Oranges in Tucson

Can you identify this fruit?

When I first moved to Tucson (long before I discovered my passion for plants), I was truly excited by the large, evergreen tree that shaded our tiny yard. I had no idea what kind of tree it was until I spotted the little green fruit; it was a lime tree. However, even after they got larger, they were the hardest limes that were ever grown on earth (ever). A couple of weeks later the fruit began turning yellow, and we laughed about having thought they were limes (of course ...they were just unripe lemons). When the fruit subsequently began turning orange, we just felt dumb.

I've since gotten a little better at citrus ID, and I've also learned a bit about the cultivars that grow well in a Tucson garden. I've recently been compiling a list of the most commonly grown and sold cultivars for the home garden market, and the merits of each.

This is the first of a multi-part series of posts I'll be making throughout the month of March to share some of what I've learned, starting with oranges! Of course, this list isn't all-inclusive, and I expect to update it from time to time as my research continues. I've started the list with a focus on cultivars that have proven themselves in our climate, and are easily found for sale here in town.

So, without further ado, here's my list of oranges to grow in your Old Pueblo garden:

Navel orange is one of the best types of citrus for eating fresh because it’s easy to peel and seedless. This is the type of orange that we typically buy at the grocery store. The juice is also tasty, but it doesn’t store well, so there are much better choices for juicing. Navel orange is often sold as just navel orange, with no cultivar name.

Washington navel orange blossom

• Cara Cara is the only pigmented navel orange; its flesh looks similar to red grapefruit, but it tastes and peels like a navel. Originally from Venezuela, this is a relatively new variety that was discovered as a mutation on a Washington navel. Ripens December to January.  
• Washington is the variety you likely have if you purchased a navel orange with no cultivar name. It’s the most popular variety of navel orange we grow. The large fruit has good quality and flavor, ripening from November to January in the low desert.

Arizona Sweet is a name we use to describe several kinds of oranges that have a delicious sweet flavor. They do have some seeds, and they’re slightly harder to peel, but these oranges are still great for eating fresh. They’re better for juicing than the navel oranges, but not quite as juicy as a Valencia. Although there are several cultivars, these oranges are often sold with the name ‘Arizona Sweet’ rather than the true cultivar name for the tree.

Cluster of blossoms on a Trovita orange

• Hamlin has lower acidity, which gives it a very sweet flavor. The tree is vigorous and productive, medium large in size, and slightly more cold-tolerant than some of our other oranges. The medium sized fruit has 0-6 seeds, and though they’re smaller in the low desert than in more humid environments, the flavor is still excellent. Harvest from November to January in the low desert.
• Marrs ripens earlier than the other sweet oranges (from late October to early February in our climate), but the fruit holds well on the tree, and will have better flavor if harvested in November rather than October. The medium to large fruit is sweet and juicy, low in acidity, with a moderately thick rind and 7-10 seeds (though seediness can vary depending on cross-pollination…more cross-pollinators will make more seeds). It has a tendency to bear fruit in heavy clusters that can give branches a “drooping” appearance.
• Trovita is one of the best sweet oranges for the low desert. It’s more productive in our high heat and low humidity, but this variety does tend to bear heavily in alternating years. Fruit is medium to large and juicy with a thin rind and 6-10 seeds. The flavor is mild but sweet, ripening from December to February.

This will be delicious juice some day...(Valencia orange blossom)

Valencia is the best orange if you want oranges for juicing. The fruit is full of bright orange juice with lots of sugary goodness that stores well. It ripens late, reaching maturity around March. Though there are several cultivars of Valencia, the home garden market usually lumps them all together as ‘Valencia’ and sells them as such. The medium-sized fruit is extremely juicy with 0-6 seeds.

Blood Oranges don’t always give us Arizonans the deep red pigments that they’re so famous for (they often require a bit of a cooler climate for good color), but the fruit is just as tasty! Their red pigmentation comes from the presence of anthocyanin, which is the same pigment that gives pomegranate its deep red color. There are several cultivars that are easy to find here in Tucson.

• Moro is the most reliable blood orange for darker color in our warm climate. The rind and flesh can turn anywhere from bright red to dark purple in January to February (when they ripen). The flavor is berry-like with a hint of spice, and it has few seeds (if any). It tends to bear fruit in clusters, and produces larger crops on alternating years. 
• Sanguinelli ripens a little later than the other blood oranges (Jan to March). It has medium-sized, egg-shaped fruit with only a few seeds. ‘Sanguinelli’ is said to be slightly less acidic (sweeter) than ‘Moro’, and more reliable than ‘Tarocco’ for red pigmentation. Small to medium sized tree that is very productive.
• Tarocco originally came from the Mediterranean, so it’s well adapted to our climate. The rind is fairly easy to peel, and its color is primarily orange with little (if any) red pigmentation. The fruit is the largest of the blood oranges, and many agree that it’s also the sweetest. There are few to no seeds, and harvest time is in January or February. Though it’s said that ‘Tarocco’ is less productive than other blood oranges, it will still produce more than enough fruit for a home garden.

More Fertilizing Tips for Citrus

In February's issue of Tucson Happenings, I discussed fertilizing citrus trees in spring, and I decided that this is an important enough topic to impart a few extra tips on this month's blog.

To begin, I'd like to link University of Arizona's Fertilizing Citrus Chart, which I mentioned in the Tucson Happenings article.  It's a no-nonsense way of figuring out how many pounds of fertilizer you should apply to get the right amount of nitrogen to your tree. Keep in mind that this is the amount of fertilizer you should use in a year, and you'll be feeding 2 more times this year (so you only need 1/3 of the recommended amount each time).

Lisbon Lemon branch with swelling leaf bud.

I also mentioned in my article that the best timing of springtime fertilizer is just before the buds begin to swell, but I didn't have enough room to describe a citrus bud in any detail.

It's easy to notice swelling buds on deciduous trees with barren branches. Citrus trees hold leaves through the winter, however, so their buds are less exposed. You can see the swelling bud on this lemon stem just above the leaf scar.

Citrus trees will begin actively growing when temperatures are staying above about 55° F, so it might be easier for beginners to keep an eye on the weather than it is to watch the buds. Though it's common to hear that you should feed on Valentine's Day, it's better to watch for the right weather and these first signs of new growth. After a warm winter, like the one we've just experienced, you may want to feed a bit earlier in February to give your tree some great fuel for making new foliage.

If you're working with an organic fertilizer, you probably already know that it's not necessary to be so precise in the timing of application. You can feed with organic fertilizers even earlier in the year, because it takes them longer to break down in the soil.

The dripline of the tree is where many of the
absorbing roots are concentrated.

The last thing I'd like to touch on in this post is exactly where and how to spread the fertilizer once you've used the Fertilizing Citrus Chart to figure out how much of your fertilizer to use. You'll have to excuse my extremely rudimentary photo editing skills, but the red lines coming down from the ends of the branches at the right and left are meant to show the dripline of the tree. The dripline is the area under a tree where water would drip off of the ends of the branches (if we ever got any rain...).



If you have gravel, use a hard rake to expose the soil under the tree and a few inches beyond the dripline, avoiding the area just around the trunk. Spread the fertilizer evenly over the exposed soil, and use the rake to scratch it about an inch into the soil.  Make sure you water deeply and thoroughly all the way out to that drip line and beyond, everywhere that you spread the fertilizer (again, avoiding the area just around the trunk). Wait until the soil has dried out a bit before raking gravel back into place or walking under the tree.

Now you can put away the citrus fertilizer until Memorial Day!

Geeking Out: Why does a well-hydrated plant survive colder temperatures than a thirsty one?

As I prepare for my upcoming Chemistry class, I thought it would be good practice to do a little bit of research into what happens to a plant at the cellular level when it freezes. Having no idea how complex the subject would be, I promised to share this information with any of you other geeks who might be interested (I know there are at least a couple of you out there). 

There are a lot of online articles covering the practical things you need to know about protecting your plants from frost damage, including a great publication by Linda Reddick, published by the Arizona Cooperative Extension.  While some people wouldn't consider it to be practical knowledge, this post is about how the plant's cells are damaged.  Why does a well-hydrated plant resist freezing better than one that is thirsty?

I've read this fact in numerous publications, and I've witnessed it to be true, but I had little understanding of exactly why it's true.  The answer is much more complex than I had thought. Beyond the fact that a well-hydrated plant simply functions better than one that is thirsty, there's a lot to know about how hydration relates to frost damage.

There are several ways that plant cells can be damaged by colder temperatures, but they do have a few tricks up their...uhh...cell membranes.  One of these tricks takes advantage of osmosis to decrease the temperature at which its insides can freeze.

This happens when ice crystals begin to form in the spaces between plant cells.  The solutes that were previously suspended within the water molecules are left behind when these crystals form.  Water molecules outside of the cells continue to form crystals, until the concentration of solutes (sugars, etc) in the water left behind lowers the solution's freezing temperature enough to keep a small amount of (highly concentrated) solution lingering between plant cells.  *Phew, that was a long sentence*

This extracellular solution begins to draw water from inside of the cell because its osmotic pressure is higher than the solution within the cell.  In turn, the solute concentration increases within the cell and lowers the freezing temperature.  

With this in mind, it’s easy to see how a well-hydrated plant cell would be better prepared for freezing temperatures than one that has little water to begin with.  A well-hydrated cell can give up more water without suffering damage associated with severe dehydration.

Of course, there are other ways that freezing temperatures can damage plant cells…but we’ll have to save that for another edition of Geeking Out.  For now, we've answered the question of exactly why a well-hydrated plant has better chances than a thirsty one when the temperatures get low.  

I'll leave you with one parting thought that should make a little bit of sense if you've read through this post.  Plants that are resistant to freeze damage are often also fairly tolerant of drought.