Sorry for really late response.
The following information mainly covers the whole Commonwealth Period (prior to 1262/63) and not Viking Age proper, since it would be very difficult to date the environmental circumstances as well as the food precisely solely based on the literary text.
"They owns all that grows in his own land. A man may warrantably eat berries and dulse in another man's land, but if he takes them away without leave, the penalty is a fine of thee marks. If he takes angelica, then the penalty is a fine of three marks, but if it is worth three ells, the owner has the right to decide what charge to bring." (Grágás, Konungsbók, Kap. 196. The translation is taken from: [Dennis, Foote & Perkins trans. 2000: 114]).
Thus, Old Icelandic law collection from the Commonwealth Period, Grágás, mention these three, berries, dulse (palmeria palmata- a special kind of see weed) and angelica primarily as edible plants for people.
Recent pollen analysis of the ruin of Monastery of Þingeyrar in northern Iceland also confirms the presence of bilberries, crowberries and angelica between the late 9th to the 11th centuries, before the establishment of the monastery itself (1112/ 1133 CE) (Riddel et al 2018: 7), so I suppose that they had already available to the Norse settlers during the Viking Age.
While not in the Viking Age in a narrow sense, several medieval Icelandic texts also tell us the interesting event on the Icelanders and berries around 1200:
- "Berry wine was first made in Iceland" (Annales regii, a. 1202, in: [Storm utg. 1888: 122])
- "Bishop Jón (of Garðar, Greenland) came from Greenland and stay in Austfjord during winter.......Bishop Jón taught people how to brew wine from crowberry, based on what King Sverre [of Norway] had told him. Next summer it happened that this berry was almost everywhere in Iceland. There was a man called Eiríkr, near Skálholt, living in the farmstead called Snorrastaðir, made some berry wine in that summer and it turned out to be good." (Páls saga, Kap. IX, í: [ÍF XVI: 301]).
As for dulse, I posted with excerpts of some more relevant texts before in: Why didn't Europeans eat seaweed like Asians do?
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On the other hand, modern Icelanders are also famous (?) for their tradition of eating edible lichens (fjallagrös), but we have difficulty in tracking this practice into the 12th and early 13th century. While Byock assumes the settlers ate it based on later evidence (Byock 2001: 51), Gunnar Karlsson makes a notes that all the extant evidence comes from post-Commonwealth Period since the late 13th century (Gunnar Karlsson 2009: 196).
It is also said that some farmers especially in southern Iceland also cultivated barley in a small scale during medieval warm period, though harvest could quite be unpredictable (Byock 2001: 54).
References:
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