March is the last call for gardeners who want a truly spectacular summer border. As the soil begins to warm and the days stretch longer, there is a narrow but very real window to get summer bulbs into the ground — and television gardener Alan Titchmarsh is making sure no one misses it. His advice is direct: plant now, or accept a summer without the bold, generous flowers these bulbs can deliver.
Titchmarsh, who has spent decades guiding British gardeners through the rhythm of the seasons, consistently champions dahlias, gladioli, and begonias as the summer bulbs worth prioritising in March. Each of these requires a relatively frost-free start, which is why late March — when overnight temperatures are climbing — marks the last sensible planting moment before the opportunity closes. What follows is everything you need to plant them confidently, from soil preparation to aftercare.
| Preparation time | 30 min |
| Planting time | 1–2 hours depending on plot size |
| Time to first flowers | 10–14 weeks after planting |
| Estimated bulb lifespan | Dahlias and gladioli: lift and replant annually; begonias: 3–5 years with care |
| Difficulty | Beginner |
| Recommended season | Late winter to early spring — plant in March for summer flowering |
Which bulbs does Alan Titchmarsh recommend planting in March?
When Titchmarsh talks about summer bulbs in March, he focuses on three main groups: dahlias (technically tubers), gladioli (corms), and tuberous begonias. Each one has a different personality in the border, but all share the same requirement: they need soil that has begun to wake up from winter, with a temperature ideally above 10 °C at a depth of 10 cm.
Dahlias are the showstoppers. From dinner-plate varieties with blooms reaching 25–30 cm across to the more elegant ball dahlias and the spiky cactus types, they flower prolifically from July through to the first frosts of October. Gladioli produce their characteristic tall spikes — often reaching 90–120 cm — and are planted in succession through March and April to extend the display across the whole summer. Begonias are the shadier alternative, thriving in containers and beds that receive only dappled light, producing waxy, jewel-coloured flowers from June onwards.
Materials and supplies
- Dahlia tubers, gladioli corms, or tuberous begonia tubers — sourced from a reputable nursery or garden centre
- Horticultural grit or coarse sand (for heavy clay soils)
- Garden compost or well-rotted farmyard manure
- Balanced granular fertiliser (5-5-10 or similar, potassium-rich to encourage flowering)
- Slug pellets or organic wool pellets (slug deterrent)
- Wooden or bamboo stakes (at least 120 cm for dahlias)
- Soft garden twine or plant clips
- Horticultural fleece (for late frost protection)
Tools
- Garden fork (for breaking up and aerating the soil)
- Hand trowel or bulb planter
- Soil thermometer (optional but helpful)
- Watering can with a fine rose head
- Measuring tape or a planting stick marked at 10 cm intervals
- Kneeling pad
Steps
1. Choose healthy tubers and corms
Before anything goes into the ground, take time to examine every bulb carefully. A healthy dahlia tuber is firm to the touch, plump, and shows no signs of shrivelling or rot — press gently with a fingertip and you should meet resistance. Gladioli corms should feel dense and solid, not spongy, with a papery outer skin that clings tightly. Begonia tubers are slightly softer by nature, but there should be no visible mould or black patches on the surface. Reject anything that has turned soft or developed a musty smell; one diseased tuber can spread rot to its neighbours underground. If you are buying in late March, check the packaging date and ask staff about arrival stock — fresher stock performs significantly better.
2. Prepare the planting site
Summer bulbs dislike sitting in waterlogged soil, particularly in the first weeks after planting when temperatures can still dip. Use a garden fork to loosen the bed to a depth of at least 30 cm, breaking up any compacted layers that could trap water around the tubers. On heavy clay, work in a generous layer of horticultural grit — roughly a bucketful per square metre — alongside two or three shovelfuls of garden compost. On sandy, free-draining soil, compost alone will improve moisture retention enough to sustain a long flowering season. The finished bed should crumble loosely when you squeeze a handful; if it packs into a solid lump, it needs more grit.
3. Dig the planting holes at the correct depth and spacing
Depth and spacing are where many gardeners go wrong in their enthusiasm to fill the border quickly. Dahlia tubers should be planted with the crown — the point where the tuber meets the old stem — sitting approximately 10–15 cm below the surface. Space individual tubers at least 45 cm apart for compact varieties and up to 75 cm for the large-flowered dinner-plate types; they are vigorous plants that need air circulation. Gladioli corms go in at roughly 10–12 cm deep and 10–15 cm apart, though planting them 20 cm apart makes staking and lifting far easier in autumn. Begonia tubers sit almost at the surface — no more than 2–3 cm of soil above them — with the hollow, concave side facing upwards. This orientation is easy to overlook but makes a real difference to shoot emergence.
4. Add fertiliser and back-fill
Sprinkle a small amount of granular, potassium-rich fertiliser at the base of each planting hole before setting the tuber in place — this feeds the root system as it develops without the risk of direct contact burning the tuber. Place the tuber carefully, avoiding snapping any necks or breaking off the small growth buds that are already beginning to swell on dahlias by late March. Back-fill with the improved soil, firming it gently with the flat of your hand rather than stamping, which would compact the structure you have just worked to create. Water in thoroughly but without flooding; the soil should be evenly moist to a depth of 10 cm.
5. Stake dahlias at planting time
The single mistake most beginners make with dahlias is waiting until the plant is already tall before inserting a stake — at which point, driving a cane through the soil inevitably damages the tuber beneath. Insert a sturdy bamboo stake or wooden post of at least 120 cm immediately after planting, positioning it 5–8 cm away from the crown. Mark it with a label so the planting location is visible before shoots emerge. Gladioli may also need staking later, particularly taller varieties or those in exposed positions, but this can be done once the spike reaches 30 cm.
6. Protect against late frosts and slugs
Late March in the UK sits firmly in the season of unpredictable frosts, and a single cold night below -2 °C can set back or kill emerging dahlia shoots. Keep a roll of horticultural fleece close at hand and lay it directly over planted areas whenever frost is forecast. Slugs are a significant threat the moment soil temperatures rise; they will find newly emerging dahlia shoots with remarkable speed. Apply wool pellets or iron phosphate-based slug deterrent around the planting area immediately after watering in, and reapply after rain.
The professional's tip
Experienced gardeners often start dahlia tubers into growth indoors in late February or very early March, potting them into 3-litre containers in a cool but frost-free greenhouse or even a light windowsill. By late March, these tubers have already produced shoots 10–15 cm tall and can be transplanted outdoors during a mild spell, giving a two to three week head start on direct planting. This technique is particularly effective in northern regions of the UK, where spring soil warms more slowly. Watch the overnight forecast carefully — harden off the young plants by leaving them outside during mild days for a week before their final planting.
Aftercare and long-term maintenance
Once shoots are visible above the soil, begin tying dahlias loosely to their stakes every 15–20 cm as they grow, using soft twine in a figure-of-eight loop that cushions the stem. Pinching out — removing the central growing tip when the plant reaches approximately 40 cm — is a technique Titchmarsh recommends consistently: it forces the plant to branch and produce three to five flowering stems rather than one, multiplying the eventual flower count significantly. Deadhead spent blooms regularly throughout the summer to maintain continuous flowering; gladioli spikes should be cut once the lower three-quarters of florets have opened.
In autumn, once the first frost has blackened dahlia foliage, lift the tubers carefully with a fork, shake off excess soil, and store them in a cool, dry, frost-free place over winter in boxes of dry compost or horticultural sand. Gladioli corms should also be lifted, dried, and stored. Inspect stored tubers each month for rot, discarding any that have softened. A single healthy tuber planted in March can multiply into a clump of four or five viable tubers by October, steadily building your stock.
Varieties worth seeking out
Among dahlias, 'Café au Lait' — its warm blush-cream tones the colour of milky coffee — has dominated garden trends for several years and flowers with extraordinary generosity. 'Bishop of Llandaff' remains a classic, its deep crimson blooms set against near-black foliage creating one of the most dramatic contrasts in the summer border. For gladioli, the 'Atom' variety is more compact and wind-resistant than standard types, while 'Priscilla' produces white flowers edged with soft pink. Among tuberous begonias, the 'Non-Stop' series lives up to its name and is widely available from UK garden centres in March.
Budget-conscious gardeners will find the best value in buying tubers loose from garden centres rather than pre-packaged — you can inspect each one before purchase and select the largest, healthiest specimens. Larger tubers reliably produce larger, more floriferous plants in their first season.
Is it too late to plant summer bulbs after March?
Gladioli can be planted in succession through April and even into early May without significant loss of performance — in fact, staggering planting dates by two weeks across March and April extends the flowering display well into September. Dahlias planted in April will flower, but expect a slightly later start, typically pushing flowers back to August rather than July. Begonias are the most forgiving and can be started into growth indoors as late as April. The key deadline is avoiding a first frost with young, unestablished growth in the ground — in most parts of the UK, this becomes a serious risk after October.
Do summer bulbs need a lot of watering?
Immediately after planting, water moderately to settle the soil and initiate root growth, but avoid keeping the ground saturated — tubers will rot in consistently wet conditions before shoots appear. Once plants are actively growing and in full leaf, dahlias in particular are thirsty and may need watering two to three times a week during dry summer spells. A 5–8 cm mulch of garden compost laid around the base of established plants in June significantly reduces moisture loss and suppresses weeds at the same time. Gladioli and begonias require less water than dahlias but should never be allowed to dry out completely once in active growth.
Can summer bulbs be grown in containers?
Tuberous begonias are arguably better suited to large containers than to open ground, and a single large begonia tuber in a 30–35 cm pot filled with a peat-free multipurpose compost will flower continuously from June through to October. Dahlias can be grown in containers — choose a pot of at least 45–50 cm in diameter for medium-sized varieties — though they require frequent feeding with a liquid tomato fertiliser every ten days once in bud. Gladioli perform less well in containers than in open ground because their tall, wind-exposed flower spikes become unstable, but dwarf varieties such as 'Nanus' types are an exception.
Why are my dahlia tubers not producing shoots?
The most common reason is cold soil. Dahlia tubers remain dormant until soil temperature at root depth reaches a consistent 10 °C, and in a cold March the process can take three to four weeks from planting before any movement is visible above ground. Patience is usually the right response. If after five weeks there is still no sign of growth, carefully unearth the tuber to check: a healthy dormant tuber shows no growth but remains firm; a rotted one will have turned soft and may smell unpleasant. Planting too deep — more than 15 cm — can also delay emergence significantly.
Should summer bulbs be fed, and with what?
At planting, a slow-release, potassium-rich granular fertiliser incorporated into the base of the hole gives a sustained start. Once plants are in active growth and approaching the bud stage — typically June — switch to a liquid high-potassium feed such as a tomato feed (typically around 4-4-8 NPK) applied every ten to fourteen days. Avoid high-nitrogen feeds once flower buds have set; excessive nitrogen at this stage pushes leafy growth at the expense of flowers. Gladioli generally require less supplementary feeding than dahlias if the soil was well prepared with compost before planting.