Ponds will be starting to warm up from April/May. It is at this time of year you need to be careful that you do not overfeed your filters and fish, so I would recommend feeding wheatgerm or an appropriate multi season food until your pond temperature is above 14 Celsius preferably 16 c before switching to a summer food.
If you start feeding your fish with a summer food at the beginning of the season you may overload your filters resulting in a rise in Nitrite NO2 which will harm your fish, especially after surviving the winter. Feeding fish on a low protein food will help prevent this and help your filters build up a health population of good bacteria.
Koi Hut stocks Hikari and NTLABS all season foods which I would recommend being used up to the end of May.