When to plant geraniums: When the frosts have passed, the flowers come fast

It’s the same question every year: when is the best time to plant geraniums on your balcony? Traditionally, geranium lovers mark mid-May, sometimes known in the UK as Blackthorn Winter, in red on their calendars. This is when an unexpected cold snap can occur, usually as blackthorn is blooming. After that, frost and cold in the UK aren’t usually a risk factor anymore, so your geraniums can go outside.

When we advise planting geraniums only after Blackthorn Winter, we’re not actually referring to country folklore; we’re talking about frost and cold. As soon as they’ve disappeared, nothing stands in the way of a beautiful balcony full of geraniums.

Why wait until after the last frost to plant geraniums?
By when should you have planted geraniums?
What is Blackthorn Winter?
How to get geraniums you bought too early through the cold safely
SOS: How to know when it’s too cold for your geraniums
Geraniums in autumn and winter
Bonus: find the right location for your geraniums

Geraniums originate in sunny South Africa. This gives them their natural characteristics: They like it warm and not too humid.

At temperatures below 12°C their growth slows down and any lower than 5°C will stop them altogether. A surprise frosty night in spring can have even more serious consequences for geraniums just out of the greenhouse and spoilt with warmth; it can kill them.

Even if the worst-case scenario doesn’t usually occur straight away: once their growth has been arrested, it takes time for geraniums to really get going again. This is why late frosts or low temperatures in spring can be real flower killers for these hardy geraniums.

There is nothing to be gained by planting too early.

Blackthorn Winter is a period when winter says goodbye to spring with one last frosty embrace before summer arrives. According to the old peasant rule, this happens in mid-May, just as the blackthorn is blooming, often just as it seems winter is over and spring has arrived.

According to this country folklore, gardeners should wait until the Blackthorn winter has passed before planting frost-sensitive plants such as geraniums.

Blackthorn Winter is a weather event associated with the beginning of summer. The sun rises higher and higher in the sky during the month of May. It gets warmer in Central Europe, but cold polar air still hangs in the far north. When high-pressure systems and low-pressure systems come together at the right time in the right place, the system pushes polar air southwards. A cold snap arrives. This is more likely at the beginning of May than at the end of the month.

Weather data from the last few decades show that in more than half of these years there was no ground frost from May 11-13 in Germany. However, daily lows below 12°C were widely recorded. In locations not known for being Germany’s warmest, the thermometer often fell below 5°C.

You have already learnt above about the growth pause your geraniums can undergo at these temperatures.

The temperatures in weather forecasts refer to the air temperature at an altitude of 2m above ground. The ground temperature can differ significantly from this.

So pay attention to the ground temperature in the weather report. It is measured at a height of just 5cm. You can find information on the ground temperature on weather portals or farming apps.

Warm air is lighter and rises upwards, which is why a hot air balloon flies, for example. The air around us isn’t always the same temperature either.

During the day, the sun warms solid, dry surfaces because they conduct heat better than air. At night, this effect is reversed and surfaces, such as balcony tiles, cool down especially quickly. You can feel this when it gets “freezing cold” outside. Cold air collects on the floor, forming a pool.

Also, cold air can hold fewer water molecules than warm air, so moisture condenses on the cooled surfaces. Dew forms. If the temperature drops below 0°C, this dew freezes to form frost.

We know it’s tempting to buy colourful geraniums in April.

• Take good care of them when you bring them home early.

• Get them used to the sun outside during the day,

• but bring them into the warmth at night when frost and cold threaten.

You can tell it’s too cold for your geraniums when they stop growing. You barely need to water them and take great care to prevent them getting waterlogged. If the plants have been exposed to frost, the leaves and flowers will droop.

If your geraniums are exposed to slightly sub-zero temperatures, this is not yet a disaster. They can recover as long as only the leaves are affected and the stems and roots remain healthy. However, it will still take a while for them to flower freely.

And after the season is over? Climate change is prolonging autumn and the first frost often doesn’t arrive until November. If you want to overwinter your geraniums, leave them outside for as long as possible. Fresh air and sunlight are better than moving them into their winter home prematurely.

If the temperatures remain permanently in the single digits, cut the shoots back to a length of 15cm and place the pots in a cool spot (5 to 10 degrees). This location must be bright, otherwise the geraniums will die. If you can’t provide these conditions, you’re better off looking forward to the growers’ new offerings next spring.

Repot in fresh compost in March, cut back any new shoots that have formed while in their winter spot, place the plants in a warmer place and acclimatise them to the light. You will need to be more patient than with new plants until your geraniums bloom magnificently again.

Overwintering is especially worthwhile for rare varieties or for collectors of scented geraniums.

Geraniums generally love the sun.

Upright geraniums with double flowers also benefit from a place where they are protected from heavy rain. Their magnificent ball-shaped flowers and water from above don’t mix particularly well!

Long trailing geraniums will give you extra pleasure if they can grow in a spot protected from strong winds. Where gusts constantly blow the stems, they don’t perform as well.

But no matter which summer beauty you choose:

Don’t plant them outdoors too early and protect them from frost and cold if necessary.

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