Perhaps you should try some spelling variations:
"vegetative spread" » "vegetative spross", "vegetative based", "vegetative viral"
"negative spread" » "negative strand", "negative real", "negative areas"
"negative tread" » "negative trends", "negative real", "negative areas"

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("negative tread") OR ((("negative spread") OR ("vegetative spread")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"negative tread\") or (((\"negative spread\") OR (\"vegetative spread\"))))".
• Removing quotes may allow a broader search: (( negative tread ) or ((( negative spread ) OR ( vegetative spread )))).
• Adding a wildcard symbol may retrieve word variants: (("negative tread") or ((("negative spread") OR ("vegetative spread"))))*.