Tips for late summer – Bonsai Tonight

When everything goes well, there isn’t a lot of bonsai work this time of year. Deciduous trees are recovering from their last pruning of the season, decandled pines are filling in, and tropical bonsai are getting their final cutback of the season.

That said, the next month is pivotal for improving and maintaining tree health. It’s the last opportunity for fertilizing trees before they go dormant or applying systemic pesticides and fungicides. It’s when wires cut in and caterpillars are hungry. In short, it’s the last chance of the year to steer our trees toward greater health and away from subtle decline.

Here are some of the late summer tasks I’m currently working on.

Targeted fertilizing

I like giving most trees in the garden ample fertilizer before they go dormant to help them grow strong the following spring. But for some species like black pines, I dole out fertilizer based on each tree’s needs.

Here’s a decandled pine with small summer buds. Because the buds are so small, I’ve already started to fertilize.

Small summer buds on black pine

When the summer buds are really vigorous, I hold off until later in the season before I start fertilizing.

Strong summer growth

I’ll start fertilizing the pine above in mid- to late-September.

For the rest of the trees in the garden, I’m fertilizing regularly with liquid and/or solid fertilizers.

Pest control

Aphids, thrips, and mealybugs can be trouble year-round, but some insects are most active in summer. I’m always on the watch for spider mites as they like hot and dry conditions and reproduce quickly. To keep their numbers down, I overhead water conifers as this can help wash the mites away.

Another pest that can be problematic this time of year is the caterpillar. Here’s a photo showing typical caterpillar damage.

Caterpillar damage on flowering quince

Leaves that are stuck together are a sure sign of caterpillars. If you peel the leaves apart in time, you might find the culprit.

Can you see the caterpillar? (It’s on the margin between the dark and pale portions of the leaf)

I like to keep the insect numbers down this time of year as plants don’t respond as well to insect damage when they’re dormant. Late summer and fall are also the last chances we get to apply systemic pesticides as trees don’t absorb pesticides as well when they are dormant.

Fungal pathogens

Late summer is also a great time to get ahead of fungal pathogens. Some of these attack the foliage, others make trouble for the roots.

Here’s an example of damage caused by a foliar pathogen.

Spots on crabapple leaves likely caused by a foliar fungus

When leaf damage is random on the leaf surface, fungal pathogens are a likely culprit. When the tips of leaves are damaged, the problem is usually in the soil. Here are examples of symptoms that likely result from “root unhappiness,” as a local plant pathologist put it.

Brown tips

Curled leaves

Yellow leaves that curl downward

Chlorosis is a related problem as iron deficiency can result from a variety of root problems ranging from pathogens to poor drainage.

Yellow leaves

The above examples are all from plums planted in the same container. As many of the leaves are in poor shape, now is the time to fertilize and treat any underlying problems before the leaves completely fall off.

Wire scars

Any trees that are actively growing can cause swelling that leads to wire scars. Limited scarring is OK in some cases, but deep scars are rarely acceptable.

Wire scar on black pine

Using flags or colored tape to indicate trees with wire can be a good reminder to check and see if swelling branches need attention.

Watering and heat protection

Late summer can bring high temperatures that require careful watering. Some trees need more water when it gets hot while others shut down to conserve resources. I check the soil more often on hot days as I’m often surprised which trees dry quickly and which don’t need as much water.

When the temperatures reach summer highs, I take a variety of steps to protect them from sunburn or general stress. For details, see “How to care for bonsai during heat waves.“

Unsolved mysteries

Late summer and fall are often our last chances for the year to use leaves as a clue to figure out why a tree might be unhealthy. Deciduous trees lose their leaves in fall and many conifers turn yellow in winter which makes troubleshooting difficult.

I found several yellow pines in a flat that was otherwise healthy which made me suspect root aphids, but after a quick check, I didn’t find any pests in the soil.

A healthy pine and an anemic pine

Given that the pines are in the same soil, on the same table, and getting the same water and fertilizer tells me it’s time to investigate further. For starters, I’ll isolate the yellow trees and water them less to rule out over-watering. I’ll also check more closely for root aphids. Beyond that, I’ll make sure these trees receive ample fertilizer to help with the chlorosis.

The above items are only a partial list of things to check in late summer. Feel free to add your own suggestions in the comments below!

News and Updates

• The upcoming U.S. National Bonsai Exhibition in Rochester is sold out. The event will be held as planned but tickets will not be available at the door. For a complete update about the show plus information about two private bonsai collections coming up for sale, see the recent update from the Valavanis Bonsai Blog.
• A new episode of the Bonsai Wire podcast is available: catch the discussion between host Andrew Robson and Sergio Cuan at bonsaiwirepodcast.com.
• My annual tree sale will run for one more week – shop the sale here.