--Sample Data
DECLARE @Foo TABLE (Plant CHAR(1), Plant2 CHAR(1))

INSERT @Foo
VALUES
('a', 'b'),
('b', 'a'),
('a', 'd'),
('s', 'y'),
('a', 'b')

-- Using MIN/MAX with VALUES clause
SELECT
	P1 AS Plant,
	P2 AS Plant2,
	COUNT(*) AS [Sum]
FROM
	(
		SELECT
			(SELECT MIN(P) FROM (VALUES (Plant), (Plant2)) AS Foo(P)) AS P1,
			(SELECT MAX(P) FROM (VALUES (Plant), (Plant2)) AS Foo(P)) AS P2
		FROM
			@Foo AS F
	) AS T
GROUP BY
	P1, P2

--Using Case expression
SELECT 
	CASE WHEN Plant <= Plant2 THEN Plant ELSE Plant2 END AS Plant,
	CASE WHEN Plant > Plant2 THEN Plant ELSE Plant2 END AS Plant2,
	COUNT(*) AS [Sum]
FROM @Foo
GROUP BY
	CASE WHEN Plant <= Plant2 THEN Plant ELSE Plant2 END,
	CASE WHEN Plant > Plant2 THEN Plant ELSE Plant2 END