Growing Under the Simmer Dim: What Shetland and Highland Daylight Changes

Shetland's "simmer dim", the long pale twilight of midsummer, is a matter of latitude: Lerwick has nearly 19 hours of daylight on the longest day, and that changes the timing of boltingA plant switching early into flowering and seed production, usually triggered by stress, after which leaves, roots or bulbs turn bitter or unusable. Full page → and how much of the extra light turns into growth. The mechanism itself, photoperiodism, is covered in the day length and plant growth guide. What's specific to Shetland and the far north Highlands is how extreme it gets this far north, and what it means for a real garden.

How extreme, exactly

Shetland sits at roughly 60°N, and its midsummer daylight is a consequence of that position rather than anything unique to the islands. The same effect, less pronounced, applies further south in the Highlands.

All times are local (BST for June, GMT for December). The sun does still set in Lerwick at midsummer, around 10:34pm, and rises again around 3:38am, so it isn't continuous 24-hour sun. But the sun stays so close to the horizon that the sky never gets properly dark, and that lingering twilight is the "dim" in simmer dim.

Sunshine is a different measurement from daylight

Shetland's sunshine hours, the bright sun recorded rather than the time between sunrise and sunset, are unremarkable by UK standards, because the islands sit exposed to cool, cloudy North Atlantic weather even at midsummer:

May, not June, is Lerwick's sunniest month on this measure, even though June has the longer days: day length peaks at the solstice, but cloud-free sunshine doesn't.

Bolting risk arrives earlier here

The day length guide covers the day-length bolting trigger for long-day annuals like lettuce and spinach, and the separate cold-spell trigger for biennials like onions, leeks, carrots and beetroot. Both are recognised bolting triggers, along with heat and drought. The timing is what differs here. Days are already very long before the solstice arrives, and spring cold snaps are a normal feature of a cool Atlantic climate, so both triggers are likely to come earlier and more abruptly than a grower used to Central Belt conditions expects. The practical response is the same succession-sowing and slow-to-bolt-variety advice (the RHS suggests bolt-resistant cultivarsA plant variety that arose or was bred under cultivation and is kept true by deliberate propagation, rather than a naturally occurring wild species. Full definition → such as 'Boltardy' beetroot, and keeping soil moisture steady), just started earlier. For biennialA plant that completes its life cycle over two years: growth in year one, then flowering, seeding and dying in year two. Full definition → seedlings, keep young plants under cover, in a cold frame or under fleeceA lightweight, breathable fabric laid or draped over plants to trap a little warmth and keep off frost, wind and some flying pests. Full definition →, until the worst spring frost risk has passed rather than transplanting into an early cold snap.

Where the extra hours are a straightforward gain

Day-neutral plants aren't triggered into flowering by the length of day or night, so they get the benefit of the extra daylight without the flowering risk. Tomatoes are the RHS's example. For these, extra daylight hours should translate into more photosynthesis time, though temperature, soil conditions and wind still set the ceiling. Strawberries show the split within one crop: June-bearing varieties are short-day plants that set their flower buds in the short days of autumn and fruit the following spring, so they aren't chasing the summer daylight total at all. Day-neutral varieties form buds regardless of day length, and everbearingA strawberry type that produces smaller flushes of fruit continuously through the season, rather than one large crop over a few weeks. Full definition → varieties form buds in the long days of summer as well as in autumn, which is why they can crop later in the season.

Wind exposure and a shorter frost-free growing season are the other defining features of Shetland and far-north Highland gardens, and they probably cap how much of that extra light turns into growth: a tomato plant checked by wind or cold soil won't out-grow a Central Belt plant just because its days are an hour or more longer. The daylight advantage is one factor among several, not a guarantee. It helps most at the season's edges, where the long light either side of the solstice gives seedlings hardening offGradually getting young plants raised indoors used to outdoor conditions over 7–14 days before planting out, so cold, wind and direct sun don't shock or damage them. Full definition → outdoors more usable growing hours per day than they'd get further south.

Common questions

Sources