The environment department says what should be a carpet of blue at this time of year is now fringed with white.
Commonly known as wild garlic, the alium was introduced as a garden plant but it escaped and is now rampant in woods and hedgerows.
Plant expert Jane Gilmour from the Biological Records Centre is advising gardeners to get rid of the invader.
Its leaves appear in the autumn which means the department can use contact weed killer to tackle the problem without harming the bluebell bulbs.
Ms Gilmour said: "Sometimes called stinking onions, this is from the Mediterranean. It has increased massively in numbers in the past few years.
"The milder weather and generally milder winters have suited it brilliantly and it is out-competing bluebells as it comes into leaf just a little bit earlier.
"It is one of those plants people are staring to realise is a thug."
Bluebell woods in Guernsey are being invaded by a non-native species, environmentalists say.