An environment group will have to wait until the New Year to hear if luxury housing at two reservoirs in Cardiff can be stopped by rare mushrooms there.
A number of different types of wax caps have been found
A legal challenge has been mounted against the claims that Lisvane and Llanishen reservoirs are Sites of Special Scientific Interest (SSSIs).
Western Power Distribution claims the Countryside Council for Wales' decision is "premature, arbirtrary and unfair".
A High Court judge reserved judgement and his ruling is expected next year.
Lawyers for the Countryside Council for Wales (CCW) urged judge Mr Justice Andrew Collins not to quash its decision to protect what is claims is an "internationally important" fungi population on the banks of the reservoirs.
It follows a long campaign against the proposed development, which was first put forward in 2002.
Landowner Western Power Distribution wants to reduce the 60-acre Llanishen reservoir in size, and build luxury homes on its banks.
Western Power has asked the court to quash the SSSI decision and order the CCW to have the matter reconsidered
CCW counsel John Howell QC argued that Western Power's challenges were "misconceived".
Mr Howell claimed the reservoirs were designated an SSI for their "diverse assemblage of grassland fungi including over 25 species of waxcap Hygrocybe", the decline of which has been reported in the grasslands of northern and central europe.
Llanishen reservoir is used for sailing
Mr Howell said; "Outside the UK, the best waxcap grasslands are found in Scandinavia, though even there, with a longer history of intensive recording, Denmark and Norway have only one site each with 28 or more Hygrocybe - the number which this site has."
He said the Llanishen site was recorded as having the highest number of waxcap fungi species in the area, the joint fifth highest in Wales and the 10th highest in Great Britain.
He said: "The opinion that the site is special by reason of the diverse assemblage of grasslang fungi, it is know to support, is one CCW was entitled to hold having regard to the specialist advice available to it."
Mr Howell said that if any further information came to light that indicated that the site was not of special interest, then it may be denotified.
He added that the notification was confirmed by the CCW because it was considered to be of specail interest, not because of any threat from development.
He argued that any claim on predisposition on the part of CCW was "misconceived legally and factually".