Some areas of the world such as England, the Benelux countries, and Eastern United States have had considerable experience with these apple varieties having three sets of 17 chromosomes (51 total), triploids.
Unlike the more common diploid (two sets of chromosomes, 34 total), these triploids produced much bad pollen and many defective ovules; however, for nearly 300 years they have been important to the apple grower because of their quality and often large size.
My former horticulture teacher will probably turn over in his grave when I call these sterile, but for practical terminology we are not far from wrong, so let us do it; however, the old Thompkins County King variety does have some self-fertility despite being triploid.
Perhaps a little apple history is in order before getting to Jonagold. On my experimental orchard, there have been 13 triploid varieties at one time or another.
The oldest probably is Gravenstein (triploid) with a known history to about 1670 in South Denmark, but almost surely an Italian variety whose scions were given to the Duke of Gravenstein in the middle of the 17th century.
Then there is Ribston Pippin long the classic apple of England before its seedling (Cox Orange) was widely grown. Our new Gala (a diploid) variety has Golden Delicious, Kidd’s Orange Red, Cox Orange and Ribston Pippin in its genetic background. No wonder it tastes so good.
Ribston Pippin (triploid) was planted in 1708 at Ribston Hall, Yorkshire and the original trunk did not die until 1835. It then sent up a new shoot and on the same root lived until 1928, 220 years!
Bramleys Seedling (triploid) is yet quite important commercially in Ireland and England as a cooking apple, and is one of my favorite pie and sauce apples, with its one percent acid and huge size. The original tree was planted probably in 1813 and was still alive in 1956 at Southwell, Notts and may still be living 171 years later. A famous triploid which has no peer as a cooking apple.
Belle de Boskoop (triploid), commercially important in Holland, Belgium, Denmark, etc, with many modern offspring it is an old English variety still grown in backyard orchards.
Early American triploid seedlings were Baldwin the most important commercial red apple in the Northeast for 75 years until a few days of minus 40 degrees F. killed then out in 1933-34. They were replaced by McIntosh.
Rhode Island Greening (triploid), still grown commercially in New York – Stayman (triploid), resulting from a seed of Winesap planted in 1875.
Some of the newer apple crosses are also triploids – notably Jonagold, Mutsu (Crispin), Spigold, and Suntan (European).
Mutsu (Crispin) and Spigold often have apples too large unless sold in gift boxes, if one leaves the king blossom* on. I have seen Spigold picked from the king bloom only, that ran 35 to 40 per box but were still of good flavor. What a sight on a dwarfing tree.
Jonagold also sizes well and fruiting the king bloom is not that important unless one wants considerable apples in the 64 to 88 size. There are at least two ways to pollinize ‘Jonagold’ depending a great deal on the apple growing area.
First, no matter what geographical area you want to grow them in, you cannot use Golden Delicious as a pollen source. Golden delicious is cross-unfruitful with Jonagold.
Remember we usually need two other varieties with variable pollen to pollinize Jonagold. Don’t use just one variety, because even though that variety may give a good fruit set to the Jonagold – what is pollinizing the pollinzer. If you want apples from your pollenizers, Jonagold won’t do it. It’s virtually sterile.
Depending somewhat on climate, Jonagold is usually about in the middle to the late part of the mid-bloom. Suggested varieties as pollinators are Akane and Spartan.
For the latter part of the bloom, Melrose and Paulared would cover it well. Akan and Melrose would be my choice, as Melrose is an outstanding keeper and an all purpose apple.
Another alternative is for those who like Yellow Newton. Since Yellow Newton is quite self-fertile and has a good overlap in bloom period, it would work fairly well. But triploid should normally have pollen from two sources.
However we do it, there is a way for any area – but doing our homework is important.
*The king blossom is the central, first opening blossom in the flower cluster of apples. It will produce the largest fruit. Many times all other blooms and/or small fruit, but the king, is removed during tile thinning process.