Flower Bulbs Guide: Planting Depth, Spacing and Honest Perennial Verdicts

Contents

• Bulb, corm, tuber, rhizome: what is the difference?
• Fall-planted and spring-planted bulbs: which go in when?
• How to buy good bulbs and what the size numbers mean
• How deep should you plant flower bulbs?
• The full planting chart, species by species
• Soil, drainage and site
• How to plant flower bulbs step by step
• Naturalising bulbs in grass
• Bulbs in pots and the bulb lasagne
• What do you do with bulbs after they flower?
• Which flower bulbs come back every year?
• Lifting and storing tender bulbs over winter
• Voles, deer, squirrels and rot
• Zone and timing chart
• Common mistakes
• Frequently asked questions

Bulb, corm, tuber, rhizome: what is the difference?

Everything in a bulb bag gets called a bulb, but these are four different organs botanically, and the difference decides how you plant and whether you can divide it.

True bulb

A compressed stem wrapped in fleshy scale leaves, like a kitchen onion. Point up, flat basal plate down. Tulip, daffodil, hyacinth, allium, snowdrop, grape hyacinth and lily are true bulbs. They multiply by offsets at the plate, which is why a daffodil becomes a clump while a tulip becomes fewer.

Corm

A solid, thickened stem base with no scales, often flat and disc-shaped. Crocus, gladiolus, freesia. The corm is used up over the season and replaced by a new one on top, plus cormlets around the edge, which is why crocus travels upward in the soil.

Tuber and tuberous root

A dahlia has tuberous roots: thickened roots that store food but carry no buds. All new growth comes from the crown, so split a clump without a piece of crown and you get no shoot. Tuberous begonias have flat tubers with buds on top, planted dished side up.

Rhizome

A thickened stem growing horizontally just under the surface, with buds along the sides. Canna, calla and most irises. Planted flat and shallow.

Fall-planted and spring-planted bulbs: which go in when?

Bulbs split into two groups by planting season, and mixing them up costs a year.

Fall-planted, spring-flowering

Tulip, daffodil, crocus, hyacinth, snowdrop, grape hyacinth, squill, allium and camassia go in during autumn and flower the following spring. They need ten to sixteen weeks below 9 °C (48 °F) to break dormancy; without the chill you get leaves and no flower.

The window opens when the soil cools to around 15 °C (60 °F): late August in zone 3, late September in zone 6. Bulbs need four to six weeks with soil above 5 °C (40 °F) to root before freeze-up, so early beats late. Timing by zone is covered in the guide on when to plant spring bulbs.

Spring-planted, summer-flowering

Dahlia, gladiolus, begonia, canna, calla and acidanthera go in after the last frost and flower from July. None survive a zone 3 to 6 winter in the ground, so you either lift the tubers or treat them as annuals. Lilies sit in between: true bulbs with no tunic, hardy to zone 3, planted in October or April.

Fall-flowering

Colchicum and autumn crocus go in during August and flower the same autumn. They need planting within a week or they bloom in the bag. Leaves follow next spring.

How to buy good bulbs and what the size numbers mean

Bulb quality is visible and tactile, and ten seconds per bag is the biggest difference between a good spring bed and a mediocre one.

Four things to feel for

The bulb should be firm all the way round; if it gives under your thumb it is rotting inside. The basal plate should be dry and solid. Splits in the tunic are cosmetic, grey-blue mould is not. And shoots should be short: a centimetre is fine, five means the energy went into store heat.

Bulb size is circumference, not diameter

The numbers on the bag, 12/14 or 11/12, are the bulb's circumference in centimetres. Bulbs are graded over plates of holes and drop through the one that matches, so 12/14 is roughly 4 cm (1.6 in) across. Every genus has its own scale: 8/9 is large for a crocus and tiny for a hyacinth.

What size means in practice

A bigger bulb carries more stored food and gives larger flowers in year one. A 14/16 daffodil often throws two stems, a 12/14 throws one, a 10/12 gives leaves only. Same cultivar, same care. The difference is the pantry.

Bargain bulk packs are rarely bargains. Fifty tulips at 9/10 for the price of thirty at 12/+ looks better until half the small ones only make foliage. The exception is bulbs planted to naturalise, where small grades build themselves up in two or three years.

Buy early

Bulbs reach stores in late August and are best then. If you cannot plant at once, keep the bags open at 5–10 °C (40–50 °F), never sealed in plastic and never in a fridge with fruit. Ethylene from apples kills the flower bud inside.

How deep should you plant flower bulbs?

The rule of thumb says three times the bulb's height. Easy to remember, roughly right for half of all bulbs, badly wrong for the rest.

How the rule works

Measure the bulb from basal plate to tip and multiply by three. Dig the hole that deep from the surface down, and set the bulb with its plate on the base. A tulip 5 cm (2 in) tall wants a 15 cm (6 in) hole, leaving about 10 cm (4 in) of soil over the tip.

What matters is measuring to the bottom of the hole, not to the top of the bulb. The difference is a third. A label saying "cover with 10 cm of soil" means something else entirely.

Where the rule holds

Tulip, hyacinth, daffodil, grape hyacinth, squill and mid-sized allium all land correctly at three times height, which covers true bulbs where height and width are roughly equal.

Where the rule gives the wrong answer

Crocus is flat. Three times the height of a 2 cm corm gives 6 cm, too shallow for frost heave and voles alike, so plant crocus at 8–10 cm (3–4 in). Lilies have no tunic and root above the bulb as well as below: set them at 18–20 cm (7–8 in), nearer four times height.

Crown imperial has a hollow bulb that rots if water collects inside, so lay it slightly on its side or on grit at 20 cm (8 in). Anemones are irregular lumps with no measurable height: 8 cm (3 in). Dahlias and begonias sit outside the rule, the dahlia crown 5 cm (2 in) down and the begonia tuber level with the surface.

Deeper in sand, shallower in clay

In sandy soil add 3–5 cm (1–2 in), because sand insulates poorly. In heavy clay take 3 cm (1 in) off, since a bulb at 18 cm in wet clay rots. In zone 3 and 4 add 3–5 cm and mulch the first winter.

The full planting chart, species by species

Depth is measured to the bottom of the hole, spacing in centimetres. The right-hand column is the honest verdict on how long each species flowers with no help from you.

Two things stand out. Close relatives behave very differently: woodland crocus increases for twenty years, Dutch crocus loses a third of its stand each year. And the most reliable species are usually the cheapest.

Soil, drainage and site

Bulbs ask for little, but the one thing they ask is absolute: they rot in standing water. Nearly every failure comes down to drainage.

Testing the drainage

Dig a hole 30 cm (12 in) deep and fill it with water. Draining within an hour means the soil suits bulbs. Three or four hours means plant shallower and add grit. Water still standing next morning rules out tulips and hyacinths. Camassia, snake's head fritillary and spring snowflake take damp ground.

Improving the soil and feeding

On clay, work 3–5 litres of coarse grit per square metre into the top 25 cm (10 in) and put 2 cm of grit in each hole. That stops water sitting against the basal plate, where rot starts. On sand, add 5 litres of compost per square metre instead.

Fresh manure must never touch a bulb; nitrogen against the basal plate causes bacterial rot within a fortnight. Most bulbs want pH 6.0–7.0, lilies nearer 5.5–6.5. No feeding is needed in year one. After that, work bone meal in at planting and feed when shoots are 5 cm (2 in) tall.

Site

Six hours of sun during flowering suits almost every spring bulb, which is easier than it sounds because deciduous trees are still bare in April. A bed in deep shade in July can be full sun in March. That is why bulbs under deciduous trees work and the same spot under a spruce does not. Planning a whole bed? Read about how to plan a flower bed first.

How to plant flower bulbs step by step

This order covers every fall-planted bulb. Depth and spacing come from the chart above.

1. Check the bulbs and throw out anything soft, mouldy or paper-light.
2. Test the drainage if the site is new: an hour for 30 cm of water.
3. Loosen the soil to 30 cm (12 in) and pick out stones and root weeds. A bulb that meets a stone grows crooked and stays small.
4. Add grit to clay or compost to sand, and work bone meal into the base.
5. Dig one shared trench rather than single holes for more than ten bulbs. Three times faster, and the depth is even.
6. Put 2 cm of grit in the base if the soil is heavy.
7. Set the bulbs point up, plate down. If you cannot tell, lay the bulb on its side and the shoot finds its own way.
8. Keep the chart spacing, but plant in irregular groups of seven to ten. Rows look planted, groups look grown.
9. Backfill and firm with your hand, not your foot.
10. Water once, about 10 litres per square metre, to close the soil and trigger rooting.
11. Label every group, or you dig them up yourself next spring.
12. Mulch with 5 cm (2 in) of leaves in zone 3 and 4, or anywhere snow cover is unreliable.

Naturalising bulbs in grass

Bulbs in turf return more per hour of work than any other planting. A crocus carpet improves yearly with no care at all, but only with the right species and mowing routine.

Species that work in grass

Woodland crocus, snowdrop, winter aconite, Siberian squill, glory-of-the-snow, snake's head fritillary, camassia, miniature daffodil and species tulip. All flower and die back early, so the lawn can be cut on a normal schedule. Dutch crocus, hyacinth and Darwin hybrids need the grass uncut into late June.

How to plant them

Scatter the bulbs by hand and plant them where they land. It sounds careless and that is the point, because natural colonies have uneven density. Cut an H-shaped slit in the turf, fold back the flap, set the bulb at depth and press the turf down. Allow 50 to 100 small bulbs per square metre for a full stand the first spring, or 20 to 30 if you let them multiply over five years.

Mowing is the whole difference

Wait six to eight weeks after flowering before you cut. The leaves charge the bulb over exactly that period, and a bulb cut too early stops flowering within three seasons. For crocus flowering in April that means a first cut in early June at a mower height of 7–8 cm (3 in). Never apply nitrogen to turf where bulbs grow, or the grass crowds them out.

Bulbs in pots and the bulb lasagne

In a pot you can plant far tighter than in the ground, because the bulbs need not build themselves up for next year. One bulb width apart is enough.

What the pot needs

At least 30 cm (12 in) deep for large bulbs, with drainage holes, and frost-proof. Terracotta and thin plastic split when wet soil freezes. Put 5 cm of clay pebbles in the base and use ordinary potting compost.

The bulb lasagne, layer by layer

Three levels in one pot. The latest to bloom goes deepest, the earliest nearest the surface, and shoots from the lower layers find their way past the bulbs above. In a pot 40 cm (16 in) deep:

1. Add 5 cm of drainage, then compost to 22 cm (9 in) below the rim.
2. Bottom layer: tulips or large daffodils, close but not touching. Cover with 5 cm.
3. Middle layer: hyacinth, allium or smaller daffodils, offset from the bulbs below. Cover with 5 cm.
4. Top layer: crocus, grape hyacinth, snowdrop or squill, almost shoulder to shoulder. Cover with 5 cm.
5. Water once and stand the pot against a house wall.

Flowering then runs from March to late May in one pot. Pick cultivars with overlapping seasons or you get blank weeks.

Overwintering pots

Compost in a pot freezes right through, unlike soil in a bed. Bulbs tolerate freezing but not repeated freeze-thaw, and not standing in ice. Put the pot against a north or east wall, pack leaves around the sides but not the top, and stand it on two bricks so the drainage holes stay clear. An unheated garage at 0–5 °C (32–40 °F) works too. More in the guide to growing tulips.

What do you do with bulbs after they flower?

This decides whether you get flowers next year, and it is the step most often done wrong. Between faded bloom and yellow leaf lies the bulb's entire working year.

Remove the flower, keep the leaves

Snap off the spent flower as soon as the petals drop, but leave the stem and every leaf. A bulb allowed to set seed puts much of the season's food into seed rather than storage. The exceptions are species you want to self-sow: winter aconite, squill and snake's head fritillary.

When you may cut the leaves

Once they have yellowed completely and come away with a gentle pull, six to eight weeks after flowering. That is late June for early crocus and mid July for late tulips. Tying or braiding daffodil leaves cuts the light-catching area and measurably shrinks the bulbs.

If yellowing leaves bother you, plant the bulbs among perennials that come up in May: daylily, hosta or catmint. The foliage is hidden while it still works.

Tulips worth lifting

If you grow tulips among perennials it is often easier to lift them once the leaves yellow, dry them airy for two weeks and store them cool until October. You avoid the yellow foliage in June and can replant deeper.

Which flower bulbs come back every year?

Here is the answer rarely given straight: most modern tulips do not come back, and that is not your fault. They were bred for one perfect flower.

Group 1: increase on their own

Snowdrop, winter aconite, woodland crocus, Siberian squill, glory-of-the-snow, grape hyacinth, spring snowflake, Spanish bluebell, camassia and most daffodils. They make offsets, self-seed or both, and the planting thickens every year. Grape hyacinth and Spanish bluebell increase so willingly they may need containing. See also the guide to growing daffodils.

Group 2: five to ten years

Species tulips, meaning Tulipa greigii, kaufmanniana, tarda and praestans, plus allium, snake's head fritillary, dog's tooth violet, lilies and colchicum. They do not multiply, but nor do they decline, if they sit dry through summer.

Group 3: three to five years, then fading

Darwin hybrid tulips, Dutch crocus, hyacinth, giant allium, crown imperial and reticulated iris. Hyacinths keep flowering but the spike loosens yearly, which reads as disease and is only ageing. Top up every third year.

Group 4: effectively annual

Triumph, double, parrot, fringed and lily-flowered tulips, plus anything sold as a forced pot plant. They carry energy for exactly one bloom, and the bulb left behind is too small to flower again. That is how the breeding works, not a failure.

Three factors that decide within each group

Drainage matters most: a Darwin hybrid in sharply drained soil can last seven years, the same bulb in clay is gone after two. Summer dryness is second, since these bulbs come from Mediterranean climates with a dry rest. Depth is third: plant a tulip at 20 cm rather than 15 and it splits less and lasts longer, the most underrated trick here.

Lifting and storing tender bulbs over winter

Dahlia, gladiolus, canna, begonia and calla turn to mush in zone 3 to 6 ground. Either buy new each spring or lift them. The temperature is not approximate: five degrees out gives rot one way, desiccation the other.

Handling dahlias

Wait until the first frost blackens the leaves, but no longer. Cut the stem to a hand's width, lift the clump with a fork clear of the stem and shake off loose soil. Turn it upside down for a few hours so water in the hollow stem drains out, or the tubers rot inside. Pack in dry peat so they do not touch, and check monthly. Divide in March. More in the guide to growing dahlias.

Finding a frost-free spot

A root cellar, an insulated shed or a fridge set to 5 °C (40 °F). A warm basement at 15 °C (60 °F) does not work: tubers shoot in January and are spent by April.

Voles, deer, squirrels and rot

Four problems account for almost everything that goes wrong, three of them animals.

Voles

Meadow voles eat tulip bulbs, crocus corms and lilies from below, so you see nothing until gaps show in spring. The protection that works is mechanical: a cage of 13 mm (half inch) hardware cloth in the hole, or a couple of centimetres of coarse grit around each bulb. Daffodil, allium, grape hyacinth, snowdrop and squill go untouched, because they carry bitter alkaloids.

Squirrels and chipmunks

They dig up newly planted bulbs the same week, mostly because the soil smells freshly turned, and crocus is the favourite. Lay hardware cloth or evergreen boughs over the surface for six weeks. Watering in and raking level also removes the cue.

Deer

Deer browse tulip buds the night before they would have opened, and they eat lilies whole. Daffodil, allium, crown imperial, spring snowflake, snowdrop, grape hyacinth, squill and camassia go untouched. If you have deer, build the spring bed on daffodils and alliums.

Rot and tulip fire

Soft grey rot inside the bulb comes from wet soil or manure against it. Tulip fire (Botrytis tulipae) gives brown spots on leaves and buds that spread and turn grey and fuzzy in damp weather, and it overwinters in the soil. Bin affected plants with the bulb, and keep tulips out of that ground for three years.

Zone and timing chart

The window shifts by roughly a week per zone. Bulbs need four to six weeks with soil above 5 °C (40 °F) before freeze-up, so colder zones plant earlier, not later.

In zone 3 and cold zone 4 the species list shrinks. Safe choices are snowdrop, Siberian squill, daffodil, grape hyacinth and species tulip, set 3–5 cm deeper than the chart and mulched with leaves. Crown imperial, reticulated iris and Grecian windflower are marginal there. For bulb flowers indoors instead, see the guide to amaryllis care.

Common mistakes

• Planting too shallow. The commonest error. A bulb at half depth splits into many small ones that do not flower.
• Bulbs in wet ground. Standing water is the usual cause of death. Test the drainage, or choose camassia.
• Manure touching the bulb. Fresh manure against the basal plate causes bacterial rot within two weeks.
• Cutting the leaves too soon. Bulbs recharge from foliage for six to eight weeks. Cut in May and you get no flowers two springs later.
• Braiding or tying daffodil leaves. It reduces leaf area and shrinks the bulbs. An old tip that does not hold up.
• Too few bulbs per group. Three tulips look like a mistake. Seven to ten in an irregular clump look like a planting.
• Bulbs bought in November. Leftovers are dried out or forced into growth by store heat.
• Dahlia tubers in a warm basement. At 15 °C they shoot in January. Store at 2–7 °C.

Frequently asked questions

How deep should you plant flower bulbs?
Two to three times the bulb's own height, measured to the bottom of the hole. That is 8 cm (3 in) for crocus, snowdrop and squill, 10–13 cm (4–5 in) for reticulated iris and species tulips, 15–18 cm (6–7 in) for tulip, hyacinth and daffodil, and 20 cm (8 in) for crown imperial. Add 3–5 cm in sand and in zone 3.

When is it too late to plant fall bulbs?
As long as you can dig, it is worth doing. Bulbs need four to six weeks with soil above 5 °C (40 °F) to root, so the window is late August to late September in zone 3 and late September into November in zone 6. Later plantings often flower anyway, just later and shorter.

What do the numbers 12/14 on a bulb bag mean?
The bulb's circumference in centimetres, not its diameter, so 12/14 is roughly 4 cm (1.6 in) across. Larger bulbs hold more stored food and give more and bigger flowers in year one. Aim for 11/12 in tulips, 12/14 in daffodils, 16/17 in hyacinths and 8/9 in crocus.

Which flower bulbs come back every year?
Snowdrop, winter aconite, woodland crocus, Siberian squill, grape hyacinth, camassia and most daffodils increase on their own. Species tulips, alliums and lilies hold five to ten years. Darwin hybrids, hyacinth and Dutch crocus manage three to five. Triumph and double tulips are effectively annual.

Can you plant flower bulbs in spring?
Fall bulbs no, summer bulbs yes. Tulip, daffodil, crocus and hyacinth need ten to sixteen weeks below 9 °C (48 °F) to form a flower, and without that chill you get leaves only. Dahlia, gladiolus, begonia, canna and lilies go in during spring. Found forgotten fall bulbs in March? Plant them anyway.

Do you have to lift tulip bulbs every year?
No, but it extends their life. Left in place, a Darwin hybrid manages three to five years in free-draining soil and a Triumph often one. Lift them once the leaves yellow, dry them somewhere airy for two weeks and store them cool until October. You also clear the yellow foliage from the bed.

How do you protect bulbs from voles and squirrels?
Mechanically. A cage of 13 mm hardware cloth in the hole, or a few centimetres of coarse grit around each bulb, stops voles. Against squirrels and chipmunks, wire mesh or evergreen boughs over the surface for six weeks is enough. Simplest is choosing what they avoid: daffodil, allium, grape hyacinth and snowdrop.

When can you cut back bulb foliage after flowering?
Once the leaves have yellowed completely and come away with a gentle pull, six to eight weeks after flowering. That is late June for crocus and mid July for late tulips. Snap off the spent flower straight away, but leave stem and leaves. Never tie or braid daffodil foliage.

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