Carya californica

• California Walnut

Seed - requires a period of cold stratification. It is best sown in a cold frame as soon as it is ripe^[2]. Stored seed should be kept moist (but not wet) prior to sowing and should be sown in a cold frame as soon as possible^[2]. Where possible, sow 1 or 2 seeds only in each deep pot and thin to the best seedling. If you need to transplant the seedlings, then do this as soon as they are large enough to handle, once more using deep pots to accommodate the tap root. Put the plants into their permanent positions as soon as possible, preferably in their first summer, and give them some protection from the cold for at least the first winter[78, K]. Seed can also be sown in situ so long as protection is given from mice etc and the seed is given some protection from cold^[3] (a plastic bottle with the top and bottom removed and a wire mesh top fitted to keep the mice out is ideal)

Prefers a deep moisture-retentive loam in a sunny sheltered position, requiring a good summer for best development^[4][5][6][3]. Slow growing^[3].

This species is not hardy in the colder areas of the country, it tolerates temperatures down to between -5 and -10°c^[3]. Plants are strongly tap-rooted and should be planted in their permanent positions as soon as possible^[4][6]. Sowing in situ would be the best method so long as the seed could be protected from mice^[4][3]. Trees are late coming into leaf (usually late May to June) and lose their leaves early in the autumn (usually in October)^[6]. During this time they cast a heavy shade. These factors combine to make the trees eminently suitable for a mixed woodland planting with shrubs and other trees beneath them^[6]. Plants in this genus are notably resistant to honey fungus^[3]. Most species in this genus have quite a wide range of distribution and, in order to find trees more suited to this country, seed from the most appropriate provenances should be sought^[6]. Most trees growing in Britain at present tend to only produce good seed after hot summers^[6]. This species is closely related to J. hindsii.

Trees are self-fertile but larger crops of better quality seeds are produced if cross-pollination takes place^[7].