15 Flowers to Start Indoors in January: Fastest Germination Times and Best Varieties
Every year, US gardeners spend hundreds of dollars on nursery transplants that could have cost pennies. A tray of zinnias at the garden center runs $4–7 for six plants. A packet of zinnia seed? Under $3 — enough for 50 or more plants. For annuals you replant every year, that math adds up fast.
Starting flowers from seed indoors extends your growing season by 6–10 weeks, depending on your USDA zone. In Zone 5, the last frost date falls around mid-May — but seedlings started indoors in February are already the size of nursery transplants by the time the soil warms. You get earlier blooms, more blooms, and access to varieties that your local garden center will never stock.
This guide covers the 15 best flowers to grow from seed indoors, organized by difficulty. Whether you have a sunny south-facing windowsill or a spare shelf in the basement, there are flowers here for you. We have included sowing months, germination temperatures, equipment essentials, and a full hardening-off protocol so nothing dies in the transition from indoors to garden.
What You Need to Know Before You Start
Why Windowsills Alone Are Not Enough
The single biggest mistake beginner seed-starters make is relying on a windowsill. Even a south-facing window in January or February delivers only 6–8 hours of weak, angled light. Seedlings respond by stretching toward the light source and growing tall, thin, and floppy — a condition called etiolation. These leggy seedlings are weaker, more susceptible to damping off, and less productive than stocky, compact transplants grown under artificial light. We will cover the grow light solution in the equipment section below.
Seed Compost vs. Potting Mix
Seed compost is not interchangeable with potting mix. Seed-starting mix is finer-textured, sterile, and deliberately low in nutrients — seeds contain all the nutrients they need for germination, and high-fertility compost encourages fungal diseases. Use a purpose-formulated seed-starting mix or make your own by blending equal parts fine perlite and coir.
Reading the Sowing Calendar
Sowing dates in this guide are given as weeks before your last frost date (LFTD). To find your LFTD, enter your zip code at the USDA Plant Hardiness Zone Map or search your county extension website. A quick reference: Zone 4 (LFTD mid-May), Zone 5 (LFTD mid-May), Zone 6 (LFTD mid-April), Zone 7 (LFTD mid-March), Zone 8 (LFTD late February), Zones 9–10 (minimal frost risk — start even earlier).
For detailed month-by-month sowing schedules, see our guides on what to plant in January and what to plant in February.
The Easiest Flowers to Start from Seed Indoors
These five flowers are beginner-proof. They germinate quickly, tolerate a range of temperatures, and forgive mistakes. If you have never started seeds before, start here.
1. Zinnia (Zinnia elegans)
Sow: 4–6 weeks before last frost | Germination temp: 70–85°F | Days to germinate: 5–7
Zinnias are among the fastest, most reliable flowers you can grow from seed. Large seeds, quick germination, and zero fussiness make them the ideal first seed-starting project. Sow 2–3 seeds per cell in a module tray, thin to the strongest, and transplant after last frost. Note: zinnias do not like root disturbance — use biodegradable pots or handle the root ball carefully. Direct sowing also works, but starting indoors gives you blooms 3–4 weeks earlier.
Seed cover? Yes, cover lightly with compost. Surface sow? No.
2. Marigold (Tagetes spp.)
Sow: 6–8 weeks before last frost | Germination temp: 70–75°F | Days to germinate: 5–7
Marigolds are the quintessential beginners’ seed, fast and forgiving. African types (Tagetes erecta) grow tall and produce large pompom flowers; French types (Tagetes patula) are more compact and prolific. Both germinate readily at room temperature without any supplemental heat. Marigolds also double as pest deterrents — plant them at the border edges to discourage aphids and whitefly.
Seed cover? Cover lightly. Surface sow? No.
3. Cosmos (Cosmos bipinnatus)
Sow: 4–6 weeks before last frost | Germination temp: 65–75°F | Days to germinate: 7–10
Cosmos is so easy it borders on invasive in warm climates. The feathery foliage germinates in a week and grows quickly. Cosmos actually performs better when slightly pot-bound — avoid over-potting. One important note: cosmos flower best in poor soil. If you feed them with high-nitrogen fertiliser, you get lush foliage and few flowers. Withhold feed and let them struggle a little.
Seed cover? Cover lightly. Light requirement: Full sun.
4. Sunflower (Helianthus annuus)
Sow: 2–3 weeks before last frost | Germination temp: 70–85°F | Days to germinate: 5–7
Sunflowers grow so fast that there is minimal benefit to starting them early — 2–3 weeks is enough. Sow one seed per biodegradable pot since sunflowers resent root disturbance. The main advantage of indoor starting is getting tall varieties (such as ‘Russian Giant’ or ‘Mammoth’) established before the heat of summer arrives. Transplant carefully without disturbing the taproot.
Seed cover? Yes, cover 1 inch deep. Note: Do not start more than 3 weeks early.
5. Nasturtium (Tropaeolum majus)
Sow: 2–4 weeks before last frost (direct into final container) | Germination temp: 65–70°F | Days to germinate: 7–14
Nasturtiums are the one flower on this list that is actually best sown direct into their final pot indoors, rather than transplanted. They hate having their roots moved. Sow 2–3 large seeds into a 6-inch pot or hanging basket, thin to 1–2 plants, and move outside after last frost. Like cosmos, nasturtiums flower better in poor soil — a rich mix produces leaves, not blooms. Flowers and leaves are both edible.
Note: No transplanting. Sow direct into final container.
Moderate Difficulty: Flowers That Need Some Attention
These five flowers are well within any gardener’s reach but reward close attention to temperature, light, and sowing technique.
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→ View My Garden Calendar6. Sweet Pea (Lathyrus odoratus)
Sow: 10–12 weeks before last frost (January in most zones) | Germination temp: 50–60°F (cool) | Days to germinate: 7–14
Sweet peas need cool germination — around 55°F is ideal, which means a cold frame, unheated garage, or cool basement in January. Soak seeds for 24 hours in warm water before sowing, or nick the seed coat with a nail file to speed germination. Root trainers (tall, narrow cells) are strongly preferred over standard modules because sweet peas develop a long taproot that resents being confined. Sow two seeds per cell, thin to one. Do not move to warm growing conditions until the seedlings are actively growing.
Key tip: Cool germination is essential — warmth causes failure.
7. Snapdragon / Antirrhinum (Antirrhinum majus)
Sow: 8–10 weeks before last frost | Germination temp: 55–65°F (cool) | Days to germinate: 10–14
Snapdragon seed is dust-fine and needs light to germinate — surface sow onto moist compost and do not cover. Press seeds gently onto the surface with your finger to ensure good contact. Like sweet peas, snapdragons prefer cool germination around 60°F. A humidity dome helps maintain moisture around the tiny seeds. Once germinated, grow on in cooler conditions (around 55°F) to produce stocky plants. Snapdragons are hardy to Zone 7 as annuals and can tolerate a light frost.
Surface sow? Yes — needs light to germinate.
8. Celosia (Celosia argentea)
Sow: 6–8 weeks before last frost | Germination temp: 70–85°F (warm) | Days to germinate: 7–10
Celosia is heat-loving and needs consistent warmth of 70–80°F to germinate well. A heat mat is helpful. Surface sow onto moist seed compost under a humidity dome and place on a heat mat. Once germinated, celosia grows quickly in warm conditions. Do not rush to move them outside — even a brief cold snap below 50°F can set the plants back significantly. Wait until nighttime temperatures are reliably above 55°F.
Surface sow? Yes. Heat mat recommended.
9. Rudbeckia / Black-Eyed Susan (Rudbeckia hirta)
Sow: 8–10 weeks before last frost | Germination temp: 65–70°F | Days to germinate: 14–21
Rudbeckia is a perennial (or short-lived perennial, depending on variety) that takes more time than annuals. Be patient — germination can take 2–3 weeks. Surface sow or cover very lightly. Once up, rudbeckia is robust and forgiving. The reward is a plant that will return for years with minimal care and produces the quintessential summer daisy from July through September. Varieties such as ‘Goldsturm’ are reliably perennial across Zones 3–9.
Note: Slower germination — do not give up before day 21.
10. Petunia (Petunia x hybrida)
Sow: 10–12 weeks before last frost | Germination temp: 70–75°F | Days to germinate: 7–14
Petunia seed is extremely fine — many commercial seeds are sold as pelleted seed (coated in clay) to make handling easier. Surface sow without covering and maintain moisture with a humidity dome and bottom watering. Petunias need bright light from the moment they germinate, so grow lights are essential. Without adequate light, petunias become leggy and rarely recover fully. Start them early — 10–12 weeks — because they are slow to reach transplant size.
Surface sow? Yes — tiny seed needs light.
Challenging but Worth It: Slow and Fussy Flowers
These five flowers test your patience, but the results justify the effort. Each produces something a garden center cannot easily replicate.
11. Lisianthus (Eustoma grandiflorum)
Sow: 20–24 weeks before last frost (November–December for spring planting) | Germination temp: 75°F for 4 weeks, then 65°F | Days to germinate: 14–28
Lisianthus is the Everest of indoor seed starting. It requires a two-stage temperature protocol: maintain 75°F for the first four weeks to break dormancy, then drop to 65°F once seedlings emerge. From sowing to transplant-ready size takes 120 days or longer. Seeds are sold pre-pelleted. Surface sow onto sterile seed compost, maintain consistent moisture, and cover with a humidity dome throughout germination.
Why bother? Because a single established lisianthus plant in your garden produces $15–20 worth of blooms (at florist prices) in a season. The ruffled, rose-like flowers last 2–3 weeks as cut flowers and come in colors from pure white to deep purple. Once you’ve grown them, you’ll never pay florist prices for them again.
Start in November or December for the following summer.
12. Pelargonium / Geranium (Pelargonium x hortorum)
Sow: 12–16 weeks before last frost (January for most zones) | Germination temp: 70–75°F | Days to germinate: 7–14
Bedding geraniums (actually pelargoniums) are expensive to buy as transplants — $3–6 each is typical. Growing from seed saves significantly, but requires a long lead time. Start in January for May transplants. Seeds germinate readily at 70°F with a humidity dome. The challenge is the long grow-on period: 12–16 weeks under grow lights, with regular liquid feeding once true leaves appear. Pinch the growing tips at 3–4 inches tall to encourage bushy, branching plants.
Start in January for Memorial Day planting.
13. Begonia (Begonia x semperflorens)
Sow: 12–14 weeks before last frost | Germination temp: 70–75°F (minimum) | Days to germinate: 14–21
Begonia seed is dust-fine — the finest of any flower on this list. Mix the seed with dry horticultural sand to help distribute it evenly, then surface sow onto moist compost and press down gently. Cover the tray with a clear lid or cling film to maintain 100% humidity. Do not let the surface dry out even slightly — the seed is so small that drying kills it instantly. A heat mat at 70–75°F is essential. Once seedlings emerge (tiny, fragile), ventilate gradually and increase light.
Mix with sand for even distribution. Never allow surface to dry.
14. Lobelia (Lobelia erinus)
Sow: 12–14 weeks before last frost | Germination temp: 65–70°F | Days to germinate: 14–21
Lobelia seed is also extremely fine — mix with dry sand before sowing, just as with begonias. Surface sow onto moist seed compost under a humidity dome. Lobelia is slow-growing and needs full 12–14 weeks under lights before it is ready to transplant. Thin seedlings in clusters rather than individually to avoid disturbing the fragile root systems. Plant out as clumps and they will fill in quickly once established.
Mix with sand. Thin in clusters, not individually.
15. Delphinium (Delphinium elatum)
Sow: 8–10 weeks before last frost (or in autumn for spring germination) | Germination temp: 55–65°F (cool) | Days to germinate: 14–28
Delphinium seed benefits from cold stratification: place seeds in a sealed plastic bag with slightly damp vermiculite and refrigerate for 2–3 weeks before sowing. This mimics winter conditions and significantly improves germination rates. After cold stratification, sow in cell trays at 55–60°F. Delphiniums prefer cool conditions throughout — heat causes poor germination and weak seedlings. Once established, they are among the most spectacular perennials you can grow, with spires reaching 4–6 feet in shades of blue, purple, and white across Zones 3–7.
Cold stratify 2–3 weeks in the fridge before sowing.
Essential Equipment for Indoor Seed Starting
Seed Trays and Modules
Standard 1020 trays are the backbone of indoor seed starting. You can use them with loose seed compost (broadcast sowing) or fit them with plug trays or module inserts for individual cells. For most flowers, 72-cell or 128-cell plug trays are ideal. Seed starting trays are inexpensive and reusable for years with proper cleaning.
Soil blocks are an excellent alternative for plants that dislike root disturbance (sunflowers, cosmos, zinnias). A soil block maker compresses damp seed compost into compact blocks that hold their shape without a plastic cell. Roots stop at the air gap between blocks (air pruning) and transplant shock is virtually eliminated.
Grow Lights
This is the single most important equipment investment for indoor seed starting. Without adequate light, seedlings etiolate (stretch) and become weak. A full-spectrum LED T5 grow light placed 2–4 inches above the seedling canopy and run for 14–16 hours per day is the standard setup. Set it on a timer and forget it.
LED grow lights are now affordable enough to justify even for hobby seed starting — a basic single-shelf setup runs under $50. The difference between leggy, light-starved seedlings and compact, stocky transplants is almost entirely down to adequate light intensity and duration.
Heat Mats
Soil temperature, not air temperature, determines germination speed. Most seeds germinate significantly faster when the soil temperature is at or above the stated optimum. A seedling heat mat with a thermostat allows precise bottom-heat control. Essential for: begonias, celosia, lisianthus, petunias, and any tropical species. Less important for: sweet peas, snapdragons, and delphiniums, which prefer cool germination.
Humidity Domes
A clear plastic dome over your seed tray maintains the high humidity that surface-sown seeds need to germinate. Once the majority of seeds have germinated, remove the dome and increase air circulation to prevent damping off — a fungal disease that collapses seedlings at the soil line.
Seed-Starting Mix
Use a purpose-formulated seed-starting mix, not regular potting compost. It is finer-textured for good seed contact, sterile to reduce fungal pressure, and low-nutrient so new seedlings are not overwhelmed. Once seedlings develop their first true leaves, begin light liquid feeding (half-strength balanced fertiliser weekly).
Quick-Reference Comparison Table
| Flower | Difficulty | Start (wks before LFTD) | Germ Temp (°F) | Days to Germ | Surface Sow? | Heat Mat? |
|---|---|---|---|---|---|---|
| Zinnia | Easy | 4–6 | 70–85 | 5–7 | No | No |
| Marigold | Easy | 6–8 | 70–75 | 5–7 | No | No |
| Cosmos | Easy | 4–6 | 65–75 | 7–10 | No | No |
| Sunflower | Easy | 2–3 | 70–85 | 5–7 | No | No |
| Nasturtium | Easy | 2–4 (direct) | 65–70 | 7–14 | No | No |
| Sweet Pea | Moderate | 10–12 | 50–60 | 7–14 | No | No (cool) |
| Snapdragon | Moderate | 8–10 | 55–65 | 10–14 | Yes | No (cool) |
| Celosia | Moderate | 6–8 | 70–85 | 7–10 | Yes | Yes |
| Rudbeckia | Moderate | 8–10 | 65–70 | 14–21 | Lightly | No |
| Petunia | Moderate | 10–12 | 70–75 | 7–14 | Yes | Optional |
| Lisianthus | Challenging | 20–24 | 75 then 65 | 14–28 | Yes | Yes |
| Pelargonium | Challenging | 12–16 | 70–75 | 7–14 | No | Optional |
| Begonia | Challenging | 12–14 | 70–75 | 14–21 | Yes | Yes |
| Lobelia | Challenging | 12–14 | 65–70 | 14–21 | Yes | No |
| Delphinium | Challenging | 8–10 + stratify | 55–65 | 14–28 | Lightly | No (cool) |
Hardening Off: The Step Most Gardeners Skip
Hardening off is the process of gradually acclimatising indoor-grown seedlings to outdoor conditions. Plants grown indoors have thin cell walls adapted to still air and consistent temperatures. Moving them straight outside exposes them to wind, UV intensity, temperature swings, and cold nights that can cause transplant shock or outright death.
Follow a 7–10 day protocol:
- Days 1–2: Move seedlings outside to a sheltered spot (no direct sun) for 1–2 hours in the middle of the day. Bring in before evening.
- Days 3–4: Increase outdoor time to 4–5 hours. Allow some dappled sun.
- Days 5–6: Move to direct sun for the morning hours. Leave out until late afternoon.
- Days 7–8: Full day outside in their final position, bring in only if frost is forecast.
- Days 9–10: Leave out overnight once temperatures are reliably above 40°F.
Skipping hardening off is the number one cause of transplant failure. A seedling that has spent 10 weeks growing perfectly indoors can be killed in a single windy afternoon if hardening off is skipped.
If you also grow herbs on your windowsill, the same hardening-off approach applies — see our guide to growing herbs indoors for more detail on transitioning plants outside.
Frequently Asked Questions
How many weeks before the last frost should I start seeds indoors?
It depends on the flower. Quick-growing annuals like zinnias, marigolds, and cosmos need only 4–6 weeks. Slow-growing flowers like lisianthus, pelargoniums, begonias, and lobelia need 12–16 weeks or more. Always count backwards from your last frost date (LFTD). The USDA Plant Hardiness Zone Map gives LFTD by zip code.
Can I use regular potting compost for seed starting?
Not ideally. Standard potting mix is too coarse-textured and too high in nutrients for germinating seeds. The fine texture of seed-starting mix is important for good seed-to-compost contact and water retention around tiny seeds. The low nutrient level prevents fertiliser burn on delicate emerging roots.
Why are my seedlings leggy and falling over?
Almost always a light problem. Leggy seedlings are stretching toward an inadequate light source. Move seedlings closer to a grow light (2–4 inches from the canopy) or increase photoperiod to 14–16 hours. A small fan running for a few hours a day also strengthens stems by simulating outdoor breeze.
What is damping off and how do I prevent it?
Damping off is a fungal condition that causes seedlings to collapse at the soil line. It is caused by overwatering, poor air circulation, and using non-sterile compost. Prevent it by: using sterile seed-starting mix, watering from below (bottom watering), removing humidity domes as soon as seeds germinate, and running a small fan for air circulation. Do not overwater.
When should I pot up seedlings?
Pot up when roots start emerging from the bottom of the cell or when the seedling has developed 2–3 sets of true leaves. Do not pot up too early (roots need some restriction to develop properly) or too late (root-bound seedlings suffer a check in growth). Move to a container one size larger, not straight to a large pot.
Are grow lights necessary?
For most flowers, yes. The possible exceptions are cosmos, zinnias, and marigolds on a bright south-facing windowsill from March onwards — but even these benefit from supplemental lighting in January and February when days are shortest. Any flower on the challenging list absolutely requires grow lights.
