Summer seed starting is the quiet work that makes the fall garden possible. The brassicas, kale, chard, peas and carrots that will feed you in autumn are sown in summer, when the soil is hot, the sun is strong, and a seed that dries out after germination will not recover. The challenge is therefore not the seed itself but the conditions around it: keeping it moist, keeping it cool enough to germinate, and protecting the seedling until it is strong enough to cope with the heat. This guide covers starting seed indoors and outdoors in summer, the crops to sow, and the simple tricks that make summer germination reliable. Dates are averages and vary by place, so check your local extension service for the sowing dates in your area.
What to sow in summer
Summer sowing falls into two groups: the quick crops that will be eaten in summer, and the fall crops that will be eaten in autumn. Quick crops include beans, lettuce, radishes, carrots, beets, chard and herbs such as cilantro and dill, sown in succession every two or three weeks so the harvest keeps coming. Fall crops include brassicas — cabbage, broccoli, cauliflower, kale and Brussels sprouts — peas, carrots, beets, chard and spinach, sown from midsummer onwards so they mature in the cooler weather of autumn, when their flavour is at its best.
Start the fall crops that need a long season — brassicas in particular — indoors or in a shaded nursery bed, and transplant them out as space opens up when summer crops finish. Direct-sow the crops that dislike transplanting, such as carrots, peas and beans, where they are to grow. Choose varieties described as suitable for summer sowing or for fall harvest, because they are bred to cope with the conditions, and check the days to maturity on the packet so you can work back from the first frost in your area. The seed starting calculator can help with timing, and our summer planting guide covers what to plant and when.
Starting seed indoors in summer
Starting seed indoors in summer has one big advantage: you control the temperature and the moisture, and the seed does not have to cope with a baking seed bed. Use fresh seed compost, sow thinly, cover lightly, and water from below or with a fine spray so the seed is not disturbed. Keep the trays in bright light but out of direct sun, which can cook a seed tray in minutes on a hot day, and ventilate them so the seedlings do not damp off — the fungal disease that kills young seedlings in warm, still, damp air.
Harden seedlings off before planting them out, by placing them outdoors in shade for a few hours a day and building up over a week, so they adjust to sun, wind and temperature changes. Transplant in the evening or on a cloudy day, water them in well, mulch immediately, and shade them for their first few days. A seedling raised indoors and planted out without hardening off will wilt, scorch, or simply stop growing, and a week of gradual hardening is the difference between a plant that thrives and one that struggles. Our summer transplanting guide covers the move outdoors in detail.
Sowing outdoors in hot soil
Sowing outdoors in summer needs more care than sowing in spring, because the surface of the soil dries out so fast. Water the drill or row thoroughly before sowing, sow the seed, cover it, and water gently again. Cover the row with a board, a piece of cardboard or a length of shade cloth to hold moisture in until germination, and lift it as soon as seedlings appear — check daily, because seedlings left under a board will go pale and leggy. Sow a little deeper in summer than in spring, because the surface dries first, and sow in the evening so the seed has a cool night to settle in.
Some seed germinates poorly in heat. Lettuce and spinach seed in particular struggle when soil temperatures are high, and they may germinate better in a tray indoors, or in a shady spot, or sown in the evening and kept moist. Carrots are slow to germinate in summer and need the surface kept moist for up to three weeks — a board over the row, checked daily, is the reliable method. If a sowing fails, sow again with better watering and shade rather than assuming the seed was at fault. For crop detail, see growing lettuce and growing carrots.
Keeping seedlings alive in the heat
A seedling's first week is its most vulnerable. Water gently every day, or twice a day in very hot weather, using a fine spray or a watering can with a rose so the seedlings are not washed out of the soil. Shade the row with a length of shade cloth or a board propped above it, so the seedlings get filtered light rather than full sun, and remove the shade in the evening and on cloudy days. Mulch around the seedlings once they are big enough to handle it, keeping the mulch clear of the stems, so the soil stays moist and cool.
Thin seedlings promptly, because crowded seedlings compete for water and light and grow weak and leggy. Thin to the spacing on the packet, doing it in the evening when the plants are cool, and water after thinning so the remaining seedlings settle back into the soil. Protect young seedlings from slugs, which are at their worst in a wet summer, and from flea beetles on brassicas, which are at their worst in a hot, dry one — a fine mesh cover over the row protects against both, and it also shades the seedlings a little. Our summer pest control guide covers the pests to watch for.
Seed saving and seed care in summer
Summer is also the season to think about seed for next year. Let a few plants of each variety you want to save go to seed — a lettuce, a tomato, a bean — and collect the seed when it is ripe and dry, storing it in a paper envelope in a cool, dry place, labelled with the variety and the date. Do not save seed from hybrid varieties, because the plants grown from it will not come true to type, and do not save seed from a plant that was diseased, because the disease may carry over.
Store bought seed in a cool, dry place through the summer, because heat and humidity shorten its life. A sealed jar in a cool cupboard is ideal, and a packet of silica gel in the jar helps keep it dry. Check the date on packets, and use the oldest seed first, because germination falls with age. If you are unsure whether old seed is still viable, sow a few seeds on damp paper and see how many germinate — a simple test that saves a wasted sowing. The germination time calculator gives the usual germination times for common crops.
A summer seed-starting checklist
Plan the fall garden first, working back from the first frost in your area to find the sowing dates, and start long-season crops such as brassicas indoors. Sow quick crops in succession every two or three weeks, water seed rows before sowing, cover them to hold moisture, and check daily for germination. Harden seedlings off before planting out, transplant in the evening, mulch and shade them for their first few days, and thin promptly so the remaining seedlings have room to grow.
Summer seed starting rewards the gardener who is organised and attentive rather than the one who sows and forgets. A seed sown at the right time, kept moist and shaded until it is established, will grow into a plant that feeds you in autumn, and that is a good return for a few minutes of care on a summer evening. For the rest of the summer routine, read our summer transplanting guide and browse the guides hub and the planting calendar for the full seasonal series.
Frequently asked questions
What seeds can I start in summer?
Quick crops such as beans, lettuce, radishes, carrots, beets and chard for summer eating, and fall crops such as brassicas, kale, peas, carrots and spinach for autumn harvest. Start long-season brassicas indoors and direct-sow crops that dislike transplanting, such as carrots and peas.
Why won't my seeds germinate in summer?
The usual cause is the seed drying out after germination has started. Water the row before sowing, sow a little deeper than in spring, cover the row with a board or shade cloth to hold moisture, and check daily. Lettuce and spinach seed germinate poorly in hot soil and may do better started indoors.
Should I start seeds indoors or outdoors in summer?
Start long-season fall crops indoors, where you control temperature and moisture, and direct-sow quick crops and those that dislike transplanting outdoors. Harden indoor-raised seedlings off over a week before planting them out, and transplant in the evening.
How do I keep summer seedlings alive?
Water gently every day with a fine spray, shade the row with shade cloth or a board, mulch around seedlings once they are big enough, thin promptly, and protect them from slugs and flea beetles with a fine mesh cover.
When should I sow for a fall garden?
From midsummer onwards, working back from the first frost in your area using the days to maturity on the seed packet. Check your local extension service for fall sowing dates in your zone, because they vary widely.