In Zone 9, there will often times be winters that stay rather warm deep into the season. There have been a few winters that frost only shows up 2 or 3 times in a year. Those winters are perfect for extending warm weather crops deep into the winter. Here are a few tips to help do so.
Build a Cold Frame
A cold frame is basically a micro greenhouse that uses the heat of the sun to keep the inside warm. A basic cold frame can be as simple as small PVC pipe curved over a garden and covered with a clear tarp. Keeping the cold frame in place during cold spells will help keep the temperature inside warm and somewhat stable. Once the warmer temperatures return, remove the cold frame.
There are many plans for many different types of cold frames that can be found with a simple google search.
A tried and true method of keeping plants warm during a cold spell is to simply cover the plants with blankets. Using a cotton (or similar) blanket, gently cover the plants before the cold arrives and remove when the warmer temperatures return. If the daytime temperatures are acceptable but overnight is too cold, remove the blankets from the plants during the day to allow them to get sun and replace them before sun down.
Make sure that the blankets are made out of a breathable material, any type of plastic could quickly kill the plants.
Using a dark covered mulch will help keep the soil warm. The dark color will cause the mulch to absorb the warmth of the sun and transfer it to the soil. Mulch will not be very effective by its self on very cold nights, but used in concert with covers or cold frames it will work great.
Add a Heat Source
Using any of the above can be very helpful, but there may come a time when the overnight temperatures get to cold. Adding a heat source will help keep the air temperatures around it in an acceptable range. Placing a hand warmer inside a cold frame or beneath a blanket as the sun goes down will help ensure warmer temperatures over night.
Make sure the hand warmer is not touching any of the plants!