When to Plant Tulip Bulbs in Ontario: October, Not Frost

Plant them in October. That's the answer for most of the Greater Toronto Area, and the reason is night temperature rather than the calendar. Landscape Ontario, the province's horticultural trades association, says the best moment is "when nightly temperatures hover around five to 10 C". We ran that band against the monthly overnight low at all seven Environment and Climate Change Canada stations inside the GTA. In autumn, October is the month inside the band at five of them and September at the Georgina-area station. At Markham's Buttonville the band falls between the two months. The first frost isn't your cut-off, whatever the packet says. The real deadline is soil you can still dig, and here that usually arrives long after Halloween.

The rule is a temperature, and three GTA sources give it a number

A tulip bulb needs to grow roots before winter, and it needs to do that without being fooled into sending up a shoot. Warm soil triggers the shoot. Cool soil triggers the roots. That's the whole reason fall bulbs have a window at all.

Three sources serving GTA gardeners put a number on "cool". Landscape Ontario names nights of five to 10 C. Sheridan Nurseries, which runs eight garden centres, says the window opens "when soil temperatures fall below 15 degrees (Celsius)". Lakeside Garden Gallery in Brampton is stricter and says to wait until soil is "consistently below 10 degrees Celsius at planting depth". Outside the GTA, the National Capital Commission in Ottawa, which counts close to one million spring blooms in its gardens each spring, splits the difference at "below 10 to 15 C".

None of those is a date, and none of the seven GTA weather stations publishes a soil temperature. So we used the closest published measure: the average overnight low, month by month.

Five of the seven GTA stations put the window in October

Environment and Climate Change Canada publishes 1991-2020 normals for seven stations inside the GTA, and every one carries the monthly Daily Minimum Temperature row. We read all seven, counting a month as inside the band when its normal overnight low falls between 5.0 C and 10.0 C.

Read the September column, then November. Six stations sit above Landscape Ontario's 10 C ceiling in September, and all seven sit below its five-degree floor in November. That leaves October as the autumn month inside the band at five of the seven.

Two stations sit outside that pattern. Udora, nearest Georgina, is already down to 9.7 C in September, so September is its month. Buttonville's September average of 10.4 C is 0.4 above the ceiling, and its October average of 4.3 C is 0.7 below the floor. No month sits inside the band there, so it is crossed during those weeks. In practice that pushes a Markham gardener to the front of October rather than the back.

The test doesn't pick October out of the whole year on its own. May sits inside the same band at all seven stations, so we read the autumn months only: bulbs go in when soil is cooling, not warming.

One caution on the frost figures. Each station's average covers a different run of years, printed beside it, so those four are not a like-for-like set. The letters ECCC prints with them are record length inside the window: C means at least 20 years of data and D at least 15. These are not 30-year averages. The overnight-low column is the sounder comparison, because six of the seven stations run the full 1991 to 2020 there and Burlington Piers runs 1994 to 2020.

The first frost is not the deadline

Those four averages span 24 days on the page, from Nov 06 downtown to Oct 13 at Buttonville in Markham. Treat that spread as rough rather than exact, because Nov 06 is a 1991 to 2011 figure and Oct 13 a 1991 to 2020 one. It is still the wrong deadline to work to.

Five of the nine GTA sources we read end the window at the ground instead of at the air. Three of them say the ground freezes. The Toronto Master Gardeners say tulips go in "from mid-September until the ground freezes". TERRA Greenhouses, which has GTA stores in Vaughan, Milton and Burlington, says bulbs "can be planted at any time until the ground is frozen". Hank Deenen Landscaping in Toronto says to plant "before the ground freezes". The other two use a spade test: Vandermeer Nursery in Ajax says you can plant "as long as you can dig", and Buying Thyme Designs in Durham Region says the same.

Two different events, and only one is a deadline. Frost is an air temperature at head height and it kills soft leaves. The ground freezing is a soil condition, it arrives weeks later, and it's what stops your spade.

Your hardiness zone will not tell you the date

"When to plant tulip bulbs in Ontario zone 5" is a common way people search this. It's the wrong tool for the job.

We looked up all 25 GTA municipalities on Natural Resources Canada's Plant Hardiness Zone by Municipality lookup, one at a time, on the 1991-2020 map.

Four zones, 5b to 7a, inside one region. Four municipalities sit in 7a, nine in 6b, nine in 6a and three in 5b.

Now look at what a zone measures. Natural Resources Canada builds it from a formula with seven climate variables. Three of them are about cold: the coldest month's mean daily minimum, the mean frost-free period above 0 C, and a winter factor. The other four are the warmest month's mean daily maximum, June to November rainfall, mean maximum snow depth, and the biggest wind gust in 30 years. The result is scored from 0 to 100 and cut into zones, so 50 to 54 is zone 5a and so on.

That's a survival score. It answers "will this plant live through my winter", a fair question for a shrub. It won't tell you which weekend to dig. Every source in our set plants tulips outdoors, in 7a Toronto and in 6b Brampton alike, so the zone isn't the constraint here. Timing is.

Nine GTA sources, and seven weeks between them

Here is why gardeners get confused. We read the fall bulb page of nine GTA garden centres, gardens and garden businesses, and they don't agree on the start date.

Six of the nine name a calendar start. Those six run from mid-September at the Toronto Master Gardeners to the first week of November at Buying Thyme Designs, about seven weeks apart. The other three don't give a date at all: Landscape Ontario and Sheridan publish a condition, and Hank Deenen Landscaping publishes a season.

They aren't really contradicting each other. They're weighing two risks. Plant early and the soil may still push a shoot that winter then kills. Plant late and you may run out of diggable ground. TERRA names a third, and it's the one GTA gardeners feel most: squirrels. It says "foraging squirrels are more likely to find your planted tulips in September and early October". It adds that "planting later in October (or even November) can help to deter critters".

If you want one date rather than a range, take the middle of October. Only one row in that table rules it out: the Toronto Botanical Garden, which starts once Halloween is over.

How deep, and why depth is really about squirrels

"How deep to plant tulip bulbs" draws more searches in Canada than the timing question does. Five of the nine GTA sources give a depth you can act on, and they do it two different ways.

Three give a number in centimetres. The Toronto Master Gardeners and Lakeside Garden Gallery both say 15 to 20 cm, and Lakeside adds the useful detail that you measure from the base of the bulb. The Toronto Botanical Garden says a flat 20 cm for larger bulbs including tulips.

The other two give a multiplier, and the two aren't the same rule. Sheridan says two to three times the bulb's diameter. Hank Deenen Landscaping says three times its height. A tulip bulb is taller than it is wide, so those dig different holes for the same bulb. The shallow end of the Sheridan rule is the shallowest number on the list.

That matters because of what lives in a GTA garden. Lakeside says "squirrels typically dig 8 to 10 centimetres down". The Toronto Master Gardeners say voles "generally do not tunnel as deep as 20 cm (8 inches)", and follow it with two words: "Plant deep." Put those together and the fixed 15 to 20 cm isn't fussiness: it's the depth that puts the bulb under the animals.

What tulip bulbs cost in the GTA

Sheridan Nurseries lists 36 tulip varieties in its online fall bulb shop. Thirty-four are $9.99 and two are $14.99. Twenty-two print a pack size, and 21 of those hold six bulbs.

The listing prints no tax line, so treat those as the prices shown. Fourteen of the 36 don't print a pack count, so check the label in the store.

You can turn that into a rough cost per square foot, with one caveat. Sheridan's density is a general bulb-grouping guideline, not a tulip specification, and it's worded as a minimum: plant "in big, irregular groupings of at least five to seven per square foot". At the common price, $9.99 divided by 6 bulbs = $1.665 a bulb. Then 5 x $1.665 = $8.325, about $8.33 and 7 x $1.665 = $11.655, about $11.66. So about $8.33 to $11.66 of bulbs per square foot at that shop, and because the guideline says "at least", read that as a floor rather than a range.

If it is already November

You haven't missed it. Buying Thyme Designs, a cut flower grower in Durham Region, plants in the first week of November on purpose, when the mild days are done and the squirrels are less busy. Its cut-off is a spade test, not a date: "If you can still dig the soil, you can still plant your spring bulbs." A cold snap doesn't end the season either, because "even if we've had freezing temperatures, only the surface of the soil will be frozen". It puts the point of no return anywhere from the end of November to January, and says mid-December is usually the last chance.

Vandermeer puts the ground freeze at "normally late October or November". So November planting is normal here, not a rescue.

Whenever you plant, water the bed afterwards. Four of the nine GTA sources tell you to, and it settles soil around the bulb and starts the roots.