That is part of the difficulty that we have encountered, although section 1 says in brackets, after the phrase that you have quoted,
“(referred to in this Act as a ‘high hedge’)”,
so we were speaking specifically about high hedges.
We did not want to have a definition that referred to individual species, because that would have created loopholes that people could have exploited. For example, we discussed the possibility that if the definition referred specifically to leylandiis, a case in which leylandiis had another species planted in between them might be excluded from consideration, even though such a hedge could have the same effect as one made up entirely of leylandiis. As I recall, at stage 3 of the bill’s consideration, we accepted an amendment from Anne McTaggart that removed from the definition the reference to evergreen trees or shrubs so that the bill would cover deciduous trees or shrubs, because we recognised that they could also form a barrier to light. That was part of our consideration. We tried not to be overly prescriptive on the basis that we wanted to ensure that the widest number of cases could be considered under the legislation.
However, it might be the case that, as a consequence of that, local authorities have chosen to use the broader flexibility that the act provides in the opposite direction, to enable them to rule things out. I freely admit that that might have been an unintended consequence.