Adrian Higgins on growing trees in containers, and all things gardening

By Jennifer Heffner For The Washington Post
Nov 08, 2018

Washington Post gardening columnist Adrian Higgins took questions on gardening.

For several years now, I've had a birdbath on the concrete patio just beyond my living room (I have a heating element for the winter). It's very convenient for me to dump out the water on the grass and refill it with the hose. This fall, the birdbath has been incredibly busy, with swarms of migrating birds (blackbirds? starlings?) gathering to drink and splash around. I've had to change the water every day, and it's very entertaining to watch. I highly recommend birdbaths.

Greetings to all. I hope you've had a chance to read my piece about growing hardy trees in planters, but don't worry, there won't be a test. Surprisingly, these trees need watering during dormancy, and so do the birds. The biggest need for birds in the cold months is water -- there's an abundance of berries and seeds for them to eat, but water is scarce. Provide water, keep it changed, and you will be rewarded. 

Producer here, with a link to Adrian's most recent column, on growing trees in containers.

I have two lovely Meyer Lemons that have winter residence on my heated sun porch. I also have grow lights from dawn til dusk. Generally I wouldn’t fertilize during the winter but these both have growing fruit. Fertilize? What kind? Thanks.

You cut back on fertilizing to induce dormancy but if you have them growing year round, that is not quite the same. However, with reduced light levels, I would not feed them heavily and hold back with the nitrogen. You could add a little slow release citrus food. Watch for mealybugs and scale insects.  

I live in the temperate climate of Ireland where tropical plants do not appear of their own accord. However, this summer we had a heat wave and it just so happened that I had turned over a plot of garden, previously and for centuries, untilled parkland, this spring. Situated close enough to the sea, this land suddenly produced a number of curious flowering species, the most interesting to me being a vigorous, black stemmed, Chilean, Potato plant with it's beautiful blue flowers. How did it get there? Can seeds lie dormant for a very long time and, given heat and light, germinate? As a gardener I have never seen one of these before and it certainly is not a native. What to make of this?

As you know, the West of Ireland is very temperate (if windy) and you find hedgerows of self seeded fuchsias, so your story doesn't surprise me. If this is the ornamental potato vine (Solanum), it's entirely possible that someone grew one in your vicinity, the seeds were dispersed and it was just waiting for the right conditions to germinate. Seeds can indeed lay dormant for many years until conditions are right. Germination is preceded by embryonic stimulation, which the heat wave may have initiated. 

what size is good for a tree on a patio?. I have heavy support roof but I planted trees in containers that are already too small for the tree - where can I get large containers?

I would get a container that is 24 inches square, and you will find cedar planters at garden centers. One brand is Bar Harbor Cedar.  (call first). That will easily accommodate a tree etc. with a rootball 12 to 16 inches across and allow for years of growth. 

What can I pour on the soil of my patio plants to kill pests etc. before bringing them inside for the winter

First establish that they have pests. The least toxic approach is to spray them with a light horticultural oil before bringing them in, making sure you cover all surfaces, including the undersides of leaves. Follow the directions on the label. 

Still wondering when to trim and grow raspberries in Northern Ohio.

There are two basic forms of raspberry. Everbearing types such as Heritage fruit in late summer and fall and produce on the current season's growth. Older varieties that fruit in early to mid summer develop from canes that grew the previous season. If you are not sure which type you have, you should now take out all the old canes (two years or more -- signaled by their darker color) and leave a few well spaced canes from this year. Tip them back to tidy them. That's about it. 

Plant seeds are, ahem, excreted by birds. In our case, an otherwise dear, sweet neighbor with bird feeders in her backyard would buy a seed mix that included thistle seeds. We live just above her property, and although we didn't have thistles in our garden plot before, once our neighbor began bird-feeding, we began a (decades-long) war to eradicate thistle plants from our garden.

Thistle seeds are fed to finches, but I'm not sure they are of a species that would grow here. Worth checking out. When thistles are young, they are quite easy to pull out (with gloves). After a winter in the ground putting down that tap root, that's another story. Stay on top of it. 

we have a lovely tree peony in front yard- but after it blooms and drops leaves it is quite ugly. Can stalks be cut down like a regular peony without losing the plant?

No, unlike herbaceous peonies, you cannot simply cut it back to the ground. For one thing, the buds for next spring are developing now. After it blooms next April, you can do some restorative pruning, removing branches that are congested, crossing or weak. But don't over do it. 

We just had major drainage work done on our yard (we live in Ellicott City, need I say more) and the regrading and re-seeding and covering with straw was done just before two mature maples shed their leaves onto the straw. Help, how do we do this? We usually "vacuum" the leaves up with a mulcher/blower and put them in our compost or use them as mulch, but how to we avoid ruining the new seeding?

This is one of the rare moments when a leaf blower (the scourge of autumnal peace) can be used to remove the leaves without disturbing the grass seedlings. Keep it on a low speed (if you can) and keep the nozzle as far above the soil level as you can while still shifting leaves. Be patient and recognize the maple may keep on dropping leaves for another month. Don't try the vacuum in this circumstance. 

We planted an edgeworthia in the front yard and need to prune it. What is the best time of year to prune it? We get so many questions about the bush; its got a lovely shape and winter/early spring display is quite interesting. Just had no idea how much it would grow each year.

Edgeworthias develop into large and tropical looking shrubs, but this is a winter bloomer and the buds are ornamental through the winter. Again this is  a shrub to prune in the spring after flowering. 

There were a couple of lovely peach trees in a lot near my house in Baltimore. The benighted owner had them cut down a few months ago. However, they bore a lot of little peaches and I found a seedling and potted it up. It's about a foot tall and in a 12-inch pot. Should I leave the pot outside for the winter or bring it in the house? I'm not sure if it needs a dormant period. Should I keep it inside for a winter before planting it outside? I am not expecting to get a crop of edible fruit from this; I just liked the trees and I hope that the seedling will be a pretty small tree like they were. (I know this takes time. I can wait.)

I admire your peach recycling effort, but it's not something I would do because there is no guarantee that the seedling will produce fruit worth growing. The parent variety is cloned but the seedlings could be too small, too bland, etc. Peach trees don't live terribly long, maybe it was time for your neighbors' trees to go. I would plant a named variety if you want to cultivate peaches. 

Hi Adrian, What fruit trees do you recommend growing in a pot/wood planter and what size would you recommend building the planter box? I am planning a roof garden that will get full sun and I would like to have a few larger fruit trees that will survive the extreme temps of winter and summer. Do you have any recommendations on growing vines or a wall covering in a pot? What size pot would you suggest for that and which plants grow the best? Would a grape vine do well? Thank You!

Again, I would use a standard 24 inch planter. There are dwarf apples you could try and some now grown as single columns. They bear early but don't live for decades. You will need to figure out how many varieties you may need for good fruit set. You could try some more obscure things, such as the medlar. A Grapevine would work but do select a variety tolerant of hot, humid climates. I would not recommend blueberries (too stressed in that setting) or Asian persimmons, which might not be hardy enough in a pot. A hardy kiwi might be a good choice for wall covering. 

What would cause flowering plants to overgrow but in some cases have limited blooms. I planted cosmos in my west-facing garden that grew to monster like proportions (thick stems and lots of leaves) but barely bloomed. Same with the black-eyed susans. The crepe myrtles droop their heavy branches instead of staying upward like their sister in another part of the garden. The soil is compacted sand but amended heavily. What condition would do this and how to I fix it?

"Amended heavily" may be the operative words. If the soil is too rich, plants that like lean soil will produce a lot of growth at the expense of flowering. The other issue is that this year we had rainfall measured in feet. Try again, but next year starve the cosmos a little. 

Hi, I would appreciate some suggestions as I have clover and strawberries growing in my grass. What can I do to get rid of the clover and strawberries? I live in Rockville, MD with my dog. Thanks!

Clover is pretty benign but I admit the wild strawberry is a pain and can take over a lawn. You can remove it with a weeding knife, but it's laborious work. The bigger problem is that the weeds are invading space left by the retreating turf. It's really late now to be doing lawn renovation. You could have applied a herbicide in August and re-seeded a few weeks later. A herbicide now would impede any seed germination. If you can't stand it, I would take a thatching rake and slice out the strawberries as much as you can, make sure you rake up all the pieces, and then put down some turf type tall fescue seeds (assuming you live here or more north). Do a second seeding in March. 

Good afternoon, we have a false indigo plant in our front garden. It handles the hot afternoon sun and is also deer resistant so has been great in its location. The plant is beautiful in the spring when it blooms with pale yellow blossoms. However, after blooming this year, the leaves turned black and the plant looked unsightly for the rest of the summer. Is that from all of the rain? New leaves started showing up this fall. Is our area amenable to false indigo? I can't decide whether to pull it out or not. Thank you very much.

False indigo or baptisia is a perfect native perennial for our region, but it grows to the size of a small shrub and is best regarded as a woody for planning combo purposes. Yes, the leaves will look rough after flowering in a really wet year like this one. There were all sorts of foliar issues this summer. If you can't stand it, remove them. In the season now upon us, in which perennials take on a ghostly beauty, baptisias provide an incredible dusky black ornament. 

I found a praying mantis egg case on our large orange tree. We bring it indoors for the winter. What do i do?? It is (of course) on a main branch so cutting it off and leaving it outdoors is not an option.

Maybe you could carefully scrape it off the branch and place it in a sheltered, elevated spot outdoors. I imagine you don't want dozens of bewildered mantises hatching out indoors this winter, all dressed up with nowhere to go. 

I have a teeny tiny garden in front of my condo which I am planting with flowering perennials. Every year I find I want to move some around -- should I do it now or wait until early spring?

September to November is a good time to move hardy perennials. Check them through the winter to make sure that freeze-thaw action hasn't exposed the roots to the air. 

I have such problems finding ones big enough for trees but light enough to be able to move indoors. any suggestions?

My article was about trees that stay outdoors year round. I agree, a large planter with a tree is too heavy to haul around, especially unaided. It is important to locate it in the right spot before planting.  

Winter is deep in Ottawa, Ontario. I generally take in my large potted oleander in the Fall, but it invariably gets, I think, aphids (white, cottony bugs) on the burgeoning flower buds, come January. In order to avoid infestation in my house this year, would it be possible to overwinter the oleander in my cold garage -- minus 5 degrees Celsius-- with the pot burlapped to prevent freezing? There is light from 2 windows in the garage.

It might make it at - 5C. If the alternative is to ditch it, give it a try.  

My house has a sheltered patio that is deeper than the surrounding lots and that has no external egress (its former egress to an alley was removed many years ago). As such, it creates a little micro-climate. I have a hibiscus that was beautiful this spring and summer. It's still healthy but not blooming right now. Should I bring it indoors for the winter? If I leave it outdoors, will it come back?

The tropical hibiscus won't make it outside. Alas, it really doesn't like the dry, overheated and dark environment of the winter room. If you can find a cool, bright spot for it, keep it there, accept that it may drop leaves, but should perk up in mid-spring, when it can be repotted and placed back outdoors in a sheltered, shady location. 

My mom has a built-in planter area beside her front door stoop. Probably measures 30" x 30" or less with about the same headroom. Any suggestions on something low maintenance that would work there? Was thinking of blue juniper but that seems dull.

If it only has 30 inches of top space, you would have to grow some sort of hugger. A juniper would be okay, but a little dull as you say. What about a prostrate yew or a Grey Owl juniper. Sadly, we have run out of time and I'm sorry I couldn't answer all your questions. Thank you and see you here again soon.

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Adrian Higgins
Adrian Higgins is The Washington Post's gardening columnist. Read his latest columns and stories here and follow him on Twitter.
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