British Garden Centres says potatoes and quick-growing carrots can be planted or sown during August, while young plants can be used for crops which would normally have been started earlier in the year.
The garden centre group says vegetables can also be grown in pots and containers, allowing people without large gardens to grow some produce.
Potatoes planted during August can be ready to lift in around 12 weeks. British Garden Centres recommends varieties including Charlotte, Duke of York, Maris Peer and Pentland Javelin for a Christmas crop.
A container around 15 inches in diameter and depth can be filled initially with 10cm of multi-purpose compost, with up to five chitted seed potatoes placed on top and covered with another 10cm of compost.
More compost should then be added around the green shoots as they grow until the container is full.
There is also time to sow short-rooted carrots, according to the group, with varieties including Chantenay, Oxheart and Round taking around 12 to 16 weeks to mature.
Carrots can be grown in a container at least 30cm deep containing light, well-drained compost. Three or four seeds can be sown in holes around 1.5cm deep and 1.5cm apart before weaker seedlings are thinned out after they emerge.
Gardeners who missed the spring sowing period for other vegetables can instead use young plants.
Brussels sprouts are normally sown between February and April for a winter harvest. Young plants can still be put into a large, deep container during August and left outside through the winter.
Winter cabbage varieties such as Tundra can also be grown from young plants in containers. British Garden Centres advises watering them at the roots rather than over the leaves and harvesting when the heads feel firm.
Leeks would normally be sown between March and April but young plants can be transferred into containers at least 18 inches deep. They can remain growing through the winter and be harvested as required.
Sage can also be bought as a young potted plant rather than grown from seed. The perennial can be grown in sun or partial shade in well-drained compost.
Cranberry plants provide another option for Christmas food preparations.
Young potted cranberry plants can be harvested from late September to mid-October, allowing the fruit to be used to make cranberry sauce which can then be frozen for Christmas.
The plants can be grown in containers around 12 inches in size and British Garden Centres recommends using rainwater where possible because cranberries prefer acidic soil.
Gardeners wanting to plan further ahead can also prepare for Christmas 2027.
Garlic cloves can be planted from October in pots around 10 inches wide, with eight to 10 cloves in each pot. The cloves should be placed just beneath the soil with their pointed ends facing upwards.
Parsnips require a longer growing period and are best sown between February and June before being left in the ground and harvested when needed.
Julian Palphramand, head of plants at British Garden Centres, said: “August is still a great time to sow quick crops like potatoes and carrots, and for anything else on the Christmas dinner list. There's a real satisfaction in bringing something you've grown yourself to the Christmas table, and you really don't need a huge garden to do it.”
British Garden Centres describes itself as the UK's largest family-owned garden centre group. It has 80 centres around the country and employs more than 3,500 people across its garden centres, restaurants, nurseries and other operations.
