seed, geraniums and fuchsias are usually grown from cuttings, and bulbs are bought as bulbs rather than grown from seed, which can take years to flower. There is no shame in buying the difficult ones and sowing the easy ones — a garden is judged by its flowers, not by how every plant started. If you are unsure about a diagnosis, a product, or a local rule, check your local extension service — its advice is tailored to your soil, climate, and the pests and diseases active in your area.
Frequently asked questions
When should I start flower seeds indoors?
Most tender annuals four to six weeks before your last frost date; hardy annuals six to eight weeks before; slow perennials eight to twelve weeks before. Check the packet and your local extension service for your frost date.
Why are my seedlings tall and floppy?
Not enough light. Seedlings need a grow light a few centimetres above them for fourteen to sixteen hours a day; a windowsill is rarely enough.
Can I sow flower seeds directly outside?
Yes — sunflowers, nasturtiums, calendula, cornflowers, poppies, and cosmos all do best sown where they are to grow, once the soil has warmed.
What is damping off?
A fungal disease that makes seedlings collapse at soil level. It is caused by wet, still conditions and dirty containers. Use fresh mix, water from below, and ventilate the tray.
How do I harden off seedlings?
Over seven to ten days, put them outside for a few hours a day in a sheltered spot, increasing the time and exposure gradually until they stay out overnight.