August 2020 – The Garden Professors™

A time to deadhead!

Summer is here in the west in a big way. We are just coming off of one of the largest heat waves ever recorded, and while temperatures are down they are not done. Its hot. Depending on where you live your gardens may have suffered. In the East Hurricanes are starting and extreme rains are occurring. I have images of bent over palm trees in Florida. No matter the season, plants respond with their own growth stages providing they are not blow away or burnt up by raging wildfires. Here in Arizona we have had moderately hot weather in my location but the garden is surviving with irrigation. My Iris plants remind me that it is long past time to deadhead and remove spent flower stalks. Deadheading is second nature to most gardeners and other than making the garden look better, you may not realize why you have or have not adopted this common garden practice.

Deadheading involves removing the spent flowers or inflorescences have withered. Sometimes pruning back to a lower leaf or adventitious bud in the case of roses, or completely removing flower stalks in the case of German Iris is required. The immediate result is a neater looking garden and an emphasis on remaining blooms. When the dead flowers are gone the remaining flowers look better the garden is refreshed. Depending on the plant there can be other benefits if deadheading is done consistently and is well timed.

This portion of the flower garden needed deaheading weeks ago. Even though I won’t advantage the plants by deadheading it will look better if I do.

We grow many kinds of plants in our gardens and deadheading has varied physiological impact depending on the subject being pruned. Properly timed, deadheading can extend the bloom of some plants for example Calendula. However, Calendula produces lots of flowers and removing spent flowers can become an enduring task if you have a lot of Calendulas. Deadheading some garden plants seems pointless such as impatiens which just regenerates flowers on its own. Deadheading soon after a flower passes prevents the plant from investing energy in seed development. If the plant has a long enough bloom cycle, so that energy can be put into other flowers then trimming back the flowering stem stems that are destined to fruit production often releases other buds to grow more flowers. Since photosynthate (sugars) flows in plants on a source-sink model, taking away the “sink” or developing fruit allows energy to be used for growth elsewhere in the plant. The trick is to remove spent flowers soon because seed begins to form immediately after flowering and the plant will rapidly allocate its energy to reproduction once the flowers are pollinated.

with dead heads removed this corner of the garden looks a bit better

Not all garden plants respond to deadheading–the number of flowers some plants present is genetically regulated and dead flower removal does not promote more flowering (many bulbs produce only one set of flowers). Other garden plants will re-bloom if given a chance, and with deadheading (no matter what the flowering habit) the garden will look better without the dead flowers. Some bulbs can be deadheaded to prevent seed formation so that the energy is put back into the bulb or bulblets for next year’s display. Many roses will re-bloom after deadheading. This is not a wild-type characteristic of roses but a quality that has been selected for after years of plant breeding.

Deadheading can also be an excellent method of excluding diseases. Botrytis on rose blossoms and petal blight on Camellia are both controlled to some degree by removing infected blooms as soon as they are observed and disposing of them away from the garden.

This bag contains Penstemon with ripe seed heads ready for harvest and seeds for planting somewhere else in the garden.

Sometimes deadheading results in seed collection. Left too long, some plants go to seed but have not yet released their seed. If you want to save seed for propagation, strategic deadheading will allow you to collect seed while redirecting the plants growth patterns for more vegetation or more flower shoots. It is also helpful with our more ruderal garden friends to remove flower stalks to prevent their reproduction and taking over of smaller garden spaces that endure frequent cultivation or soil disturbance. Some plants are desirable but their progeny are a bother….

Some like it hot… but most do not: How high temperatures delay pollination and ripening

Ah, summer – vacations (pre-COVID), swimming pools (pre-COVID), ice cream, vegetable gardens, and, in many places, really high temperatures. These things all go hand-in-hand (or at least they did before the pandemic). Many gardeners feel that the heat of mid-summer goes hand in hand with garden production; those high temps driving production on those fruiting plants like tomatoes and peppers. But…..could they be wrong?

We’ve had lots of extra hot days this summer in Nebraska, so it stands to reason that we should have really great production on those garden favorites like tomatoes, right? Then tell me why our extension office has received numerous questions this year about why tomatoes aren’t setting on or ripening. Heck, we even had a Facebook post about tomatoes not ripening in the heat go viral (well, for our standards – 300,000 views/2,000 shares). Could it be a disease? Nope – it’s the heat. High daytime temperatures can have a big effect, but the effects are compounded when nighttime temperatures are high as well.

Tomatoes not ripening? You're not alone. Temperatures above 85 degrees will slow down the ripening process. Temperatures above 95 can stop the process all together. #NebExt #NeWX
Posted by Nebraska Extension in Douglas-Sarpy Counties on Wednesday, July 22, 2020

It turns out that high heat does two things in many of those fruiting vegetables (and of course fruits) that we grow. First, it inhibits pollen production, which in turns reduces fruit set. Second, heat inhibits gene expression for proteins that aid in ripening/maturation of the fruit. Heat stress also reduces photosynthesis (Sharkey, 2005) in many different plants, which would slow down plant processes (such as fruit development and ripening) as it reduces the availability of sugars to fuel these processes. So high heat can not only reduce the number of fruits developing on the plant, but also slow down the ripening process for fruits that have already set. And if you think that these effects only happen at super extreme temps, most of the research studying temperature effects of this nature use a common “high ambient temperature” of 32°C/26°C for daytime/nighttime temperatures. For us U.S. Fahrenheit-ers, that’s 89.6°F/78.8°F, which isn’t really all that hot for most of us.

Many studies show that application of this “high ambient temperature” to crops such as tomatoes, beans, and corn during the pre-fertilization phases of reproduction (ie – flower/pollen development) can negatively effect fruit set. The introduction of Porch and Jahn (2001) gives a pretty good overview of literature detailing the effect in beans (Phaseolus vulgaris). I’ll sum it up here: heat stress while the pollen is forming (called sporogenesis) led to pollen sterility and failure of pollen to release from the anthers (dehiscence). It also led to flower abscission (basically the plant aborts the flower) and reduce pollen tube formation (how the pollen nucleus gets through the stigma to the ovule for pollination) when applied during the period of pollen sac and ovary development. And application during flower opening (anthesis) resulted in pollen injury (sterility) and reproductive organ abscission. All of these effects lead to reduced fruit/seed set in beans. (Interestingly, heat stress at the ovary development phase also led to parthenocarpy – basically the pods developed, sans seeds, without fertilization).

However, we get the most calls about tomatoes (they’re the top crop for most home gardeners). Is it the same issue? Yep. Numerous studies (Sato, et al., 2000; Pressman, et al., 2002; Abdul-BAki, 1992) show the same effect in tomatoes. Pressman, et al. (2002) linked the effects on pollen to changes in carbohydrates in the anthers (reduced starch storage and carbohydrate metabolism).

Tomato pollination and how to increase it in high tunnels — Tomato floral structures

To add insult to injury, high temperatures also slow down or stop ripening of crops like tomatoes. Picton and Grierson (1988) found that 35°C (95°F) temperatures altered the gene expression in tomato fruits – inhibiting the expression of polygalacturonase, which softens cells walls, allowing the fruit to ripen. Reduced photosynthesis would also reduce the availability of sugars for fruit development and ripening.

But there’s hope, both this season and in the long term! The effect on the plants is not permanent. When temperatures drop below that “high ambient temperature” threshold pollen production, and therefore fruit set, will return to normal (as long as the plant is healthy). Sato, et al. (2000) found that pollen release and fruit set resumed within a few days after heat stressed plants were “relieved” and temperatures dropped back into the optimal range of 26-28°C/22°C (78.8-82.4°F/71.6°F). So many of those plants will become productive again (good news for my own tomatoes and beans, which had an initial flurry of production then went on vacation), especially as we head into fall. And efforts are under way to develop and test heat stress resistant cultivars.

This last point may be more important than you realized. These production problems plague many areas around he world at current climactic norms. Many fear that increasing temperatures will limit the productive capacity of many areas of the world that are already struggling. It is easy to see how the difference in just of just a few degrees can take your veggie production from prolific to paltry.

You can also try to reduce the heat a bit yourself for an immediate fix. Shade cloth can help reduce temperatures a little bit, which may make all the difference in your garden if you’re just slightly over the “high ambient temperature” threshold.

Tomatoes under shade cloth | Tomatoes under shade cloth | Flickr — Tomatoes under shade cloth | Source: flickr.com

But in the meantime, if your vegetable garden has taken a summer siesta it will get around to producing again one day. You’ll just have to take good care of the plants in the meantime. And perhaps it’s a blessing in disguise – when its that hot I don’t want to be out working in the garden much, either.

Sources

Abdul-Baki, A. A. (1992). Determination of pollen viability in tomatoes. Journal of the American Society for Horticultural Science, 117(3), 473-476.Porch, T.G. and Jahn, M. (2001), Effects of high‐temperature stress on microsporogenesis in heat‐sensitive and heat‐tolerant genotypes of Phaseolus vulgaris . Plant, Cell & Environment, 24: 723-731. doi:10.1046/j.1365-3040.2001.00716.x
Pressman, E., Peet, M. M., & Pharr, D. M. (2002). The effect of heat stress on tomato pollen characteristics is associated with changes in carbohydrate concentration in the developing anthers. Annals of Botany, 90(5), 631-636.
Sato, S., Peet, M. M., & Thomas, J. F. (2000). Physiological factors limit fruit set of tomato (Lycopersicon esculentum Mill.) under chronic, mild heat stress. Plant, Cell & Environment, 23(7), 719-726.
Sharkey, T. D. (2005). Effects of moderate heat stress on photosynthesis: importance of thylakoid reactions, rubisco deactivation, reactive oxygen species, and thermotolerance provided by isoprene. Plant, Cell & Environment, 28(3), 269-277.

What’s wrong with my tree? You won’t find the answer in a book.

This tree suffers chronic drought stress every summer. Why?

It’s the middle of summer, and maybe you’re wondering what’s wrong with your landscape tree (or shrub) that just doesn’t seem to be putting on the growth that you’d expect this time of year. Before you take any “corrective” action, let’s figure out what the problem might be. Here’s a short checklist that we will start with. (NOTE: This is just a start. You can go so many different directions once you have some specific concerns to explore.)

Do you have one of these? If not, you can’t adequately diagnose problems.

Soil information. Have you had a soil test done in the last few years? If so, are there any nutrient toxicities indicated? Has the soil been significantly disturbed or modified in the last several years? Have you recently added any chemicals (fertilizers and pesticides, organic or otherwise) or amendments?
Plant information. When was the plant installed? Was it in a container or in a burlapped rootball? If so, were all materials removed from the roots by root washing before planting?
Planting information. Did you amend the soil (i.e., add anything to the backfill) prior to planting? If so, what did you add? Did you mulch it afterwards? If so, what is your mulch material? Did you ensure that your plant was set at grade in the landscape? (“Grade” means that the root flare is at the soil surface.) Did you water it in well and avoid compacting the soil? Are new plantings adequately irrigated during their first year in the landscape?
Environmental information. Have there been unusual weather events between time of planting and now? Is there sufficient irrigation and drainage?
Symptoms. What are you seeing that concerns you?

Intact clay rootball after 28 years (and yes, the tree died long before this photo was taken).

At least 95% of the landscape failure cases I’ve diagnosed over the last 20 years can be traced back to improper planting methods. You simply cannot pull a woody plant out of a pot and stick it in a hole. There are three major factors at play here to consider when rootballs are planted intact:

Think that this root system can straighten itself out? Think again.

The textural and structural differences between the soilless media around containerized roots (or the clay in a B&B rootball) and the soil in the landscape are significant enough that they will impair water, air, and root movement across the interface. This means roots have a difficult time establishing outside the planting hole.
Any structural flaws in the root system created during improper potting-up at the production nursery, such as circling or J-hooked roots, are undetected and uncorrected. And these woody roots will stay in a death spiral after planting.
If you cannot see the root flare of your plant, then you cannot plant at grade. Most trees and shrubs that are buried too deeply will generally fail to thrive and eventually will die.

If you can’t see the root flare, you’ve got a problem. See the next photo.

If you’re like the majority of people who are seeing problems this time of year, you know that improper planting or severe soil disturbance is to blame. But now is not the time to fix it! You’ll need to wait until the fall, when the crown has gone dormant, to dig the plant up and take corrective action. (The “corrective action” has been discussed in this blog before; you can explore the archives or wait for an upcoming post).

These are the roots of the tree at the top of the post. No root flareNo surprise that it’s chronically water stressed in the summer, given this pathetic root system.

What you want to do right now is keep your plant as healthy as possible by mulching with coarse wood chips (not bark) and supplying them with adequate water. You DO NOT want to prune them, because that just uses up stored resources as the plant then replaces pruned material with new shoots and leaves. You DO NOT want to add fertilizer, unless you know that you have a nutrient deficiency (which you can’t know unless you’ve had a soil test. And no, those cute little diagrams of what nutrient deficiencies look like in corn leaves are worthless. You’re not growing corn here.) And DO NOT add any pesticide of any sort, even if you see signs of insect or disease damage on the foliage. With few exceptions, pesticides are broad-spectrum and you will kill beneficial species as well as any possible pests. Opportunistic pests and disease attack stressed plants, and that’s why you are seeing them.

Crown pruning just results in more crown growth. Don’t do this if you are planning to move a woody plant during the current year.

In the upcoming months, I’ll do some follow up case studies that can help you learn how to diagnosis problems. If you’re interesting in having your tree or shrub problem diagnosed and can supply sufficient information (as outlined above) and clear photos, leave a comment on this post and I’ll contact you.