When to Plant Spring Bulbs: Soil Temperature, Not the Calendar

Contents

• Soil temperature decides, not the calendar
• When should you plant spring bulbs in zones 3 to 6?
• The planting window species by species
• Why are tulips planted later than daffodils?
• Zone chart with soil temperature triggers
• How late is too late to plant bulbs?
• Latest safe planting by species
• A salvage plan for bulbs you find too late
• Frozen, waterlogged and stony ground
• Bulbs in pots run on a different schedule
• Signs a bulb has started growing too early
• How to plant, step by step
• Common mistakes
• Frequently asked questions

When should you plant spring bulbs in zones 3 to 6?

Across USDA zones 3 to 6 the difference is about eight weeks. The zone number describes winter lows rather than autumn arrival, but the two track each other closely enough for a rule of thumb.

Zone 3: the Prairies, northern Minnesota, interior Scandinavia

August is bulb month here, not October. Soil is down to 13 °C (55 °F) by mid to late August and hard frost can arrive by late October, so the deadline lands in mid September. Snow compensates: under half a metre, soil temperature stays far steadier than in zone 6.

Zones 4 and 5: Wisconsin, Vermont, the Great Lakes, the Highlands

The window opens in late August for daffodils and mid to late September for tulips. Zone 4 closes in early November, zone 5 in early December, so there is room for two rounds three weeks apart and November planting usually works in zone 5.

Zone 6: the mid-Atlantic, lowland Scotland, coastal Denmark

The latest opening and the latest close. Soil often stays above 13 °C (55 °F) until late September, so October is the normal tulip month and December planting is often possible. The catch is chilling: in a mild winter the twelve to sixteen weeks below 9 °C is only just met, which is why hybrid tulips often flower well once and poorly after.

For which bulbs suit which site, start with the flower bulbs guide. The same trigger applies when you grow garlic, so both jobs fit one weekend.

The planting window species by species

Bulbs are not interchangeable, and this is the part most guides skip. What differs is the skin protecting the bulb, how fast it dries out and the temperature at which rooting begins.

Daffodils and narcissus: first of all

Narcissus bulbs have a thin, incomplete tunic and lose moisture fast in the bag. They also begin rooting at a higher soil temperature than tulips, around 13–15 °C (55–59 °F). Get them in as soon as you have them, ideally August or early September; a daffodil kept warm until November is often half dead. 'Carlton', 'Ice Follies' and 'Tête-à-Tête' are reliable in cold gardens, and there is more in the guide to growing daffodils.

Crown imperial and other fritillaria: early, and on a slant

Fritillaria imperialis 'Rubra' and snake's head fritillary (Fritillaria meleagris) have fleshy bulbs with no protective skin and dry out faster than anything else on the shelf. They belong in the ground in August, September at the latest. The crown imperial also has a hollow at the top that collects rain and rots, so set it at a slight angle or cover the hollow with grit.

Alliums: early to mid window

The easiest group to handle. Allium hollandicum 'Purple Sensation' and Allium christophii have firm, well-skinned bulbs that tolerate weeks in the bag. Plant late August in zone 3, September in zones 4 and 5, early October in zone 6. Give them the full three times bulb height or the stems come up unstable.

Crocus, snowdrop, scilla and muscari: mid window

Small bulbs are tough and forgiving, with one exception: snowdrops (Galanthus nivalis) dry out within a fortnight in warm air and flower poorly from a late planting. Crocus tommasinianus 'Ruby Giant', Crocus vernus 'Pickwick', Scilla siberica and grape hyacinth (Muscari armeniacum) can all go in through October in zones 5 and 6. Muscari sends leaves up in autumn as a matter of course, which is normal rather than a sign of bad timing.

Tulips and hyacinths: last

The tulip has a tight brown tunic, stores better than any other spring bulb and roots happily in cold soil, so it goes last, three to four weeks after the daffodils. Hyacinths follow. 'Apeldoorn', 'Purissima' and 'Queen of Night' are on the shelf every autumn, and among hyacinths 'Delft Blue' and 'City of Haarlem'. For aftercare see growing tulips.

Why are tulips planted later than daffodils?

Three reasons, all practical rather than traditional.

The first is drought tolerance. A tulip wears a tight papery tunic that slows evaporation, so it survives two months in a cool store. A narcissus has no such protection and loses weight every week it sits out.

The second is fungus. Fusarium and tulip fire thrive in warm, moist soil. A tulip sitting in 16 °C wet ground for three weeks before rooting runs a clearly higher risk of rotting than the same bulb in 8 °C soil. Daffodils are far less prone.

The third is root temperature. Tulips make roots down to about 4–5 °C (39–41 °F), so deep into autumn, while narcissus wants to start sooner.

In practice: daffodils, fritillaria and alliums in round one, tulips and hyacinths three to four weeks later. With only one weekend available, plant everything at the daffodil date and accept slightly more risk on the tulips. The reverse is worse, because by the tulip date the daffodils are already damaged.

How late is too late to plant bulbs?

Later than you think, and always better than not planting at all. The result drops off in steps, and it helps to know which step you are on.

October: fully normal in zones 4 to 6

In zones 4, 5 and 6 October is still ordinary tulip month. Bulbs root, chilling is long enough and flowering arrives on time.

November: works, with shorter stems

In zones 5 and 6, November is fine as long as a spade goes into the ground. Bulbs make less root before the freeze, so expect flowers one to two weeks late and stems a few centimetres shorter the first year. Tulips handle it better than daffodils, which have already dried in the bag. Several inches of straw or shredded leaves buys the roots extra time.

December: a gamble that usually pays

If the ground is workable, and in zone 6 it often is, plant. The bulb roots slowly in cold soil and chilling comes out just long enough. Flowering runs two to three weeks late and blooms are smaller. The alternative, bulbs left in the bag, is worse in every case.

January and February: do it anyway

A bag of tulips found behind the snow shovels in January is not garbage, but it is on the way. Firm, heavy bulbs with no mould and no soft patches are worth planting, outdoors if you can dig or in a pot if not. They often flower, just late and low. Daffodils and fritillaria do worse after four months of drying.

What genuinely does not work

Saving bulbs for next autumn. A tulip kept at room temperature for a year is effectively dead and the reserves left will not build a flower. Throw out the soft and mouldy, plant the rest now.

Latest safe planting by species

This is the chart no other bulb guide seems to publish: the last date at which each species normally still flowers acceptably in its first spring, given workable ground. Past it you are into the salvage plan.

The pattern is worth remembering: fleshy, unskinned bulbs have the earliest deadline, tight-tunicked ones the latest. Short of time, plant fritillaria and daffodils first and leave the tulips for last.

A salvage plan for bulbs you find too late

The sequence below works from November onward, the same in January as in March. Start by sorting.

1. Tip the bag out on newspaper and squeeze every bulb. Firm and heavy for its size means alive; soft, light, shrivelled or blue-green mouldy means compost.
2. Dry white fuzz on the outer skin rubs off. If the bulb is firm underneath, it is usable.
3. Test the ground with a spade in your most sheltered spot, ideally against a south-facing wall. If it goes down 20 cm (8 in), plant straight into the soil and ignore the date.
4. If it will not go in, plant in pots. Use a frost-proof pot at least 20 cm (8 in) across, ordinary potting compost and a drainage layer of grit.
5. Set the bulbs 1–2 cm (half an inch) apart with 3 cm (1 in) of compost over them. Crowding is fine in a pot.
6. Water once, lightly, so the compost is damp but not wet.
7. Stand the pot somewhere cold and dark, 0 to 9 °C (32–48 °F): an unheated garage, root cellar, cold frame or shaded balcony corner. A refrigerator takes a couple of pots, but not alongside apples, which give off ethylene.
8. Leave it twelve to sixteen weeks, checking monthly that the compost has not dried out.
9. When shoots reach 5 cm (2 in), move the pot somewhere bright and cool, around 10 °C (50 °F). Too warm and the stems come up floppy.
10. In open ground, mulch with 10 cm (4 in) of shredded leaves or straw as soon as the ground freezes, to even out soil temperature.

The result is rarely as good as an October planting, but it is flowers. Bulbs left in the bag give nothing.

Frozen, waterlogged and stony ground

The calendar is half the question. The ground is the other half, and it decides more often than the weather whether the job can be done.

Frozen ground

If only the top 3–5 cm (1–2 in) is frozen, common after a cold November night, break through with a mattock and plant underneath; below the crust the soil is often soft. If the frost runs deeper than 10 cm (4 in), take the pot route instead of forcing it.

A second option is to set the bulbs on the surface under 15–20 cm (6–8 in) of loose compost mixed with shredded leaves, then boughs on top. It fails on an exposed site, but elsewhere the bulbs root into the upper layer and flower.

Waterlogged ground

Standing water kills more bulbs than cold does. A tulip in saturated clay rots within weeks, in November as much as September. Squeeze a handful of soil: if water runs between your fingers, do not plant there.

Three fixes. Raise the bed 20 cm (8 in) with topsoil and coarse sand. Dig 5 cm (2 in) deeper and put 3 cm (1 in) of grit in the bottom as a sump. Or use pots, the safest call in heavy clay. For a new bed, the drainage part sits in the guide on how to plan a flower bed.

Stony and root-filled ground

Under trees and in old glacial till, roots set the limit rather than frost. A steel bulb planter beats a spade for fifty crocus in a lawn. Workable depth is often only 10 cm (4 in) there, so choose species that live with it: crocus, snowdrop, scilla and muscari. Tulips and alliums need their depth to stand straight.

Bulbs in pots run on a different schedule

A pot is not a small garden bed. It freezes right through, dries faster and warms sooner in spring, so timing and technique both change.

Plant later, not earlier

Pots can go in two to four weeks later than beds, because you control the temperature by moving them, which also makes pots the rescue route for anything found too late.

Pot, compost and depth

Choose frost-proof terracotta, glazed ceramic or plastic, at least 20 cm (8 in) across and as deep, with drainage holes. Thin unglazed terracotta cracks when compost freezes. Plant closer than in a bed, 1–2 cm apart, and shallower, with 3–5 cm (1–2 in) of compost over the bulb. Two or three layers in one pot works well: tulips at the bottom, daffodils in the middle, crocus on top.

Overwintering

The pot must stay cold without freezing solid: 0 to 9 °C (32–48 °F) for twelve to sixteen weeks. An unheated garage, a root cellar or a sheltered corner with the pot packed in leaves and burlap all work. On a balcony in zone 4 or colder, insulate it or the root ball turns to ice and the bulbs split. Check monthly; potted bulbs die of drought more often than cold.

Signs a bulb has started growing too early

Green growth in autumn worries most gardeners more than it should. Three cases, told apart.

Leaves above ground in October or November

This is what a bulb planted into soil that was too warm looks like, or a species that does it anyway. Muscari always sends leaves up in autumn. Tulips and daffodils do it only if soil was above 15 °C (59 °F) at planting, or if autumn runs mild.

What matters is where the flower bud sits: down inside the bulb, below ground, protected. Leaf tips can freeze brown on a cold night, but that is cosmetic, and the leaves feed the bulb, so leave them. Spread 10 cm (4 in) of shredded leaves over the surface once the ground freezes.

A white shoot in the bag

A bulb shooting in the package has been kept too warm and too damp. Do not snap the shoot off. Plant it shoot upward and slightly deeper than normal, as soon as you can. It usually flowers, later and lower.

Roots but no shoot

White roots in the bag are the best sign of all: the bulb has found moisture and is starting in the right order. Plant at once and handle the roots gently, because they snap and the bulb grows no second set.

How to plant, step by step

The sequence is the same for every species. What changes is the depth in step four.

1. Measure soil temperature at 10 cm (4 in) three mornings running. Below 13 °C (55 °F) starts the daffodils, below 10 °C (50 °F) the tulips.
2. Clear weeds and loosen to a spade's depth, working coarse sand into the top 20 cm (8 in) of heavy soil.
3. Sort the bulbs. Firm and heavy get planted, soft and mouldy thrown out.
4. Dig two to three times as deep as the bulb is tall, measured from its base: large bulbs 15–20 cm (6–8 in), small bulbs 7–10 cm (3–4 in).
5. Set the bulbs in a group, 2–3 bulb widths apart, pointed end up. Crown imperial goes in on a slant.
6. Stir a tablespoon of bone meal into the bottom of the hole, so the bulb does not sit on fertiliser.
7. Backfill and firm with your hand. Do not tread it down; that compacts the soil and ruins drainage.
8. Water once, thoroughly, if the soil is dry. Water triggers root growth.
9. Add a label. You will not remember where they are in April.
10. Mulch with 10 cm (4 in) of shredded leaves or straw only once the ground has frozen. Early mulch keeps the soil warm and invites voles.

Crocus and scilla are also among the best nectar sources in the first mild weeks of spring, and there are more options in the guide to bee friendly plants.

Common mistakes

• Planting by date instead of soil temperature. A warm September turns "plant in September" into bad advice in zones 5 and 6. Measure instead.
• Daffodils left until last. Narcissus and fritillaria dry out in the bag and belong in the ground first. Tulips tolerate waiting.
• Planting too shallow. Bulbs set at 5 cm freeze, dry out and become vole food. Two to three times bulb height is the minimum.
• Planting into waterlogged soil. Rot kills more bulbs than cold. Raise the bed, add grit under the bulb, or use pots.
• Saving the bulbs for next autumn. A bulb kept at room temperature for a year is dead. Plant in December or January rather than waiting.
• Mulching too early. Leaves and straw in October keep the soil warm, start the bulbs growing and attract rodents.
• Leaving pots uninsulated in zone 4 and colder. The root ball turns to ice and the bulbs split.

Frequently asked questions

When should you plant spring bulbs?
Once soil at 10 cm (4 in) has held below 13 °C (55 °F) for three mornings running, and below 10 °C (50 °F) for tulips. That is mid to late August in zone 3, early September in zone 4, mid September in zone 5 and October in zone 6. Daffodils first, tulips three weeks later.

Is it too late to plant bulbs in November?
Not in zones 5 and 6, as long as a spade goes into the ground. Bulbs make less root before the freeze, so flowering runs one to two weeks late and stems come up shorter the first year. Tulips cope best. Mulch after planting to buy rooting time. Saving the bulbs for next autumn is always worse.

Can you plant bulbs in December or January?
Yes, if the ground is workable. If only the surface is frozen, break the crust with a mattock. If the frost runs deeper, plant in a frost-proof pot and keep it dark at 0 to 9 °C (32–48 °F) for twelve to sixteen weeks. Flowering arrives two to three weeks late and shorter, but it arrives.

How deep do spring bulbs go?
Two to three times the bulb's own height, measured from its base to the soil surface. For tulips, daffodils, alliums and crown imperial that is 15–20 cm (6–8 in). For crocus, snowdrop, scilla and muscari it is 7–10 cm (3–4 in). Deeper is safer: shallow bulbs freeze, dry out and get eaten by voles.

Why are tulips planted later than daffodils?
The tulip has a tight tunic that slows evaporation, so it stores well, and it roots down to about 4 °C (39 °F). The narcissus has no such skin and dries out in the bag. Tulips sitting in warm moist soil also run a real risk of Fusarium and tulip fire, which makes early planting unwise.

My bulbs are sprouting in October. Have I lost the flowers?
Almost never. The flower bud stays down inside the bulb below ground, protected even in hard cold. Leaf tips can freeze brown, but that is cosmetic, so do not cut the leaves off. Spread 10 cm (4 in) of shredded leaves over the surface once the ground freezes, to even out the temperature. Muscari does this every autumn.

How long do bulbs need to root before the ground freezes?
Four to six weeks of unfrozen soil gives enough root for normal flowering. Planting can continue until a couple of weeks before the ground freezes, but the root system will be smaller and flowering later. Count back from the typical hard-freeze date in your zone and you have your own deadline.

How can I tell whether bulbs in the bag are still alive?
Squeeze them. A live bulb is firm and heavy for its size. Discard anything soft, shrivelled, unnaturally light or carrying blue-green mould under the skin. White fuzzy surface mould rubs off and is harmless if the bulb is firm underneath. White roots in the bag are a good sign: plant at once, gently.

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