Some plants flower depending in the amount of daylight hours. Long-day plants will flower always when the day is longer than a threshold number of hours of daylight. Such vegetables will not produce well in Iceland (or other polar climates such as Sweden, Norway, Canada or Alaska) because they will bolt (early flower) even when they are very young seedlings. So, you cannot expect spinach to make large leaves, because the plant will flower almost after sprouting from seed. You cannot expect radish to make a radish, because the plant will flower very fast, and skip the part of creating the nice edible bulb. This will also happen with lettuce.
Lettuce, spinach, fennel, radish, turnips (and to some extent other brassicas) will bolt if grown during the polar summer, due to the excessive amount of hours of daylight. Therefore it is not possible to grow these plants, unless in shadow or later in the summer, when the daylight is reduced from 24 to 16 hours. By then, the first frosts are rapidly approaching, and so this makes growing them very complicate.
I am not worried with the turnips: I can still eat those greens like that, but for the spinach I would like to see some nice big leaves, not young flowering seedlings. The same goes for the radish: from a whole bunch of plants, only one gave a bulb. All six fennel plants bolted. I will not even try again, as fennel can only grow without cold, and the summer is already finishing here. But I will seed now (as it is 1st August) new seedlings of spinach, radish and turnip, to see if they don't bolt so easily during early autumn (August to October): there will be already some frosts, but still no snow.
Swiss chard also bolted, but it was growing since May, and so it already produced a nice crop of leaves. The swiss chard growing indoors, in shadow did not bolt yet. This demonstrate the value of growing plants indoors to accelerate growing, and protecting from the excessive bolting-inducing sunlight. With lettuce, the case was different, some plants bolted, others they strangely stalled in growth.
The positive side was for the indoor tomatoes: with so much daylight, I was surprised to discover how sweet the tomatoes taste! The extra sunlight increases even more the level of sugar in the fruits. It's unbelievable.