Ice Saints: when to plant tender plants outdoors after May 15

The Ice Saints: May's cruel temperature surprise

Mid-May arrives and you are eager to plant tomatoes (Solanum lycopersicum) and courgettes (Cucurbita pepo) outdoors at last. The sun shines, night temperatures feel mild. Then comes the Ice Saints: a sudden frost wave precisely in this period. The Ice Saints are no joyful festival but five days when frost returns unexpectedly. Mamertus (May 11), Pancras (May 12), Servatius (May 13), Boniface (May 14) and Sophia (May 15). On these dates night temperatures drop below zero Celsius, even when days are warm. This frost kills tender plants: tomatoes collapse overnight, courgettes blacken, dahlias freeze solid.

The good news: you can organize your full planting schedule around this period. Wait until after May 15 and your tomatoes thrive uninterrupted. But wait too long and you lose precious growing days in June and July. The art is recognizing the first frost-free night after May 15 in your specific region.

What exactly are the Ice Saints?

The Ice Saints are a centuries-old farming tradition built on a real weather pattern: in the second half of May, cold polar air can still push south before Atlantic summer weather settles in for good. Modern meteorologists note that the exact dates are statistically less reliable than they once were, but a late frost around this period remains a genuine risk. Farmers noticed the pattern centuries ago and attached it to five saints' feast days so they would remember it.

In the Netherlands we typically count May 11-15. In Belgium this can be May 11-15 or May 12-16. In France the Saints de Glace usually fall May 12-14. Germany observes similar dates. The precise timing varies by region. Southern Flanders may be later than the north. Elevation and proximity to the sea play a role. A garden on a hillside has more frost risk than one in a valley with nighttime heat radiation from nearby buildings.

Tomatoes: only outdoors after May 15

Tomatoes despise frost. Even one night below 3 degrees Celsius damages them. Leaves shrivel, stems blacken. A tomato seedling you carefully grew indoors in March dies in one evening.

Therefore: plant tomatoes only after May 15. Better still: wait until May 20, especially if your garden sits on higher ground or in southern Netherlands or northern Flanders. The later in May, the greater your certainty. One extra week of waiting costs you few growing days; frost damage to 50 seedlings costs plenty.

For those who want to start earlier: place your seedlings outdoors in full sun during the day (this hardens them off), bring them inside at night. This 'hardening off' takes two to three weeks. This way plants slowly build resilience to temperature swings.

Courgette and cucumber: warm houseplants until May 15

Courgettes and cucumbers fall in the same category as tomatoes: frost kills them. They thrive only above 15 degrees Celsius. At 10 degrees virtually nothing grows; they sit dormant. One frost night and they are gone.

Plant courgettes and cucumbers only after May 15, preferably mid-May or later. They grow quickly (4 to 6 weeks to first fruiting), so even late May planting brings abundant harvests in August and September. Courgette timing: plant June 1 and you have small fruits July 1.

For both: ensure warm soil. Courgettes in cold, waterlogged soil simply rot. Warm your vegetable bed with black plastic the weeks before planting. Or plant them on raised beds.

Dahlias: wait until mid-May or later

Dahlias are tubers, not seeds. You dig them out in October, store them dry and cold over winter, then replant them in May. Dahlias fear frost: one night at minus 2 degrees and the tubers you just planted rot away.

Wait until after May 15 with dahlias. Preferably until May 20. Dahlias need considerable growing time before flowering (about 6 to 8 weeks to first bloom), so the earlier you plant them the better. But frost risk does not offset the delay.

Dahlias grow rapidly in warm soil (above 18 degrees). Plant them in deep, nutrient-rich compost. Add support stakes from planting day onward; dahlias reach considerable heights (up to 150 cm for larger varieties) and bend under their own flowers.

Tender potted plants: oleander, citrus and agapanthus

All potted plants you wintered indoors can only return outside after May 15. This includes:

• Oleander
• Agapanthus
• Citrus (lemon, orange)
• Myrtle
• Bougainvillea
• Strelitzia
• Palms
• Abutilon
• Hibiscus

These plants originate from warm regions. They cannot survive one frost night at minus 3 or 4 degrees Celsius. Place them outside after May 15; they will reward you with a spectacular summer display.

Position potted plants first in part shade outdoors for one week. This helps them adjust to harder wind and dimmer light (they may have needed grow lights indoors). After this they can move to their final spot: full sun for citrus and agapanthus, part shade for palm species.

Hardening off: the bridge from indoors to outdoors

Hardening off is the key word. This is gradually exposing your seedlings to outdoor conditions. You do this over three weeks:

• Week 1: place seedlings outside in full sun or part shade during the day, bring them inside at night.
• Week 2: leave them outside during afternoon and early evening, bring them in only at sunset.
• Week 3: leave them outside all night if temperatures stay above 10 degrees, bring them in only on very cold nights.

Hardening off prevents stress. A seedling moved directly from warmth to outdoors experiences shock: wind, UV light, temperature drop, lower humidity. Hardening off gradually trains the plant to these conditions.

Reading weather forecasts: check your app each evening

You might think that after May 15 frost risk vanishes. This is not entirely true. Late frosts can occur in June, especially at higher elevations or in France. Therefore: monitor your weather forecast regularly.

In April or May with seedlings indoors and your forecast shows frost for the coming night (below 0 degrees), bring seedlings back inside. Already in June? Better not plant them than take the risk.

A good weather model shows minimum night temperature. KNMI.nl in Netherlands, IRM.be in Belgium and Météo-France (meteofrance.com) are reliable. In Germany check DWD.de.

Your region matters: south warmer, north colder

Your region determines the date of the last frost-free night. Amsterdam (maritime warmth) may be safe by May 12. Arnhem at higher elevation must wait until May 18. Ghent in Flanders perhaps to May 16. Freiburg in Germany until May 20 due to higher elevation.

This is not exact: it also depends on your specific garden location. A garden on a hillside is colder than one in a valley. A spot against your house's south wall is warmer through radiation. Have you never experienced real frost damage in May? Start a week earlier. Had frost damage three times this year? Wait until late May. Wondering whether your garden runs warmer or colder than the regional average? On gardenworld.app you can see how your garden's sun exposure and shelter affect your ideal planting date.

Gardeners who have worked the same plot for years develop a feel for these local differences.

Step by step

Step 1: Determine your frost-free date

Check the KNMI online archive for your region. Look for the date of the last frost night (below 0 degrees) in May. This gives you a baseline. Wait four to five days after for safety.

Step 2: Start hardening off mid-April

Begin three weeks before your planting date with hardening off. Place seedlings outdoors in full sun each day (start with part shade a week earlier), bring them in at night.

Step 3: Check soil temperature

Soil temperature must be at least 12-15 degrees for vegetables. Measure with a soil thermometer at 10 cm depth. Cold soil leads to disease and rot risk.

Step 4: Plant after your frost-free date

Plant tomatoes, courgettes and dahlias only after May 15 in your region. Move potted plants outside.

Step 5: Monitor weather

Until end of May: keep watching forecasts. Frost predicted? Bring unfinished seedlings back inside or cover them with cloth.

Frequently asked questions

Can I plant tomatoes outdoors in early May? Not permanently. You can place them outside during the day for hardening, but bring them in at night. For permanent placement: wait until after May 15. One frost and you lose all your early work.

I am in France (Provence): do these same dates apply? Saints de Glace in Provence usually fall May 12-14, sometimes earlier due to warmer location. Check your local weather history for the exact date. A good Météo-France archive tells you this.

Can I bring potted plants back inside if frost threatens? Yes, if your garden is small. Large potted plants (big agapanthus, mature citrus) are difficult to move. Place them where they are not frost's first target (against your house wall, not exposed on the terrace).

How do I know if my plant has frost damage? Damaged stems turn black or grey. Leaves shrivel and become translucent. Tomatoes that experienced one frost sometimes recover. Courgettes usually do not. Check plants a few days after a suspicious cold night.

See it in your own garden first

Wondering whether your garden has a warm microclimate? Or asking how to position tomatoes and courgettes most favorably? You can view your own garden plan and see how these heat-loving vegetables relate to your location, walls and existing features. Upload a photo of your garden at gardenworld.app, view a free sample design with these heat-loving crops. The sharp image with plant list costs 7.99 euros, one-time only, no subscription.