I received several extremely large walnuts and decided to try making them germinate.
Walnuts are easy to propagate from seeds, but they grow slow at first and you don’t really know the size of the walnuts until the tree is already quite large. They also secrete juglone, a phytotoxic chemical that is toxic to many plants. All parts of the walnut tree have some level of juglone, which inhibits growth of plants around it. Juglone is also the reason, why it’s usually not a good idea to add any part of the plant into compost bin. My experience is, that when properly composted, juglone decomposes enough and doesn’t even bother tomato seedlings, which are otherwise extremely sensitive to juglone. Trees grow tall, with dense and broad crown. They can grow in most soils and can handle some drought.
To germinate the walnut, it have to be stratified for 3 to 4 month in a plastic bag with damp paper towel, moist peat or sand in a refrigerator. During the stratification, paper towel and the nuts can become mouldy. They can be washed in cold water and wrapped into new moist paper towel, before placing it into the fridge for some more time. I did not see any issues regarding the mold, all the walnuts I tried germinated into healthy seedlings, even if kept moldy for a while, before I’ve noticed and cleaned them.
After 4 months or when you feel the seeds are stratified enough to germinate, wash them in cold water and plant the nuts into well draining moist compost rich soil, 2 to 5 cm deep. They start sprouting in a couple of weeks and in a month or so, you can have your first little trees emerging. If it takes longer, seeds might not have been stratified enough and will take a bit longer to sprout. When they grow enough to handle, place them separately into large enough pots, buckets or directly into the ground. Walnuts are supposed to get larger if they get up-potted a couple of times when they are young. I have heard that story many times, but I have no idea if it is true.
When planting walnut trees outside, you have to make sure they have enough space to grow properly. If they grow close together, the overall appearance of the tree will be tall, without the broad crown. When they have enough space if you plant them 15-20 meters apart, they will grow more sturdy, thick and branched out trunk with broad and extremely dense canopy. It can get too dark under the trees for most plants to thrive, especially considering that the roots of walnut tree release juglone through the roots and keeps the competition away.