Sustainable shopping: four fruit and veg to look for in May and June

We all know how important eating a variety of fruit and vegetables is for our health. But not all fruit and veg is a sound environmental choice, especially at the wrong time of year.

For those of us trying to shop more sustainably, it can feel instinctively better to go for locally grown produce. It's not always so clear-cut, though. Something may be locally grown but in a climate it's unsuited to - tomatoes being a good example.

The majority of tomatoes labelled as British will have been grown in artificially heated greenhouses, increasing carbon emissions significantly. Tomatoes from Spain or Italy, however, will be field-grown under the Mediterranean sun, generally making them a better choice even when you factor in the transport emissions (by road).

A 2019 University of Manchester study (Frankowska et al, 2019) carried out a full life cycle assessment of 21 vegetables bought in the UK but often imported from abroad, and ranked their overall environmental performance. Tomatoes placed 18^th (with one being best and 21 worst) with only asparagus (21st), beans (20th) and peas (19th) coming behind, and sweetcorn equal 18th. That's largely because the majority of these crops consumed in the UK are imported.

It's also important to note that some British tomato growers are making significant improvements to growing methods including using renewable energy and waste heat.

The good news is that May is the beginning of a bountiful season in the UK where you should be able to eat healthily and guilt-free. So what should you be buying?

Four seasonal products to eat in May

1) British asparagus

Really make the most of this now! British asparagus has a very short season, running from late April (officially 23 April this year) to June, although Waitrose boasts its first asparagus from around the end of February/beginning of March.

When grown in the UK, asparagus is an environmentally friendly choice. But asparagus imported from abroad demands more than five times the energy according to the Frankowska et al study and means it places last (21st) in its ranking.

A large proportion of imports come from Peru and the Carbon Trust also calculates that importing a kilo of Peruvian asparagus generates more than five times the carbon of its UK equivalent (11kg per kilo compared to 2.1kg). Much of this is down to the emissions involved in air freighting it.

Asparagus is also a thirsty crop. That's not so much of an issue in the rainy UK, but in the valleys of Peru, the dry climate means water is diverted to cultivate the crop and can mean indigenous communities are deprived of precious water supplies.

Mexico and the USA are also big asparagus exporters.

All the supermarkets will stock British asparagus at this time of year, so make the most of the season. You can also freeze asparagus if you want to have supplies at other times of year - but it's best to blanch it first.

2) Lettuce and leaves

According to Sarah Bridle, author of Food and Climate Change Without the Hot Air, lettuce and other leaves and herbs are increasingly being grown using new technology called vertical farming. Plants are grown indoors under lights and stacked up high. This might sound inefficient, but indoor environments can be warmed more efficiently than greenhouses and are very space efficient.

At this time of year, though, there's no need for these clever innovations, as lettuce can be grown easily outdoors. The British Leafy Salads Association says 80% of British lettuce is grown outside. Indoor greenhouse varieties tend to be butterhead and some speciality leaves, but Iceberg lettuce is never under glass and Cos/Romaine also tends to be grown outside.

The season for wholehead lettuce begins in mid-May and continues until late October. Some other leaves may be harvested earlier. Spinach season also starts in May.

Lettuce overall ranks eighth in the Frankowska et al study - its generally low environmental energy demands are increased by the energy needed to keep it refrigerated during transport and in the shops.

In winter months, you'll likely be buying lettuce from Spain which will be field-grown rather than greenhouse-grown UK lettuce although some UK suppliers grow leaves year-round.

Bagged salad comes in for a lot of criticism when it comes to food waste. That's because past research by WRAP found that 40% of what's bought is wasted.

Wholehead salad should last longer, both because wholehead varieties such as Little Gem and Iceberg tend to be more robust, but also because the less handling the leaves have, the better for shelf life.

Find out more: keep your food fresher for longer

3) Summer berries

Summer wouldn't feel complete without fresh berries - plus, they are very good for us. Luckily, all our favourites have a UK growing season to look out for.

According to British Summer Fruits, which represents UK and overseas berry growers, the UK seasons for berries are as follows:

• Strawberries - April to October
• Raspberries - June to October
• Blueberries - June to September
• Blackberries - July to November

According to Mike Berners-Lee, author of 'How Bad are Bananas?', out-of-season strawberries can have a carbon footprint seven times higher than seasonal ones.

This is because berries are often imported by air freight - necessary because of their perishability. This won't always be the case - berries can also be frozen and then sea-shipped which has a far lower impact. But unless the packaging tells you otherwise, and they've come from a long way away, it's reasonable to assume that foreign berries have been on a long plane journey.

If you are keen to keep berries in your diet throughout the year, consider UK berries that are seasonally harvested, then frozen. While there is some environmental impact to freezing them, it isn't huge.

And while the plastic packaging feels wasteful, berry punnets are usually recyclable and do a useful job of protecting the produce from being bruised and therefore, wasted.

Find out more: how to recycle in the UK

4) New potatoes

When you think of new potatoes you might traditionally think of Jersey Royals, which are at their peak in May.

But the first UK mainland potatoes actually come from Cornwall, and will be in supermarkets from June.

A 2016 study published in the Journal of Cleaner Production found that potatoes had lower GHG (greenhouse gas) emissions and water use than pasta or rice. This only takes into account methods associated with production, manufacture and distribution of the foods, and not consumption or preparation. It also doesn't take into account different potato varieties, which have varying yields.

Sarah Bridle says that much of the energy emissions associated with potatoes come from the cooking - a jacket potato in the oven for two hours can mean some significant emissions. New potatoes score well on this front, as they aren't generally oven cooked and their small size means they cook more quickly.

Find out more: it's easy to grow your own potatoes

Should you buy out of season apples?

While apples are thought of as a late summer/autumn crop, some UK varieties can be found in the supermarkets all year. This is because they can be stored for such a long time after harvesting.

At the moment you may still be able to find British varieties in supermarkets - Braeburn, Bramley, Gala, Jazz and Kanzi. In fact, 40% of apples sold in the UK are British and growers' organisation British Apples and Pears aims to increase this to 60% by 2030.

There is an energy cost to this storage. Local apples bought at this time of year will have been in cold storage for many months, which could mean the environmental impact of importing seasonal apples from places as far flung as New Zealand is lower, says Mike Berners-Lee, although he acknowledges that studies have come to differing conclusions on this calculation. British Apples and Pears say that this energy cost is offset by the widespread use of solar energy to run cold storage.

In any case, even allowing for the energy impact of local cold storage or importation by boat, apples are still a reasonable environmental choice - the University of Manchester team also carried out a study of the environmental impact of fruits bought in the UK and found that overall apples ranked fifth best (Frankowska et al, 2019).

So while local and in season is best (UK apples are harvested in September through to the end of November), you don't need to avoid buying apples the rest of the year, whether British or not.