What to Sow in Spring: The March to May Sowing Plan

Why timing is everything: the case for a structured spring sowing plan

It is October as I write this, which is not, admittedly, the moment to start a tray of tomatoes on the kitchen windowsill. The garden is slowing down, the nights are closing in and most sensible seedlings would like a quiet lie-down. But it is exactly the right time to make next spring less chaotic. A little planning now means that, come March, you are not standing in front of a seed display with a basket full of ambition and no idea where any of it will go.

Spring sowing in the UK is wonderfully hopeful and slightly treacherous. The first bright weekend arrives. The compost smells good. You find six packets of seed left over from last year and suddenly it seems perfectly logical to sow everything. Tomatoes, runner beans, sweetcorn, cucumbers, courgettes, a whole packet of basil. By April, the windowsill resembles a small, badly managed jungle. By May, half the plants are pale, stretched and cross with you.

The problem is not enthusiasm. Gardens run on enthusiasm. The problem is that a seed packet's sowing range often gives the broadest possible window, whilst your garden has its own weather, soil, shelter and frost pattern. "Sow March to May" can mean three very different things depending on whether you have a heated propagator, an unheated greenhouse, a sunny windowsill or simply a patch of open ground still cold enough to make a worm reach for a jumper.

This plan separates each job into three places. First, indoors: a warm windowsill, propagator or heated greenhouse, for crops that genuinely need warmth and a long growing season. Second, under cover: an unheated greenhouse, cold frame, cloche or polytunnel, where plants gain shelter but not tropical conditions. Third, direct sowing: seed going into its final outdoor position, once the soil and weather make that a reasonable proposition.

That distinction removes a surprising amount of uncertainty. A tomato can be right for a March indoor sowing and completely wrong for a March outdoor sowing. Broad beans can cope with an early start under a cloche yet sulk in cold, wet ground. Courgettes look like a classic spring crop, but they grow so quickly that sowing them in March indoors often creates a plant too large, too root-bound and too impatient by the time the garden is warm enough for it.

Last frost dates are the reason a fixed national sowing date is never quite enough. Southern England often sees its last frost around mid-April, whilst northern England and Scotland can still encounter frost well into May. Gardens on exposed sites, in valleys and at higher elevations can lag behind nearby towns too. Equally, a sheltered city garden may behave like a little warm pocket. It is useful, but never wise, to assume that a forecast for a regional airport describes the temperature at the bottom of your own garden at dawn.

Think of March to May as a sequence rather than a starting gun. March is about a few slow crops, careful under-cover work and patience. April is the engine room, when longer days and rising temperatures make more options sensible. May is when the outdoor vegetable plot starts to earn its keep. Follow that rhythm and you will sow less seed at the wrong time, waste fewer pots of compost and produce sturdier plants. None of which is glamorous. All of which is deeply satisfying.

At a glance: the spring sowing plan

This is a March-to-May framework rather than a promise that every crop will work in every garden on the same date. It covers the main vegetables, culinary herbs and flowers commonly raised from seed in a UK garden, then asks you to make the final decision using local conditions. That is the useful bit. A calendar should guide your judgement, not replace it.

The plan broadly serves gardens spanning the more temperate RHS hardiness categories, from H1 to H4, but the important practical point is simpler: local frost patterns matter. If your garden routinely catches a late frost, work at the later end of each window. If you have a very sheltered, coastal or urban plot, you may be able to work at the earlier end — whilst keeping fleece, cloches or pots ready for a cold night.

It also helps to be honest about the way you like to garden. If you are away often in spring, choose direct-sown crops and avoid raising dozens of tender plants that need daily checking. If your greenhouse is your favourite place to hide with a cup of tea, then modules of brassicas, salads and herbs will fit naturally into your routine. If space is tight, sow little and often. Three short rows of salad leaves several weeks apart are more useful than one heroic row that all matures on the same Tuesday.

March: waking the garden carefully — what to sow indoors first

March is one of the most misunderstood months in the gardening calendar. The days noticeably lengthen and there may be sunshine warm enough to make lunch outdoors feel like an excellent decision. Yet the soil is often still cold. In early March, outdoor soil temperatures in the UK commonly sit around 5–7°C, below the 7–10°C minimum that many vegetables need for reliable germination. Seed in cold ground does not become more determined because the gardener has had a productive morning.

That is why indoor sowing in March should be selective. This is the month for crops that need a long season and enough warmth to start properly, not a general invitation to fill every available container with compost. The best candidates are slow-maturing tender vegetables: tomatoes, peppers and aubergines. They need time to make a decent plant before summer, particularly in northern gardens or where they will be grown outside rather than in a greenhouse.

Tomatoes germinate best at 18–24°C. Aubergines prefer 20–25°C, whilst peppers are happiest around 21–27°C. Those temperatures are a useful reality check. A chilly spare room or windowsill directly above a radiator can be bright but inconsistent. A heated propagator, or a reliably warm indoor spot followed by strong light, makes this work much more smoothly. Once seedlings emerge, light becomes critical. Warmth creates germination; light prevents the plants becoming long, thin and desperate.

Celery and celeriac deserve their own warning label. Their seed is tiny, their germination can take 14–21 days and they need consistent temperatures of 15–18°C. Scatter seed thinly on the surface of moist compost, press it in gently and leave it uncovered. The temptation is to keep checking and then water again, which can turn a carefully prepared tray into a small bog. Cover the tray with a clear lid or bag to retain moisture, but remove it promptly once green shoots appear.

March is also an excellent month for a modest sowing of herbs indoors, particularly basil if you can provide enough warmth and light. Parsley can be started now as well, although it is famously unhurried. There are few gardening experiences more educational than waiting for parsley to germinate. You learn patience, humility and the location of every other seed tray in the house.

For flowers, March indoors suits slow-growing annuals or plants that benefit from a protected beginning. The same principle applies: sow only what you can give enough light to. A crowded windowsill of pale seedlings leaning hard towards the glass is not a sign of successful propagation. It is a polite protest.

Watering is another quiet March trap. Compost should be damp, not sodden. Water from below where possible, especially once seedlings are visible, and provide ventilation on mild days. Stagnant air encourages problems just when tiny plants are most vulnerable. A clear propagator lid is useful, but it is not a permanently sealed terrarium.

March under glass: cold frames, cloches and the unheated greenhouse

"Under cover" is one of those phrases that can sound more magical than it is. In this plan, it means an unheated greenhouse, polytunnel, cold frame or cloche: something that shelters seed and young plants from wind, rain and the sharpest frosts, but does not provide supplementary heat. It is a buffer, not a tropical holiday.

That buffer matters. In March, soil in an unheated greenhouse can be 3–5°C warmer than soil outdoors. It is enough to open a useful early window for hardy and half-hardy crops, particularly when the structure is ventilated on sunny days and closed before cold nights. A cold frame is less stable than a greenhouse, and a simple cloche less stable again, but each can make a real difference to a small sowing.

Early brassicas are a good fit for this protected space. Summer cabbage, cauliflower and Brussels sprouts can be sown in modules from around mid-March, ready to establish as young plants rather than struggling as tiny seedlings in open soil. Raising them in modules also makes it easier to protect them from slugs and pigeons later. Brassicas are remarkably capable of looking robust one day and becoming a rather expensive pigeon buffet the next.

Broad beans are the stars of the March under-cover plan. They can germinate from 5°C, which makes them much more willing than tender beans to get moving early. A sowing under a cloche in early March can give a three-to-four-week head start over waiting for reliably safe outdoor conditions. Use well-drained ground or deep modules, because cold, waterlogged compost is not a friendly place for any bean seed.

Peas, hardy salad leaves, radishes and spring onions may also be possible under cover, particularly in a greenhouse border or a raised bed protected by cloches. Sow a small amount first. The aim is an early harvest, not a competition to see how much seed can be exposed to March weather at once. If a late cold spell arrives, a short row is easy to cover. A whole plot of seedlings can become an emotional administrative burden.

There is a useful distinction between sowing in modules under cover and sowing into a greenhouse border. Modules let you control spacing, moisture and slug damage, then transplant when the seedlings are sturdy. A greenhouse border gives roots room but commits the crop to that space. For early brassicas and salads, modules are often the calmer choice. For broad beans, either method can work provided the compost or soil drains well.

Do not forget the ordinary jobs that make covered sowing successful. Clear old debris, wash pots and trays, check that drainage holes are not blocked, and have fleece ready. A greenhouse does not remove the need to watch the forecast. It simply gives you more options when the forecast becomes rude.

April: the sowing engine room

April is when the spring sowing plan starts to feel busy in the best sense. Day length increases, soil gradually warms and the gap between indoor, under-cover and direct sowings begins to close. By mid-April, outdoor soil in southern England commonly reaches around 8–10°C, whilst late-April days provide roughly 13–14 hours of daylight. That does not mean every garden is frost-free or every seed can go outside. It does mean the list of realistic jobs becomes much longer.

For indoor sowing, April is the moment for courgettes, cucumbers and squash. This is later than many enthusiastic gardeners expect, but it is usually exactly right. These plants grow at an astonishing pace once they have germinated. A courgette sown in mid-April can be the right size for planting out after the last frost, typically in late May. A courgette sown much earlier is often large, congested in its pot and profoundly unimpressed by the wait.

Sow each large seed individually, rather than scattering several into one container and creating a root-separation puzzle for later. Keep the compost evenly moist and provide warmth for germination. Once the seedlings are established, give them bright light and do not overpot them too rapidly. Young plants should be sturdy, compact and leafy when they head outdoors — not floppy, flowering in a pot or negotiating with the cat for more windowsill territory.

French beans and runner beans can also be started indoors from mid-April. Sow them individually in 9 cm pots. A four-to-six-week-old plant will be ready for the garden in late May, provided frost risk has passed. Pre-soaking bean seed for four-to-six hours can speed germination, and at temperatures above 18°C it may take around five-to-seven days. Do not leave seed submerged indefinitely; the point is to wake it, not give it a long spa break.

April is also the month when direct sowing begins properly for hardy vegetables. Carrots, beetroot, peas, parsnips, radishes, lettuce, spinach, turnips and Swiss chard can all be started outdoors when the soil is workable and conditions suit. "Workable" is gardening language for soil that is neither frozen nor so wet that digging it creates sticky lumps and regrets. If it clings to boots and tools like cake mixture, wait.

Prepare a fine seed bed for smaller seed. Remove stones and perennial weeds, break down large clods and rake to a level surface. Water the drill before sowing if conditions are dry, then cover seed lightly and mark the row. Labels may feel fussy in the moment, but after a fortnight of tiny green shoots they become the difference between "promising carrots" and "possibly weeds, best not touch them".

Herbs can join the plan too. Parsley, coriander, dill and chives can be sown in pots or prepared ground as conditions improve. Coriander is particularly worth sowing little and often, because it tends to run to flower quickly once weather warms. A few seeds every couple of weeks is more productive than one vast sowing that bolts just as you develop a craving for something involving lime and a large bowl.

For flowers, April is a generous month for hardy annuals direct in borders and cutting patches. Choose a prepared patch with good light, clear weeds and soil fine enough for seed contact. Scatter or sow in shallow drills according to the seed size, label the area and keep it watered if dry. A dedicated patch is much easier to manage than a handful of seed tossed optimistically between established perennials and immediately forgotten.

In colder regions, April remains a month of watching rather than assuming. You may sow the same crops, but later in the month, in modules, beneath protection or with fleece ready. The weather is allowed to change the timetable. That is not failure; it is the whole point of having one.

May: sowing at full throttle — direct sowing dominates

May is when the vegetable garden begins to look like it has a plan. Outdoor soil temperatures have typically reached 10–15°C in much of the UK, frost risk is receding and direct sowing becomes the dominant method. There are still regional differences — especially in northern England, Scotland and exposed gardens — but the list of crops that can be sown straight into their final positions grows dramatically.

This is the month for beans, sweetcorn, courgettes, squash, cucumbers in suitable sheltered places, salad crops, carrots, beetroot, chard, peas, spring onions, radishes, kale, lettuce, spinach and a steady relay of herbs. You do not need to sow every one. In fact, you probably should not. A productive garden is more often the result of repeated small sowings than one dramatic bank-holiday blitz.

Runner beans and French beans are especially satisfying May jobs. From mid-May, once the last frost risk has passed in most of England, they can be sown directly outdoors. At 15°C or above, bean seed can germinate in around 6–12 days. Direct-sown beans often catch up quickly with earlier indoor plants because they develop roots in the soil where they will grow, rather than adapting from a pot.

That does not make indoor-raised beans pointless. They are valuable where slugs are ferocious, soil is cold or you want a few plants ready for a quick start. But direct sowing is simpler, uses less compost and avoids the task of hardening off. If you do direct sow, protect young shoots where slug pressure is high and have replacements ready. Beans are generous plants, but they are not invincible.

Late May is the direct-sowing window for courgettes and squash, once soil is warm and frost is no longer a realistic threat. Sow one or two seeds per station, placing them on their edge to reduce the chance of rotting. Courgettes need stations spaced at around 90 cm; winter squash needs around 120 cm. It is tempting to tuck them closer together when they are only seeds, but those small plants will not stay small for long.

Placing the large seed on its edge is a small but useful technique. It helps moisture drain rather than settle on the flatter surface of the seed. The result is a better chance of clean germination, particularly in a damp spring. If both seeds emerge at one station, keep the strongest plant and remove the weaker one rather than trying to separate their roots. It feels a little ruthless. It is kinder than making both plants compete.

Sweetcorn can be sown in May too, but it needs a warm, sunny site and grows best in a block rather than a single row, so plants can pollinate one another. It is a crop for gardeners with enough room and a genuinely warm patch. In a cool, shaded garden it can spend summer looking decorative and slightly confused.

Keep sowing salad leaves, radishes, beetroot and carrots in small intervals. May warmth speeds growth, but it can also dry the soil surface quickly. Water drills before sowing when conditions are dry, then keep the seed zone moist until seedlings emerge. A hard crust on the surface of dry soil can stop fine seedlings from pushing through, particularly after a warm, windy spell.

May is also when hardening off becomes part of the daily routine. Plants raised indoors or in a warm greenhouse need a gradual introduction to outdoor life. Put them outside for increasing periods during mild days, bring them under cover if nights are cold, and avoid planting out just before a sharp weather change. A plant that has lived in steady conditions needs time to adjust to wind, sunlight and temperature swings. Think of it as acclimatisation, not punishment.

Regional timing: how to adapt the plan for your garden

A national sowing plan is useful only if it leaves room for local common sense. The broad pattern is reliable: southern gardens usually warm first, northern and Scottish gardens tend to follow later, and sheltered urban plots can be ahead of exposed rural sites. But there are plenty of exceptions. A frosty valley in southern England may be colder than a protected coastal garden much farther north.

For gardens in southern England, early-to-mid spring sowings can often be made at the earlier end of the suggested windows, with close attention to late frosts. The biggest risk here is not always cold; it can be overconfidence. A balmy April weekend can be followed by a thoroughly unhelpful cold snap. Keep fleece available and do not plant tender crops out merely because the supermarket has started selling bedding plants.

In the Midlands, Wales, northern England and higher ground, use the middle or later end of the window. Under-cover sowing becomes especially helpful in March and April, and tender plants may be better held until late May or beyond if nights remain chilly. There is no prize for putting out a cucumber early and then spending three weeks trying to persuade it that it has not made a terrible mistake.

In Scotland and colder northern gardens, sow slow tender crops indoors on time — tomatoes, peppers and aubergines still need their season — but delay planting out until conditions are reliably settled. Focus heavily on hardy direct-sown crops, protected salads, peas, brassicas and beans once the soil has warmed. A greenhouse or polytunnel can make a much larger difference here, extending the useful season at both ends.

A soil thermometer is one of the simplest ways to make better decisions. It tells you what matters beneath the surface, rather than what the air temperature feels like while you are standing in a sunny patch with a mug of tea. Check at roughly the depth where seed will sit, preferably at a similar time each day. If the soil has been cold and wet for days, wait. If it has warmed and is drying nicely, that is a much more persuasive sign than the date alone.

It is also worth keeping a small garden diary. Note your last frost, the first bean germination, when you planted tomatoes out and whether a crop struggled. After a couple of years, you will have a better local guide than any general calendar can provide. Gardening is full of variables, but your own records turn some of them into useful patterns.

The practical sowing routine that prevents spring chaos

A good sowing plan is not only about choosing the right month. It is about making each sowing easy to manage afterwards. Before the season starts, check your stock of compost, labels, trays, pots, fleece and seed trays. Clean anything that held old compost or diseased material. Make sure greenhouse vents work and that your cold frame can be propped open. The boring preparation is what gives you more time for the enjoyable bit later.

Label every sowing. Every one. Write the crop, variety and date. It takes seconds, and it stops the annual guessing game when three trays contain very similar small green leaves. A waterproof label and pencil are perfectly adequate. Fancy systems are welcome if they make you happy, but a legible date is doing the essential work.

Use fresh compost for sowing where possible, particularly for small seed. It should be fine-textured, free-draining and lightly firmed rather than packed down hard. Sow at a depth appropriate to seed size, then water carefully. For tiny seed, a fine rose or watering from below avoids washing everything into one unfortunate corner of the tray.

Once seedlings emerge, give them air and light. Thin crowded direct sowings early, whilst the seedlings are small, rather than waiting until they have all begun competing for the same patch of soil. Pot on indoor-raised plants before roots become densely congested, but do not keep moving them into ever-larger pots simply because you have them. The final destination and the weather outside should drive the schedule.

Finally, do not confuse a sowing plan with an obligation. If March is cold, wet and miserable, wait. If April is unusually kind, make the most of it sensibly. If life gets busy, sow a few dependable things in May and enjoy them. A small patch of beans, beetroot and salad leaves you actually tend will always beat a plot packed with ambitious crops that have become a source of low-level guilt.

Pros, cons and the honest verdict on this approach

The great strength of a three-route spring plan is that it replaces vague instructions with a decision you can actually make. Instead of reading "sow in March" and wondering whether that means the greenhouse border, a tray in the kitchen or the open plot, you decide where the seed belongs. It is a small shift, but it prevents many of the standard spring mistakes.

For most UK gardeners, the best result will come from combining a clear monthly plan with a little flexibility. Use March for the few crops that really require a head start. Use April to build momentum. Use May for broad direct sowing once the soil has warmed and the risk of frost has eased. It is a much less dramatic way to garden than sowing everything at once, but it is considerably more productive.

Spring sowing questions, answered