I grew gherkins outside last year. First lot of plants i tried were not very successful, left uncovered, they do need a bit of heat. Tried the second lot under polythene, these came up fine. They do take up quite a bit of space. You also have to keep your eye on them. Very small one minute, oversize the next. Tried (Cucumber Gherkin National) from Terwins Seeds. Still got some seeds left over so will be setting these this year.