If you're following UK writing conventions, use spelt.
(If you're referring to the wheat, use spelt.)
Spelled and SpeltThe verb to spell most commonly means to write or name the letters that form a word in correct sequence.
To spell is one of those verbs with both an irregular form and a regular form. (See the table below for some others.) The past tense and the past participle can be written as either spelled or spelt. However, they are not interchangeable, especially in the US.
Americans Demand SpelledIn America, spelled dominates. The use of spelt as the past tense or past participle of to spell is considered a spelling mistake by many. It will certainly annoy a fair proportion of your readers.
Brits Prefer SpeltOutside America, spelt is more common, but spelled is generally accepted. (This is almost certainly a result of American influence spreading.)
Misspelled and MisspeltIt's the same with misspelled and misspelt. Americans demand misspelled (and Brits will accept it). However, Brits prefer misspelt.
Verbs with Irregular and Regular FormsThe following verbs (like to spell) can be regular or irregular:
|Verb||Past Simple||Past Participle|
|burn||burned OR burnt||burned OR burnt|
|dream||dreamed OR dreamt||dreamed OR dreamt|
|learn||learned OR learnt||learned OR learnt|
|smell||smelled OR smelt||smelled OR smelt|
|spell||spelled OR spelt||spelled OR spelt|
As with spelt, the second form (e.g., learnt, dreamt) is more common in British English.
In America, the noun spelt is used to refer to a kind of wheat.