Did you know that some of the most nutritious vegetables that you can eat come from trees?
Tree vegetables is a term you don’t often hear in our culture - indeed eating leaves or shoots off trees is still regarded as a bit odd, or the realm of the intrepid forager. But in other parts of the world it is entirely normal to use trees for food in this way.
As part of ART I've long grown and eaten tree veg but it wasn't until I read the excellent paper by Eric Toensmeier et al on the nutritional content of perennial vegetables (*) that I realised just how nutritious they are, coming in some of the top places in the nutritional league amongst hundreds of alternatives including annual veg.
Why tree leaves and shoots are quite so high in nutrients hasn't been researched, but it seems highly likely that it is because woody plants form the best associations with mycorrhizal fungi. These fungi are critically important (indeed we would not be here without them) and have many ecosystem roles. They form symbiotic associations with many plants (especially trees), attaching to and inserting themselves inside tree roots. These fungi are much better at accessing nutrients in soil than plant roots as their mycelial networks are finer (a tenth the diameter of the finest plant roots) and more extensive. The fungi pass various nutrients to the tree and are rewarded with sugars going the other way. As a result of this extra source of nutrients, tree leaves and shoots seem more likely to have higher nutrient levels.
Management of trees as a leaf or shoot crop usually entails a regular trimming or cutting regime (often coppicing or pollarding) to make the harvesting practical and to maximise the crop of young growth.
Here are the best tree veg to grow in our temperate climate:
1. White mulberry
All mulberry leaves are edible but white mulberry and its hybrids have the best texture. Very young leaves can be used raw but mostly use as a cooked leaf. You can harvest leaves right through the growing season: late into leaf, so this is usually June onwards until the first frosts. Last year we were picking all the way through to early December as the autumn was so mild.
White mulberry leaves have more protein (10% in fresh leaves) than almost any other leaf on the planet (that’s about 4 times the amount in annual spinach). They have high levels of calcium, iron, magnesium, zinc and Vitamin C.
You can eat the leaves (if you can reach them) of a larger tree grown for its fruits, but we prefer to grow trees for fruit separately, and pollard our mulberry trees at about 1.2m /4 ft height every winter, which keeps the plant as a bushy shrub 2m in diameter.
2. Toon
Gradually becoming better known, toon (Toona sinensis) is a Chinese tree whose leaves are widely used where native. They have a unique strong flavour of vegetable stock – onions, garlic and spices. The young leaves can be used throughout the growing season raw in salads or cooked. The older leaves can be used cooked (removing the central rib as it can get tough) although they are not quite as flavourful. We also dry the young leaves and rub into a leaf powder to use like bouillon powder in winter: they lose some flavour when dried but are still good.
Toon leaves are high in Vitamin A, Vitamin E and several other minerals.
Manage the trees by coppicing or pollarding – we use a one year cycle of cutting as they are very vigorous. Or you could do an all-over annual trim to keep bushy.
3. Vine
It was a revelation to me to find out just how nutritious grape vine leaves are. They are very high in many nutrients and exceptionally high in fibre.
Most of us have probably eaten vine leaves as you’ll find all around the Mediterranean, using the older leaves (which have usually been preserved in brine) as a wrapping for a parcel of food. However, younger vine leaves are a good vegetable themselves, cooked in stir fries and the like, with a slightly sour lemony flavour.
If you’ve ever grown a grape vine you will know that once established, they produce masses of foliage (and this is what vineyards spend their summers pruning off so sun gets to the grapes). If you are actually growing grapes for the their leaves they are one of the easiest plants to grow: they tolerate quite a lot of shade, and the young leaves can be harvested all through the growing season. The plants are kept as bushes by regular harvesting.
It's important to grow pure European varieties (Vitis vinifera) for leaf production, as the leaves of most hybrids (of European and American or Asiatic species) have thick furry leaves of poor eating quality. Our taste tests identified Chardonnay as one of the best for its leaves.
4. Goji
Goji berry (Lycium barbarum) is a spreading pioneering bush which can tip layer and grow into larger colonies if allowed. Best known for its edible (though bitter) fruits, the leaves are a widely used vegetable in China.
The leaves don’t have a big flavour but are very high in vitamins A and E, and are used cooked. We add them to soups, stews etc.
Because goji bushes are very vigorous, there are plenty of “spare” shoots and leaves. We mainly harvest in spring and again in autumn, finding that the leaves in summer can be a bit tatty or mildewed. We’ll often strip whole branches of their leaves if they seem excess to requirements.
Goji bushes require pruning in any case to keep as an upright shrub 1-2m high, so no extra pruning for leaf production is need.
5. Saltbush
Mediterranean saltbush (Atriplex halimus) is a grey-leaved evergreen shrub a couple of metres high. The leaves can be harvested all-year-round and a pleasant slightly salty taste, and are high in many nutrients.
The main issue with growing saltbush is the hardiness. It’s only hardy to around -10C and only then if it’s in a sandy soil. We’ve lost it several times over the years but keep on planting it as it’s such a nice leaf available when others are not.
6. Lime
All young lime (Tilia spp.) leaves are edible, and small-leaved lime (T.cordata) is our favourite for the leaf texture. They are very high in Zinc amongst high levels of various minerals.
Lime trees all get large with age, not a useful trait if you want to be harvesting lots of leaves, so are usually coppiced or pollarded to keep bushy. The plants then become bushy rounded shrubs, size about the same diameter as the cutting cycle – most of ours are on a 3-4 year cutting cycle and can get 3-4m wide and high.
All these leaves can also be dried through the season for use in winter. Dried leaves can be added whole to soups, stews etc. or crumpled into a leaf powder. Sometimes the leaf ribs can be rather tough when dried so crumbling allows these to be easily filtered out.
Because these plants are kept small by management, they can fit into any size growing areas, even small gardens. So what are you waiting for?!
*Eric Toensmeier, Rafter Ferguson, Mamta Mehra (2020). Perennial vegetables: A neglected resource for biodiversity, carbon sequestration, and nutrition. PlosOne.
Very informative. Thanks Martin.
The various edible Opuntia young stems are missing here.