“I believe that 15 years from now we’ll be standing here and we’ll be talking GEM. We won’t be talking Hass. People will go, ‘What is Hass?’” said Mary Lu Arpaia to a room of California avocado growers in 2017. Arpaia knows avocado varieties. She has been the director of the University of California’s avocado breeding program for a quarter century.

What is propitious about GEM? And more to the point for this post, is a GEM avocado tree as suitable for our yards as it might be for a commercial farm?

To answer this, we’ll look at GEM’s fruit characteristics, harvest season, the tree’s shape, bearing habit, and its tolerances and vulnerabilities. I’ll also survey the variety’s development, which occurred within the University of California’s breeding program.

Fruit characteristics

Fruit first. GEM is pretty, especially when immature or unripe. Here is an unripe and immature GEM avocado on a tree in October.

As the fruit matures, the peel gains a black background but the bumps — lenticels — remain yellow-green for some time. The fruit looks like a starry night, similar to the old Nimlioh variety from Guatemala. I find GEM’s speckled appearance attractive.

GEM avocados, unripe to fully ripe.

GEM avocados average in size slightly larger than Hass, and their shape is less pyriform and more like a fat egg.

Once cut open, you find a seed that is slightly larger than the seed of Hass and pointier too. And there often remains a fuzz of pulp coating the seed on GEM.

Compared to Hass, the skin of GEM is smoother, thicker, and grittier. It is also sometimes a touch more fragile and less pliable, which means that it doesn’t peel quite as well as Hass although I would still characterize GEM as a good peeler.

The taste of GEM? Excellent, top notch in every way. And to me, nearly indistinguishable from Hass. I doubt that I could pick out which is which in a blind taste test. The texture of GEM is sometimes slightly denser, and the flesh cracks a bit (like its mother, Gwen). And the color of GEM’s flesh is sometimes slightly paler. Otherwise, I can rarely detect anything peculiar in the flavor of GEM compared to Hass.

My four-minute video profile of the GEM avocado fruit:

Harvest season

GEM in Southern California is not harvested until March at the earliest but beginning in April more commonly, which puts it a month or two later than the beginning of the Hass harvest. GEM usually tastes best to me in May. And it remains good on the tree into July or maybe August here in San Diego County, probably later if you’re near the beach, certainly later if you’re north of Los Angeles.

Of course, when you harvest any variety also depends partly on how oily you prefer your avocados. GEM avocados don’t start dropping from a tree as soon as they’re mature so you can let them hang and accumulate high oil levels if that’s the taste you like.

Bearing habits

GEM is a good producer in a number of ways. One way is its consistency. All avocado varieties fluctuate in their fruit production from year to year, but GEM’s yearly yield doesn’t fluctuate as much as many other varieties. I have observed this on individual GEM trees, and more importantly, this has also been reported in scientific trials (see one from Irvine at 1:28 in this video).

This can be a preferred behavior if you’ve got a single tree in your yard. You’re more likely to get avocados from your tree every year.

Three-year-old GEM avocado tree carrying fruit down low and initiating flowers for a following crop up high.

While this is not the same as saying you’re going to get a lot of avocados from a GEM tree every year, GEM does also happen to be a very productive variety overall. In the same trial in Irvine mentioned above, GEM yielded more fruit in terms of weight compared to Hass and Lamb. Similar results have also been reported in a comparison between GEM and Hass in Ventura County by Brokaw Nursery.

Yet in a more recent and on-going trial in Ventura County comparing GEM to four other varieties, GEM’s yields are not the highest. Hass, Lamb, and Reed are higher on a per-tree basis. However, it’s important to note that the GEM trees were smaller than all other varieties except Reed. And on a yield-per-canopy-volume basis, GEM is the most efficient producer. (See 1:30 in this video.) 

Put another way, GEM avocado trees produce a lot of fruit on a little tree. Like GEM’s behavior of producing avocados more consistently, this attribute of efficiency can also be of benefit to a backyard grower since most of us have limited square footage in our yards.

A GEM avocado tree also wants to produce fruit right out of the gate, right at planting. In fact, GEM is dangerously precocious. A GEM tree that is only knee high can make hundreds of flowers, but an avocado tree that is knee high isn’t ready to carry even a single fruit to maturity without risking stunted growth or even collapse.

Newly planted GEM avocado tree with many flowers.

In order to encourage healthy growth of newly planted GEM trees some farmers actively remove flowers while others wait and remove any fruitlets that have set, perhaps around June. I’ve tried it both ways and haven’t noticed a clear difference in the trees’ responses.

Variety development and history

Let’s not be surprised that GEM is precocious, considering its lineage. As the GEM patent reads, “The seed that produced the new variety of the present invention was collected in 1985 from open-pollinated avocado trees of the ‘Gwen’ variety.” Gwen is likewise precocious.

Furthermore, Gwen is thought by many to be a great grandchild of the Lyon variety — Gwen is a seedling of Thille, Thille is a seedling of Hass, Hass is probably a seedling of Lyon. Wrote Bob Bergh in “Breeding Avocados at C.R.C.” of the Lyon variety: Lyon “bears so precociously and so heavily that the tree is severely stunted and sometimes killed outright.”

Bergh was in charge of the University of California’s avocado breeding program during the 1980s when the seed that ultimately produced the original GEM tree was planted. He was also in charge during the 1960s when the seed that produced the original Gwen tree was planted.

In the 1960s, the variety Fuerte was seen by many as the best eating avocado. Fuerte’s flaw was that its tree didn’t produce consistently enough. The Hass variety had come along and proven to be a tree of more consistent production, except that the Hass fruit had the flaw of blackening skin. At the time, black skin on an avocado was associated with damage to the fruit. So in the 1960s, Bergh was trying to breed an avocado variety that combined Hass and Fuerte, that had a tree that produced at least as well as Hass but whose fruit stayed green like Fuerte. Bergh’s answer was Gwen. Gwen is like a green-skinned Hass.

But breeding avocados doesn’t happen overnight, and by the time Gwen reached avocado farmers in the late 1980s, avocado eaters had begun accepting the black skin of Hass. In fact, avocado eaters in California had started demanding that their avocados ripen black. It’s almost like Gwen was conspired against.

In the mid 1980s, around the same time that Gwen was going from patent into commercial production testing, Bergh was continuing to plant seeds to discover new varieties, and some of those seeds had come from Gwen avocados. One such Gwen seed was planted in field 3, row 29, spot 5 on a ranch in Camarillo, Ventura County.

This 3-29-5 seedling tree ended up producing fruit that ripened black like Hass (check one!), and the tree produced a lot of fruit (check two!). More than that, the tree produced all that fruit on a compact canopy like its mother Gwen (check three!). It was like a black-skinned Gwen.

It was an assistant to Bergh who most often observed and collected data on this seedling tree and the others on the Camarillo ranch, and ultimately tree 3-29-5 would bear his name, via his initials: Gray Edward Martin.

Tree shape and size

GEM’s tree architecture offers mostly advantages. A grower in Somis in Ventura County told me a few years ago that his ranch has some of the oldest GEM trees, which were planted in the 1990s while the variety was still in testing. The trees were planted at a spacing of 18 feet apart. More than two decades later, the trees had not been pruned and yet were still not touching each other. That illustrates the speed of growth and tree shape of GEM: relatively slow and compact.

On another part of this same ranch, Hass trees were planted 10 feet apart in 2004, but the required thrice yearly pruning and reduction of overall yields after year eight lead the ranch manager to topwork the trees to GEM in 2019.

One year of growth on topworked GEM avocado trees.

This grower’s experience is that GEM avocado trees are manageable long term at a spacing of 10 feet between trees because of their shape and relatively slower growing speed while Hass need more like 12 or even 15 feet between trees.

GEM avocado tree, seven years old, planted 12 feet from adjacent trees in row.

I have seen farmers plant GEM trees as close as six feet apart down the rows, but I haven’t seen such close spacing maintained for many years so I’m unsure how sustainable it is.

Regardless, the take away is that GEM trees are less vigorous than Hass and most other varieties.

Tolerances and vulnerabilities

This can be of some disadvantage to home growers, however. GEM being less vigorous means it isn’t as fast to outgrow challenges. In my own yard, I’ve had slightly more trouble getting GEM trees through their first few years compared to some other varieties. They haven’t coped well with gopher attacks, root competition from larger nearby trees, and damage from heat or cold. GEM has seemed to me to not be as generally tough a variety as some others when encountering these challenges that occur in many home garden situations.

So, GEM’s slower rate of growth is mostly an advantage, as is GEM’s style of carrying its fruit. GEM often sets fruit in clusters and holds those avocados inside its canopy, hidden under foliage. The fruit is thus shaded and unlikely to sunburn.

GEM avocados protected inside the tree’s foliage.

But this comes with the vulnerability of more skin scarring. You notice it a bit in the above photo.

Below is a random group of GEM avocados next to a group of Hass picked from trees in the same grove in Fallbrook. You can see that a few more of the GEMs have skin blemishes.

GEM on left, Hass on right.

The scarring is mostly caused by fruit rubbing against nearby stems, branches, leaves, and other fruit in a cluster. Farmers are trying to figure out how to minimize this because such superficial blemishes affect how much they get paid for their avocados. (See a discussion on GEM scarring here.)

Personally, I couldn’t care less about this issue though, as I’m a home grower and I’ve never seen this minor scarring affect the underlying flesh.

While there haven’t been proper studies done on the heat and cold tolerance of GEM compared to other varieties, there have been anecdotes reported from growers in various parts of California and in other parts of the world. Some have noticed slightly more cold tolerance in GEM compared to Hass in specific ways. For example, Mary Lu Arpaia has said that the GEM trees in a planting in the San Joaquin Valley had a better spring bloom compared to nearby Hass trees after a freeze in 2007.

And for heat tolerance? A commercial farmer with decades of experience growing GEM in San Diego County told me he noticed GEM trees to be damaged a bit less than nearby Hass during the extreme heat of early September, 2020. On the other hand, I heard another farmer in the same area say that he didn’t notice any higher heat tolerance in GEM compared to Hass in his grove during the same event.

Maybe we should conclude that the jury is still out on GEM’s precise level of cold and heat tolerance. My own GEM trees have shown mixed responses to extremes in temperature.

Is GEM a good fit for your yard?

GEM is not perfect but it is an all-around excellent avocado tree. Its main comparative advantage is its size and shape, and therefore its efficiency, in avocado yield: GEM produces a lot of fruit while taking up little yard space. So if you’ve only got a 10-foot by 10-foot corner of the yard for an avocado tree, GEM fits your needs very well.

GEM avocado tree, six years old.

Will farmers be moving from a focus on Hass to a focus on GEM in the next 15 years? I don’t know. But I’m willing to make a different prediction: Home growers like us with an appreciation for avocado diversity and the desire for year-round fruit from our own yards are going to indirectly spur California farmers to include varieties even beyond GEM in their future plantings. No more single-variety output, Hass or GEM or otherwise.

This will be a win for everyone.

Here is a video profile of the GEM avocado tree:

Where to buy a GEM avocado tree? See this post.

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