Cultivate early peas during these longer periods? (4-5)
I believe the answer is:
leap-years
'periods?' is the definition.
(leap year is a kind of period)
'cultivate early peas' is the wordplay.
'cultivate' indicates anagramming the letters (I've seen 'cultivated' mean this).
'early'+'peas'='earlypeas'
'earlypeas' with letters rearranged gives 'LEAP-YEARS'.
'during these longer' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for leap-years that I've seen before include "'They happen once in every four (4,5)'" , "2012, 2016, ..." .)