Grow poppy seedheads and cut them in the summer for drying 
Nigella seedheads can be cut in the summer, dried and then sprayed in December. 
Crab apple red sentinel will work great as decorations in wreath, but do pick the apples or cut off the branches at the end of October for them to look good in December 
Hydrangea flowers are great and some varieties will keep their colour if picked in November or if they are in a sheltered position. You can also spray the flowers. prayed 
Callicarpa are one of the last berries that the birds will eat 
Chinese lanterns (Physalis alkekengi is best cut and dried in early autumn. ke by now) 
Table wreath with crapapples, rosehips and dried hydrangea flowers
If you have a garden, you may well have grown your own Christmas wreath. Below I show you how you can make it and you may also pick up some ideas for next year.
To start for any wreath you need a framework, either one with moss or one with sticks. For a moss frame people usually buy a wire ring but of course you could make your own with some strong chicken wire or similar. Next fill the wreath with moss from your garden (or buy a bag of sphagnum moss) and secure it to the framework with more wire. You can use floristry wire, but any wire will do. If the wire happens to be visible after you have made your wreath, you just put a bit more moss over it.
For a frame made with winter stems, you can use many different plants such as willow, hazel, dogwood (nice and colourful), birch, and probably anything in your garden that bends easily. We have a willow fedge, so when making a wreath I usually start by making one hoop and continue to weave in sticks, turning the wreath a little bit every time so that the thick ends of the willow are well distributed in the wreath. Let the thick ends stick out a bit while making the wreath and when finished you just cut them off.
Once you have your frame, go out in your garden again and look for greenery and other decorations
Greenery: rosemary, bay, holly, ivy, yew, box, olive, conifers, spruce, eucalyptus.
Decorations: pinecones, berries from cotoneaster, holly or callicarpa, crab apples, Chinese lanterns (Physalis alkekengi – although they may have become more skeleton-like by now), rosehips, hydrangea, poppy heads.
The above lists aren’t exhaustive and if you like something just try it out. I found that thistle heads were too brittle, but I have seen some great decorations with them. If you have made a wreath just using willow or dogwood, it may not need much as the wreath itself may be quite beautiful.
You could add anything else from lights, baubles, other Christmas decorations or decorate the garden treasurers even further. On the wreath featured above, I used purple glitter (use biodegradable glitter!) on the pinecones and sprayed poppy and nigella seedheads gold, added cinnamon sticks and some further haberdashery.
If you have made a moss wreath, you can use a spray bottle, maybe from the back side, to keep the moss and wreath greenery moist and thus fresh. For a twig frame, you could just mist the greenery. When using ivy I don’t even bother with that as the ivy will stay fresh by itself for weeks.
After Christmas, most of your wreath can be put on the compost heap and some of it can be saved for next year. If you have made a wreath with willow or other winter stems it will keep for years.
Happy Christmas crafting!