For me, the most famous of groves is one written about by Sophocles, in Oedipus at Colonus, amongst others, the grove of the Erinyes, which is entered by a spiritually polluted Oedipus, for a rest, and to relieve his suffering. It is here that his daughters tend to him and perform sacrifice to the Erinyes in his name:
"My daughter, if thou seest a resting place
On common ground or by some sacred grove,
Stay me and set me down."
Sacred groves, for which the ancient Hellenic term is 'alsos' (αλσος), plural 'aseelio' (ασυλιο) or 'alse', are religious sanctuaries. They are places of contact between the divine and human worlds. Theoretically, sacred groves differs from wild woods, flower and vegetable gardens, vineyards, and orchards, but their exact nature is hard to define, as little was written about them, and even more was lost, including many of the groves themselves. There is a description of the Roman version of the alsos, however, described by the grammarian Servius:
"The lucus [alsos] is a grouping of trees, possessed of a religious nature, the well grouping of trees, while silva [a natural forest] grows thick with trees and is not groomed."
This description is woefully inadequate; however, it does give a glimpse into their purpose. Sacred groves are often but not always associated with temples. There might be archeological bias in this, however, as temples attract archeologists, where patches of trees without (m)any ancient stone structures tend to get overlooked. A description of the grove of Asklēpiós near the asklepieion at Epidaurus can be found in Pausanias' 'Description of Greece'. It gives us another clue of the use and importance of sacred groves:
"The sacred grove of Asclepius is surrounded on all sides by boundary marks. No death or birth takes place within the enclosure the same custom prevails also in the island of Delos. All the offerings, whether the offerer be one of the Epidaurians themselves or a stranger, are entirely consumed within the bounds. At Titane too, I know, there is the same rule." (2.27.1)
It's important to note, for those not well versed in the ways of the ancient Hellenes, that 'no death of birth...' refers to a practice linked to miasma. Within Hellenic practice, miasma describes the lingering aura of uncleanliness in regards to a person or space through which contact is made with the Gods. It occurs whenever the space or person comes into contact with death, sickness, birth, sex, excessive negative emotions and bodily fluids. It also comes from a lack of contact with the Theoi. With deaths and births, it is not so much the actual acts of dying and giving birth that pollute, but the opening up of the way to the Underworld. Note that not all groves seemed to have this rule of miasma linked to death. Pausanias describes the sanctuary of Saronian Artemis at Aegina, for example, which became the burial place of the man who founded it:
"After Althepus, Saron became king. They said that this man built the sanctuary for Saronian Artemis by a sea which is marshy and shallow, so that for this reason it was called the Phoebaean lagoon. Now Saron was very fond of hunting. As he was chasing a doe, it so chanced that it dashed into the sea and he dashed in alter it. The doe swam further and further from the shore, and Saron kept close to his prey, until his ardor brought him to the open ocean. Here his strength failed, and he was drowned in the waves. The body was cast ashore at the grove of Artemis by the Phoebaean lagoon, and they buried it within the sacred enclosure, and after him they named the sea in these parts the Saronic instead of the Phoebaean lagoon." (2.30.5)
Groves were sacred places, places where ritual was held to the Theoi. The alsos was either part of the temenos surrounding the temple, or a functioning temenos in and of itself, as is hinted at by Pausanias when he discussed the offering that must be eaten within the bounds of the grove in its entirety; an attribute also of the temenos surrounding the temple. It seems that groves near temples had two ways of developing: either from a naturally wooded tract of land with sacred attributes up, where an altar was added to the grove, boundary markers were added, and eventually a temple arose, or as a designed landscape deliberately planted to accompany an existing temple or altar.
It is, of course, also possible that parts of the natural landscape were marked as sacred due to the influence of deity, and begun to attract a cult. The grove might have eventually received an altar, but a temple structure was most certainly not always feasable or required. Especially in more rural areas of ancient Hellas, groves would have existed on their own, without the need for a temple. On top of that, some altars might not have had groves or temples, and not all temples had groves.
Many groves have been discovered--again, mostly near temples--but to name a few the temple of Zeus at Nemea had cypresses; tree pits, neatly arranged in rows, were found at the sanctuary of Hephaestus above the agora in Athens; the gymnasia of Plato, Aristotle, and Epicurus had one, and part of that grove has since been replanted; the temple of Apollo Hylates at Kourion in Cyprus, where excavated channels and pits show clearly where trees were planted; and so on.
The main function of a grove was to give sacrifice to the Theoi, both blood sacrifice, as well as bloodless. Yet, sacrifice was not limited to groves, although if a grove was present at a temple, the sacrificial altar could have very likely stood inside of it. Another primary function of a grove--beyond the religious--was to serve as asyla, a safe haven. For a person to receive asylia, asylum, at the sanctuary, he or she had to openly declare what crime they had committed, and perform a special rite: the rite of hiketeia. In the context of this rite, the person in need of help sat down at the altar or at a statue of the Theos or Theia to which the temple, sanctuary or grove was sacred, while holding a certain symbol identifying him as a suppliant, either a freshly broken off twig or a strand of wool. From this moment on, he or she (male: hikétes; female: hikétis) became a supplicant, and entitled to protection from prosecution. To break asylia was a heinous crime, and was punished almost always by death. It did happen, on occasion. Pausanias, again, describes how Kleomenes--King of Sparta in the late sixth and early fifth centuries BC--violated asylia at the grove of Argos:
"Near the battlefield was a grove sacred to Argus, son of Niobe, and on being routed some five thousand of the Argives took refuge therein. Cleomenes was subject to fits of mad excitement, and on this occasion he ordered the Helots to set the grove on fire, and the flames spread all over the grove, which, as it burned, burned up the suppliants with it." (3.4.1)
Who could enter a sacred grove depended greatly on who the grove was dedicated to, and if it was a festival day or not. Gendered rules of admittance were common. Only women were allowed in the groves of Hera at Aigion and of Demeter at Megalopolis, for example, while only men were allowed to enter the grove of Ares at Geronthrai. Some groves could only be entered by initiates, while others only allowed clergy. Domesticated animals were hardly ever permitted on the grounds, even if the grove was the only shady spot available.
"Beyond what is called the Hall is a grove, sacred to the Mistress and surrounded by a wall of stones, and within it are trees, including an olive and an evergreen oak growing out of one root, and that not the result of a clever piece of gardening. Beyond the grove are altars of Horse Poseidon, as being the father of the Mistress, and of other gods as well. On the last of them is an inscription saying that it is common to all the gods.
Thence you will ascend by stairs to a sanctuary of Pan. Within the sanctuary has been made a portico, and a small image; and this Pan too, equally with the most powerful gods, can bring men's prayers to accomplishment and repay the wicked as they deserve. Beside this Pan a fire is kept burning which is never allowed to go out. It is said that in days of old this god also gave oracles, and that the nymph Erato became his prophetess, she who wedded Arcas, the son of Callisto." (8.37.10, 8.37.11)
"Meanwhile her four handmaids, who serve her round the house, were busy in the hall. One of those children of springs, groves and sacred rivers that run to the sea threw linen covers over the chairs and spread fine purple fabrics on top. Another drew silver tables up to the chairs, and laid out golden dishes, while a third mixed sweet honeyed wine in a silver bowl, and served it in golden cups. The fourth fetched water and lit a roaring fire beneath a huge cauldron. When the water boiled in the shining bronze, she sat me in a bath, and bathed me with water from the great cauldron mixed with cold to suit, pouring it over my head and shoulders till she drew the deep weariness from my limbs."
Sacred groves were special places of worship, guarded by nature spirit and Theoi alike. They offered protected, enclosed, places of worship, often with shade and a source of water--the value of which can not be underestimated while worshipping under the hot rays of Hēlios. Groves offered shelter for those of need, and sometimes even an alternative source of income for the temple associated with it, as some of the trees grew fruit. Much of that fruit would be sacrificed, or consumed by clergy and supplicants, but most likely not all.
In ancient Hellas as in modern day, groves provide places of worship that are affordable, relatively easy to set up and keep, and which are still very rewarding in a day-to-day practice. A grove does not even need to be huge: it can be of any size. Adding an altar means the opportunity to perform one's daily rituals in a space dedicated to one of more Gods, while still keeping the space inconspicuous. The possibilities are endless.