Black Walnut | Agroforestry

Black Walnut
Juglans nigra

Good things takes time - so act now!

Black Walnut grows naturally across the Eastern USA but its timber has a worldwide reputation. Walnut heartwood is strong yet relatively light, shock resistant and non-brittle. It is also durable and, most importantly, stable across a range of moisture and temperature conditions, which made it ideal for gun stocks. Whilst the wood of both walnut species can exhibit interesting and attractive grain patterns, it was colour that made the American Walnut, and hence the superior American weaponry, distinguishable. In peacetime these same qualities ensure the best figured lumber has a high value market for use in musical instruments, sliced veneer and designer furniture. Naturally, the highest prices are paid for large clean logs that are suitable for sliced veneer or the rare logs with outstanding grain patterns. Reports of individual trees being auctioned at the stump for $30,000 (Ohio, 1995), poachers using helicopters to steal individual Black Walnut trees (Kentucky), and university students using DNA testing to match stumps against stolen logs (Indiana, 2004) has certainly helped fuel a fever of expectation amongst Black Walnut growers. Unfortunately only very large defect free trees attract these dizzy prices with run-of-the-mill logs achieving more sobering prices, particularly from young plantations. When assessing Black Walnut log value size matters. Although a Black Walnut tree with a 5 metre long straight trunk 25 centimetres in diameter may have taken 25 years to grow, as a timber or veneer log, it is almost worthless. But, give it another 25 years and let it double its diameter and the same trees might be worth more than $1000!

But only if it is well-managed.

In 1939 a Canadian agricultural teacher climbed a branchy 19-year-old native tree growing on his farm in Ontario, Canada. He carried a handsaw with him up the tree and, starting high in the canopy, began pruning the branches off at the trunk as came back down to earth. His saw was sharp and none of the branches were larger than about 6 cm in diameter so it wasn't a difficult task. Within few minutes he'd transformed an open-grown branchy tree into a lollypop with its spreading crown sitting on top of an 8 metre clear bole. Fifty-two years later, when he was 84 and the 71-year-old tree was 73 cm in diameter, he sold the log for CAN$1500 to a veneer slicer. As he tells the story Andy adds that he might have got more had he not been so keen to sell and get a real price that he could use to argue his case that pruning can be quick and easy and, if you're prepared to wait, can dramatically increase the value of a timber tree, especially if it is a species that is as valuable and highly prized as the American Black Walnut.

Growing big Black Walnut trees takes time, lots of time. And time erodes enthusiasm. What starts as a sure-fire way to get rich can very soon whither to a distant, forgotten affair. Most landholders who plant Black Walnut soon lose interest and neglect their trees, allowing them to form heavy branches. A few, like Andy, maintain their enthusiasm. Well before he sold that first tree he was planting more Black Walnut and, to the curiosity of neighbours and derision of foresters, he pruned everything. He couldn't even bring himself to leave any tree unpruned just to show what happens if you don't. What wasn't pruned was culled to release growing space. If challenged, as he often was, Andy liked to point out that although everybody knew a clear straight trunk of Black Walnut was worth much more than a branchy one, only a handful of the hundreds of thousands of hardwood trees growing on farms across the state would ever produce a saleable log simply because nobody pruned! For him it was just common-sense: given time a tree would grow tall and fat, if pruned then time would grow value, if not all that would grow would be regret.

And no-one can stop time!

The secret to growing Black Walnut timber is in seeing value in the waiting! After 25 years the best of my best Black Walnuts have just surpassed 20 cm in diameter! They are a gift to the next generation. A living Black Walnut tree growing on your own land is not like a piece of antique furniture or a work of art that you can buy late in life when you've made some money doing something else and begun to worry about what you have to leave for future generations. By spending a little time now, growing and tending trees that we may never live to see mature, we are giving our children, or the next owners of the farm, a gift they can't buy at any price, a gift of time! Whether it is a worthy gift depends on how well I matched the species to the site, and my commitment to their timely management. Due to their slow growth rate and the propensity to develop epicormic shoots, I adapted my pruning standard 8 cm gauge pruning method and only removed branches on the stem below the point where the tree was 10 cm in diameter and then any branch over 2cm above that point right up to the anticipated final pruned height. This resulted in a deeper crown of small branches rather than a lolly-pop.

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Left - I'm measuring one of our 27 year old pruned Black Walnut trees. Planted as a seed into creek loams
Right: 40-year-old Black Walnut trees planted in central Victoria

Black Walnuts produce a chemical from their roots (Juglone) that kills some plant species including tomatoes, apple trees and some broadleaf pasture weeds. On some sites they have been shown to act as a natural weed control in pastures. Also, one of my masters students studied soil fertility under a Black Walnut plantation in NE Victoria and found that the trees actively moved nitrogen from the subsoil back to surface soil through their leaf drop and fine root turnover thus reducing the risk of soil acidification due to nitrate leaching and fertilising pastures. The second photo shows a pruned 40-yr-old tree in that same plantation proving that this valuable timber and veneer species can grow to a commercial size in Australia. It also indicates to me that I need to thin my plantation to give the best trees more space.

In 2015 I cut down one of our 27-year-old Black Walnut. The tree was malformed and needed to be thinned to give more space to it's neighbour. I also wanted to see how the timber was developing and get some wood for samples and wood working. The tree was only 24.5 cm in diameter. The white sapwood was about 5cm wide leaving about 8-10 cm of dark heartwood. On milling the timber appeared an olive-green colour (not what I expected) but overnight it turned a beautiful chocolate brown. Final picture shows freshly milled section on the right and a darker piece (from the day before) on the left.

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Deep soils required for Black Walnut

We can only grow Black Walnut on our creek flats where there is at least 1m of well-drained alluvial soil. I know they also grow well on deep red volcanic soils. The main factor is depth for development of the tap root. Please don't bother on shallow soils.


Direct seeding Black Walnut

I'd planted my first Black Walnut when I was 25 as seed, directly into the deep loams along our creek. I'd imported a kilo of nuts from the New Zealand Forest Research Institute. When my nuts arrived I left them in the sealed plastic bag and put them to the back of the fridge until the end of August to mimic a North American winter. Then, in the first week of spring, I mixed the nuts into a box of loose soil. As each seed germinated I careful removed it and sowed it directly into the paddock, being sure to point the emerging white taproot down into the lightly cultivated soil. For weed control I used a 1 metre square of old carpet with a small hole cut out to allow the shoot to emerge. Direct seeding allows the fragile walnut taproot to develop freely. In the first season our seedlings were barely 30 cm tall but when I dug one or two up I found that their taproot was more than 50 cm deep! I grew a few seedlings in our backyard nursery using deep containers but found that when I planted them out a year later they didn't grow at all in height over the next season, clearly negating any advantage that might be gained over direct seeding. In North America they can't rely on direct seeding because the squirrels take the seed so they had to except the extra cost and the setback associated with planting seedlings.

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Our 31-year-old trees have fruited well this year and we have seed available for sale. Black Walnut is a slow growing, high quality timber tree. The strong taproot means it is better to plant the seed directly into the paddock rather than try and grow them in pots. We planted ours by first germinating the seeds in a box of loose potting mix and then carefully planting the germinating seed directly into the paddock with the emerging root pointing down into the prepared soil. We used 1m2 of carpet for weed control and cut a hole in the middle for the seed.

Black Walnut grows slowly: the best of our trees are only averaging around 1cm in diameter per year. It should only be planted where the soil is more than 1m deep, preferably creek or river flats. Once established it can survive drought (by dropping its leaves) and floods.

These seeds were collected from a number of our well-performing trees. Seed should be stored in airtight bag in a fridge (to mimic a cold winter) until sown (in August). Mine germinated in late August/early September.

Photos: Seed (black woody nuts) and the fruit. Seedlings 3 months after germination.

How to germinate Black Walnut

To increase the probability of germination, Black Walnut seed should be stratified in cold conditions with both moisture and oxygen to mimic a winter under snow. The following advice has been collated from credible American web sites:

1. Place seed in a plastic bag then add a soil-less mixture (I now add peat moss to every bag of walnut seed we sell).

2. Fill the bag and soak the seed in water for 24 hours then drain (poke holes in the bottom of the bag to allow excess moisture to drain off).

3. Place the bags in the refrigerator for about 90 to 120 days. Do not freeze.

4. Do not to allow the seed to dry out and do not leave the seed soaking in water.

5. Seed may germinate in the fridge - remove and plant any that do.

6. Plant remaining seed in early spring. I put the seed out in a box of potting mix and regularly check to pick out the germinating seeds.

In early spring plant the nuts in soil about 5cm deep in the ground or in very deep pots where they can freely grow their tap root. The first shoot to emerge from the seed is the taproot. If so, plant with the root pointing downwards - take care not to damage it. If the seed has not germinated it doesn't matter which way it is orientated as it will turn itself.

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