Northern Colorado tree care calendar

Winter — December to February

Water. The single most under-done job in Northern Colorado. Dry, windy winters with no snow cover kill roots, particularly on trees planted in the last three years. Water once or twice a month when temperatures are above 40°F and the ground is not frozen, in the middle of the day so it soaks in before nightfall.

Prune. Dormant season is the best window for most species. Structure is visible without leaves, and cuts close before insects become active. Elms and oaks specifically should only be pruned now, to avoid attracting the beetles that spread Dutch elm disease and oak wilt.

Watch for sun scald. Winter sun on the southwest side of thin-barked young trees heats the bark, which then freezes at night and splits. Light-coloured tree wrap on young trunks from November to April prevents it. Remove it in spring — wrap left on year-round girdles trees.

Spring — March to May

Storm season. This is when Greeley's most destructive tree damage happens. Heavy wet snow on trees that have already leafed out breaks unions and takes out whole tops. There is nothing to do in the moment except stay clear of loaded limbs, but a tree that was thinned the previous winter fares dramatically better.

Finish dormant pruning early. Once buds break, the window closes for structural work.

Plant. Good season for it, provided you can water consistently through the first summer. See tree planting.

Remove tree wrap as temperatures moderate.

Watch for chlorosis as leaves emerge. Yellow leaves with green veins on new growth is iron chlorosis, and cool wet springs make it worse. See iron chlorosis treatment.

Summer — June to August

Water deeply and infrequently. A long slow soak out at the drip line every week or two beats daily sprinkler cycles. Frequent shallow watering produces shallow roots, which is exactly what you do not want going into a windy autumn.

Inspect ash. Now is when EAB symptoms are visible. Look for upper canopy thinning, sprouting from the trunk base, and heavy woodpecker damage. See emerald ash borer.

Watch for spider mites on spruce during hot dry spells — stippled bronzing on the needles.

Do not fertilise a drought-stressed tree. Water it. Fertiliser on a tree that cannot take up water makes things worse.

Avoid major pruning in high heat beyond removing hazards and deadwood.

Autumn — September to November

Best window for deep root fertilization. Roots stay active well after the canopy shuts down, so nutrients are in place for spring. See deep root fertilization.

Plant. Autumn planting lets roots establish before the following spring's demand.

Water thoroughly before the ground freezes. A well-watered tree going into winter is far more resilient than a dry one.

Wrap young thin-barked trunks from November.

Assess before winter. Deadwood and weak unions are easier to spot once leaves drop, and dealing with them before chinook season is cheaper than dealing with them after.

Mulch, two to four inches in a wide ring, pulled back from the trunk. Never piled against it — mulch volcanoes rot bark and encourage girdling roots.

Questions we get about this

Do I really need to water trees in winter?

Yes, and it is the most commonly skipped job in Northern Colorado. Dry windy winters without snow cover kill roots, especially on trees planted in the last three years. Water once or twice a month when it is above 40 degrees and the ground is not frozen.

When is the best time to prune?

Late winter to early spring for most species, while the tree is dormant and its structure is visible. Elms and oaks should only be pruned in the dormant season to avoid attracting disease-spreading beetles. Hazards get dealt with any time.

How often should I water an established tree in summer?

Deeply every one to two weeks, out at the drip line rather than at the trunk. That beats daily sprinkler cycles, which produce shallow roots and a tree that is less stable in wind.

When should I check my ash tree?

Summer, when symptoms are visible. Look for thinning at the top of the canopy, sprouts from the base of the trunk, and heavy woodpecker damage on the upper trunk.

Should I wrap my young trees?

Thin-barked young trees benefit from light-coloured wrap from November to April to prevent sun scald on the southwest side. Take it off in spring, because wrap left on year-round girdles the trunk.

When should I fertilise?

Autumn after leaf drop is usually best, or spring as growth starts. Never fertilise a drought-stressed tree in midsummer, and do not fertilise a healthy vigorous tree that is not short of anything.