Sentence:  Sow them in late spring in the greenhouse or coldframe, but don&#39;t be tempted to sow any earlier otherwise they turn into <WSD>triffids</WSD>, hog all your greenhouse space and promote fungal diseases. 
Meaning of the word marked with <WSD> tags:  Any large, fast-growing and hard-to-remove weed, such as the flowering shrub (species).