Trees to Avoid in Your Home Landscape

Pyrus calleryana

When you see these trees coming, run the other way…

We’ve all met that person – the one that seems great, but as time goes on, reveals themselves to be a gossip or a liar. Well, the tree universe has those people too. I’ve chosen a list of common trees (and a vine) that seem to cause problems with most of the homeowners I talk to. Major concerns with these trees include damage to houses and property; allergies; messiness; invasive behavior; and flammability.

The list of trees you should avoid planting changes depending on your criteria, the place you want to put the tree, your climate, and several other things. A “problem tree” may be worth the extra work to you, or they may be a fine fit for the place you’re putting them. A liquidambar tree is beautiful and fine as long as its roots can’t reach any pipes or sidewalks, for instance.

We are lucky to live in a mild climate where we have so many choices in trees and plants. Why choose a tree that’s a troublemaker?

A note about allergies: Olive and ash trees are highly allergic to many people, and I recommend not planting any but the sterile varieties in an urban or suburban setting. Pollen from your tree can make allergy sufferers in a couple block radius of you miserable for months out of the year. It’s not worth the bad karma.

They’ve got issues

These two trees have (deservedly) bad reputations. Stay clear!

Bradford Pear (Pyrus calleryana)

Bradford pears are top heavy and prone to splitting. It’s not uncommon for the Bradford pear tree to split in half, especially during wind or rain. Sure, all those white flowers are beautiful in spring, but it’s not worth losing a part of your house over.

Pyrus calleryana

Ash (Fraxinus)

Beautiful street trees that until recently were resistant to severe weather, diseases and pests, ash trees now face devastation from the emerald ash borer (EAB). The insects spread from Asia to Michigan in 2002, and as of last month to the east coast and as far west as Colorado. So far efforts to slow or stop the spread of EABs have not been successful and we expect them to reach the west coast at some point. Stay out of future trouble and don’t make an investment in an ash tree.

Emerald ash borer damage

Damage alert

Be careful of these trees, which can wreak havoc on your property with roots and branches.

Eucalyptus (Eucalyptus)

Imported from Australia and popularized for their speedy growth — some varieties will shoot up 10 feet in a year — the eucalyptus has a bad rap for suddenly and unexpectedly dropping big, heavy, resin-filled branches. In some areas of Australia, campers are warned not to pitch tents under eucalyptus trees. They are also highly flammable.

Eucalyptus alba

Liquidambar, Sweet Gum (Liquidambar styraciflua)

While it can be beautiful, especially in the fall, the sweetgum’s spiny brown balls, which come down by the thousand, are a major drawback. More importantly, this tree has a huge and destructive root system. Unless you have enough land that the roots can’t interfere with houses, fences or plumbing, leave this tree in the park.

Liquidambar seeds

Wisteria (Wisteria)

Not technically a tree, but deserves a place of honor on this list. Wisteria, with its brilliant, cascading purple blooms, is tempting for a gardener who loves flowers—but beware! Its root system can send shoots popping up far away from the main plant, engulfing trees, shrubs, and anything else in its way. It can live hundreds of years and requires serious pruning every year to keep it under control. Large wisterias have brought down trellises, pergolas, and even roofs.

Wisteria sinensis

Such a Slob

From flowers and nuts to seeds and seedlings, these trees keep you busy cleaning up after them on weekends.

Black Walnut (Juglans nigra)

Native to North America, this beautiful shade tree produces prized cabinet- and furniture-making wood. It also produces pollen and plenty of fruit that’ll drive you nuts when you have to clean it all up in the fall. Its true sinister side, however, is that it secretes growth-inhibiting toxins that kill nearby plants, wreaking havoc on anything that tries to grow nearby.

Juglans nigra

Mimosa (Albizia julibrissin)

Plant one, and you’ll be pulling seedlings up constantly—and so will everyone around you. There are some things you just shouldn’t share with your neighbors.

Albizia julibrissin

Russian Olive (Eleagnus angustifolia)

This rogue tree is thirsty and aggressive, so often crowds out surrounding plants. They are good at looking messy and thorny, are nearly impossible to kill, and drop olives everywhere (which I guess is a plus if you’re planning to harvest them).

Elaeagnus angustifolia

Gingko biloba, Maidenhair (Gingko biloba)

Female gingko biloba trees produce very stinky and messy fruits in late fall, which stick to shoes and can get tracked indoors. There’s no way to distinguish the male and female trees until they mature. If you really want a gingko biloba, buy a tree marked ‘Autumn Gold’ or ‘Lakeview,’ which are male-only varieties.

Gingko biloba fruits

Great Balls of Fire

I’ll be writing about landscaping for fire safety in the next few months, but for now I’ve included a quick list of the most flammable trees commonly found in home landscapes. I do not recommend any of these for the southern California garden.

Arborvitae (Thuja)
Acacia (Acacia)
Cedar (Cedrus)
Cypress (Cupressus)
Douglas Fir (Pseudotsuga)
Eucalyptus (Eucalyptus)
Juniper (Juniperus)
Some palms
Pine (Pinus).

Caring for your garden in extreme heat

Thermometer reading 100 degrees

Plants aren’t that different from the rest of us in many respects. When it is very hot outside, we need shade, sun protection and hydration, and so do our gardens. Plants carry some tools with them to deal with sun, heat and wind. Many develop light or grayish leaves or fuzzy leaves that act as sunscreen. Some rotate their leaves away from the sun all day like little anti-solar panels. Some just go dormant in summer, a plant’s version of hibernation. But when temperatures climb to 100 degrees and above, most plants appreciate some extra assistance. There are several things you can do to help your plants withstand the weather.

Apply mulch

Deep mulch in the garden

Bark chips, grass clippings and old leaves are all great for helping soil retain moisture. Mulch also creates shade, which keeps the soil cooler. If you use old grass clippings or even straw, the light color will even help to reflect sunlight.  Mulch also helps protect soil from wind seeking to dry it out. Use a two to three inch layer for best results.  Use mulch made out of biodegradable materials that will decay over time and feed the soil. If you use rocks as a ground cover, don’t add mulch to this. You are already helping your plants. Even rocks provide shade and help to keep moisture in the soil.

Be careful to keep the mulch away from the immediate area of the plant’s stems or tree trunks. Mulch is so good at retaining moisture that if it’s too close to the plant it may contribute to fungus or root rot. And just pour the mulch on top and rake it – don’t mix it into the soil or it will pull nitrogen (nutrients) out of the soil as it decays.

Set up a shade structure

Build shade structures for stressed plants

Shade cloth or row cloth is often seen in vegetable gardens, but I also use it for ornamental plants, particularly when the planting is less than a year old. I build tiny shade structures out of a sheet of 30% shade cloth, dowels and garden ties or staples, although I’m sure there are many ways to do this. It helps to position the cloth to the south or west side of the plant or directly above it. Make sure you’re giving the plant room to breathe, and don’t forget to remove the shade cloth when the heat wave is over.

Water early and deeply

Water as early in the day as possible, particularly if you have sprinklers since most sprinkler water is lost to wind and evaporation during hot times of the day. Additionally, watering leaves when the sun is directly overhead can cause leaf scald. Watering too late in the day is deadly for many plants. For low-water plants that are adapted to dry summers, sitting in warm soil and being damp at sunset and into the evening is a recipe for fungal infections that can lead to root rot or sudden death.

Hose watering tree

Using drip irrigation or soaker hoses (or just a hose laid on the ground) gives you a lot more flexibility. Leaves don’t get wet and if positioned correctly, the plant itself will not get as wet so watering during the day or early evening is not as harmful.

How much water to give your plants during a heat wave depends on the type of plant and how long the plant has been in the ground. If I have designed your garden, you have low water plants that will not appreciate being deluged during a heat wave any more than they would at any other time. What they will appreciate is an extra long, slow watering or two during the week. Drip irrigation, soaker hoses or just a trickling hose set next to plants are the best ways to deliver extra water.

Plants that have been in the ground less than two years are still developing a strong, deep root structure. The heat drying out the top few inches of the soil has a bigger impact on them. Take extra care to keep that top layer from drying out into an impenetrable brick. Only you know how quickly your soil dries out, but maybe increase to watering three times a week during very hot or dry weather. Young trees should get two to four inches of water per week once temperatures hit 100 degrees. Again, a trickling hose or soaker hose is best for this supplemental water.

Prepare when you can

After two years when plants are well established and their roots are deep, you should be able to water deeply one additional time per week when it’s really hot and/or dry. Many of you only water monthly in the summer, so just do one deep watering a week in extreme heat.

If you’ve done the work to water deeply and infrequently the prior two years you have helped your plants to set up deep, extensive root structures and are set up for success, so pay attention to your soil as you are establishing your garden. It will pay off later.

Plant with wilting leavesWhat about wilting leaves? Garden author and TV/radio personality Nan Sterman offers a great tip to help gauge how stressed your plants are in the heat by when their leaves wilt. She says “if they look droopy at the end of the day, don’t worry. If they look droopy first thing in the morning, worry.” Read the full article.