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The CSU Extension office in Broomfield City & County provides assistance and programs for citizens in the areas of Horticulture: Yard & Garden

Horticulture FAQs   arrow

Find answers to frequently asked horticulture questions by topic:

SpringSummerFallWinter

PerennialsLawnSoilTrees and ShrubsOrnamentalsFruits & VegetablesInsects

Have questions still? Get help from Broomfield Extension and a Colorado Master Gardener by submitting a question to the Broomfield Master Gardener Helpline.

Spring

Q: Why is sap flowing from pruning cuts on my maple tree?
A: Silver maples, birch, and elm produce large amounts of sap when pruned in the late winter or early spring. Late winter (mid-February to early March) is still an ideal time to prune these trees because they are dormant and will soon produce the spring growth needed for wound closure. The loss of sap after pruning does not harm the trees, and the sap will eventually stop flowing.
Q: When should I sow vegetable seeds indoors?

A: Starting seeds indoors depends on the type of vegetable you are planting. There are cool season crops, like peas, that can be planted outside well before the last frost date, so they don’t need to be started indoors. Cole crops (broccoli, brussel sprouts, cauliflower) take especially long to reach maturity, so they are often started inside just to get a head start on the season. For warm weather crops, time your indoor seeds to be ready to plant at the right time, noting that germination time and transplant readiness varies. Some crops, like squash and other vining plants, may be stunted by transplanting and should be sowed outdoors. Always check the information on the seed packet for each crop you intend to grow.

For more information, see Garden Notes 720: The Vegetable Planting Guide.

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Summer

Q: What is the best time of day to water my lawn in the summer?
A: In the heat of the summer, the best time to irrigate your lawn is between 10pm and  6am. Because wind and temperatures are typically lower during this time, and humidity is typically higher, less water will be lost through evaporation. Watering before dew has dried in the morning is also a fine option. If you must water later in the day, it is important that the grass has time to dry before the evening dew.

For more information, see CSU Extension Fact Sheet 7.199 “Watering Established Lawns.”

Q: Why does my cucumber plant have flowers but no fruit?
A: Cucurbits like cucumbers, melons, and zucchini have both male and female flowers on the same plant. The first visible flowers on a cucumber plant are usually male flowers, which will not produce fruit. However, within a few weeks of the first visible flowers, female flowers should follow and set fruit. If no fruit or deformed fruit is produced, it is possible that there was not sufficient pollination. These plants rely on insects including honeybees for pollination. If insect activity is rare in the area of the plant, lack of pollination may be the culprit.

For more information, see CSU Extension Fact Sheet 7.609 “Cucumbers, Pumpkins, Squash and Melons.”

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Fall

Q: When should I plant spring-blooming bulbs and corms?
A: September and October are generally safe bets for planting crocus, daffodils, hyacinth, and other bulbs and corms that bloom in the spring or early summer. This allows enough time for roots to get established before the ground freezes.

For more information, see CSU Extension Fact Sheet 7.410 “Fall-Planted Bulbs and Corms.”

Q: Should I wrap my trees?
A: Newly planted trees should be wrapped to prevent sunscald injury. This is especially important on trees with thin bark including aspen, birch, honeylocust, linden, maple and fruit trees. Injuries usually appear on the tree’s southside. Wrap late fall or early winter and remove in spring. There are commercial tree wraps available from garden centers and online that help prevent sunscald. One common and effective product resembles crepe paper, which provides protection as well as some breathability.

For more information on sunscald, see  Fact Sheet 2.932 “Environmental Disorders of Woody Plants”

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Winter

Q: Should I water my trees in the winter?

A: It’s important to water trees, lawns, and perennials during prolonged dry periods in fall and winter. This practice will prevent root damage that affects the health of the entire plant. Water only when air and soil temperatures are above 40 degrees F with no snow cover. Apply water at midday so it will have time to soak in before possibly freezing at night.

For trees, make sure water is reaching the critical part of the root zone by applying water out to the dripline (the spread of the tree’s canopy). For lawns, remember newly established lawns, whether seeded or sodded, are especially susceptible to winter damage. The chance of winter damage increases for lawns with south or west exposures. For more information, see Fact Sheet 7.211: Fall and Winter Watering.

Q: How do I care for my roses in winter?

A. Roses need added protection in winter when temperatures fall to 20 degrees fahrenheit or lower. Mulch or mound soil 8 to 10 inches around the plant to retain warmth and moisture. You may also want to wrap the plant in burlap to protect from winds. Pruning and clean-up of plants is a good practice for the end of the growing season, which will help prevent overwintering of pests and diseases. For more information, see Plant Talk 1726: Roses: Winter Care.

Q: Should I wrap my trees?
A: Newly planted trees should be wrapped to prevent sunscald injury. This is especially important on trees with thin bark including aspen, birch, honeylocust, linden, maple and fruit trees. Injuries usually appear on the tree’s southside. Wrap late fall or early winter and remove in spring. There are commercial tree wraps available from garden centers and online that help prevent sunscald. One common and effective product resembles crepe paper, which provides protection as well as some breathability.

For more information on sunscald, see  Fact Sheet 2.932 “Environmental Disorders of Woody Plants”

Q: How do I choose and care for poinsettias?
A: Look for plants with uniformly green foliage. The leaves may be light, dark, or mottled in color, but they should not be yellow or dropping from the plant. Give poinsettias at least six hours a day of indirect sunlight. Avoid direct sun. Keep soil moderately moist, checking daily and watering thoroughly whenever soil is dry. Make sure poinsettia is not sitting in standing water. Poinsettias do not need to be fertilized while in bloom, but will benefit from a balanced all-purpose houseplant fertilizer once per month after the holidays.

For more information, including reflowering, see CSU Extension Fact Sheet 7.412 “Poinsettias.”

Q: Are poinsettias poisonous to humans or pets?
A: You may be familiar with the old saying, “the dose makes the poison.” The answer here is technically yes, but not very. Eating poinsettias (or other other non-edible items) can cause varying levels of discomfort. Ingesting very large quantities of poinsettia bracts can be harmful to pets. It is always a good idea to keep plants out of reach of pets and small children.

Poinsettias are in the same family as the rubber tree, from which latex is derived. Those with latex allergies may experience irritation from handling poinsettias.

For more information, see CSU Extension Fact Sheet 7.412 “Poinsettias.”

Q: Why is sap flowing from pruning cuts on my maple tree?
A: Silver maples, birch, and elm produce large amounts of sap when pruned in the late winter or early spring. Late winter (mid-February to early March) is still an ideal time to prune these trees because they are dormant and will soon produce the spring growth needed for wound closure. The loss of sap after pruning does not harm the trees, and the sap will eventually stop flowing.
Q: How do I care for a live Christmas tree?
A: Make sure to plan ahead. You should know where you will plant the tree before purchasing it, and you may want to dig the hole in the fall before the ground freezes. Live Christmas trees (often pinyon, ponderosa, limber, Austrian, bristlecone, and Scotch pines) should not be kept inside longer than seven days; five days is even better. During this time, keep the tree away from heat sources, and make sure the root ball never dries out. The soil should be kept moist but not soggy.

For more information and a video, see “Christmas Trees: Care for Living Trees” by Planttalk Colorado™.

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Perennials

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Lawn

Q: Should I water my lawn in the winter?

A: It’s important to water trees, lawns, and perennials during prolonged dry periods in fall and winter. This practice will prevent root damage that affects the health of the entire plant. Water only when air and soil temperatures are above 40 degrees F with no snow cover. Apply water at midday so it will have time to soak in before possibly freezing at night.

For trees, make sure water is reaching the critical part of the root zone by applying water out to the dripline (the spread of the tree’s canopy). For lawns, remember newly established lawns, whether seeded or sodded, are especially susceptible to winter damage. The chance of winter damage increases for lawns with south or west exposures. For more information, see Fact Sheet 7.211: Fall and Winter Watering.

Q: When should I apply pre-emergent herbicide to my lawn to control crabgrass?

A: Pre-emergent herbicides can control seedlings of annual or perennial weeds if applied before germination. Existing weeds will not be affected. In Broomfield, pre-emergent herbicides should typically be applied in March.

For more information on controlling annual weeds like crabgrass, see CSU Extension Fact Sheet 3.101 “Control of Annual Grassy Weeds in Lawns.”

Q: What is the best time of day to water my lawn in the summer?
A: In the heat of the summer, the best time to irrigate your lawn is between 10pm and  6am. Because wind and temperatures are typically lower during this time, and humidity is typically higher, less water will be lost through evaporation. Watering before dew has dried in the morning is also a fine option. If you must water later in the day, it is important that the grass has time to dry before the evening dew.

For more information, see CSU Extension Fact Sheet 7.199 “Watering Established Lawns.”

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Soil

Q: What are good soil amendments for Colorado soils?
A: It is important to know the structure of your soil before choosing an amendment. Colorado soils are typically high in clay, but sometimes sandy. Amendments can be organic or inorganic. Organic amendments include sphagnum peat, grass clippings, compost, wood ash and manure. Wood ash should not be used in Colorado due to its high phosphorus and salt content. Sand should never be used to amend clay heavy soils.

For more information on soil testing, visit CSU Soil, Water and Plant Testing Laboratory 

For more information on soil amendments, see Fact Sheet 7.235 “Choosing a Soil Amendment”

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Trees and Shrubs

Q: Should I water my trees in the winter?

A: It’s important to water trees, lawns, and perennials during prolonged dry periods in fall and winter. This practice will prevent root damage that affects the health of the entire plant. Water only when air and soil temperatures are above 40 degrees F with no snow cover. Apply water at midday so it will have time to soak in before possibly freezing at night.

For trees, make sure water is reaching the critical part of the root zone by applying water out to the dripline (the spread of the tree’s canopy). For lawns, remember newly established lawns, whether seeded or sodded, are especially susceptible to winter damage. The chance of winter damage increases for lawns with south or west exposures. For more information, see Fact Sheet 7.211: Fall and Winter Watering.

Q: How do I care for my roses in winter?

A. Roses need added protection in winter when temperatures fall to 20 degrees fahrenheit or lower. Mulch or mound soil 8 to 10 inches around the plant to retain warmth and moisture. You may also want to wrap the plant in burlap to protect from winds. Pruning and clean-up of plants is a good practice for the end of the growing season, which will help prevent overwintering of pests and diseases. For more information, see Plant Talk 1726: Roses: Winter Care.

Q: When should I prune my trees?
A: Late winter (mid-February to early March) is the best time to prune most trees in Broomfield. The benefits of this timeframe are that the tree is still dormant but not far from producing new spring growth, which will aid in wound closure.

Although some elms, silver maples, birch and walnut trees exude sap if pruned in the late winter or early spring, this should not harm the tree.

Q: Why is sap flowing from pruning cuts on my maple tree?
A: Silver maples, birch, and elm produce large amounts of sap when pruned in the late winter or early spring. Late winter (mid-February to early March) is still an ideal time to prune these trees because they are dormant and will soon produce the spring growth needed for wound closure. The loss of sap after pruning does not harm the trees, and the sap will eventually stop flowing.
Q: Should I wrap my trees?
A: Newly planted trees should be wrapped to prevent sunscald injury. This is especially important on trees with thin bark including aspen, birch, honeylocust, linden, maple and fruit trees. Injuries usually appear on the tree’s southside. Wrap late fall or early winter and remove in spring. There are commercial tree wraps available from garden centers and online that help prevent sunscald. One common and effective product resembles crepe paper, which provides protection as well as some breathability.

For more information on sunscald, see  Fact Sheet 2.932 “Environmental Disorders of Woody Plants”

Q: How do I care for a live Christmas tree?
A: Make sure to plan ahead. You should know where you will plant the tree before purchasing it, and you may want to dig the hole in the fall before the ground freezes. Live Christmas trees (often pinyon, ponderosa, limber, Austrian, bristlecone, and Scotch pines) should not be kept inside longer than seven days; five days is even better. During this time, keep the tree away from heat sources, and make sure the root ball never dries out. The soil should be kept moist but not soggy.

For more information and a video, see “Christmas Trees: Care for Living Trees” by Planttalk Colorado™.

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Ornamentals

Q: When should I plant spring-blooming bulbs and corms?
A: September and October are generally safe bets for planting crocus, daffodils, hyacinth, and other bulbs and corms that bloom in the spring or early summer. This allows enough time for roots to get established before the ground freezes.

For more information, see CSU Extension Fact Sheet 7.410 “Fall-Planted Bulbs and Corms.”

Q: How do I choose and care for poinsettias?
A: Look for plants with uniformly green foliage. The leaves may be light, dark, or mottled in color, but they should not be yellow or dropping from the plant. Give poinsettias at least six hours a day of indirect sunlight. Avoid direct sun. Keep soil moderately moist, checking daily and watering thoroughly whenever soil is dry. Make sure poinsettia is not sitting in standing water. Poinsettias do not need to be fertilized while in bloom, but will benefit from a balanced all-purpose houseplant fertilizer once per month after the holidays.

For more information, including reflowering, see CSU Extension Fact Sheet 7.412 “Poinsettias.”

Q: Are poinsettias poisonous to humans or pets?
A: You may be familiar with the old saying, “the dose makes the poison.” The answer here is technically yes, but not very. Eating poinsettias (or other other non-edible items) can cause varying levels of discomfort. Ingesting very large quantities of poinsettia bracts can be harmful to pets. It is always a good idea to keep plants out of reach of pets and small children.

Poinsettias are in the same family as the rubber tree, from which latex is derived. Those with latex allergies may experience irritation from handling poinsettias.

For more information, see CSU Extension Fact Sheet 7.412 “Poinsettias.”

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Fruits and Vegetables

Q: Why does my cucumber plant have flowers but no fruit?
A: Cucurbits like cucumbers, melons, and zucchini have both male and female flowers on the same plant. The first visible flowers on a cucumber plant are usually male flowers, which will not produce fruit. However, within a few weeks of the first visible flowers, female flowers should follow and set fruit. If no fruit or deformed fruit is produced, it is possible that there was not sufficient pollination. These plants rely on insects including honeybees for pollination. If insect activity is rare in the area of the plant, lack of pollination may be the culprit.

For more information, see CSU Extension Fact Sheet 7.609 “Cucumbers, Pumpkins, Squash and Melons.”

Q: Why are blossoms dropping from my tomato plants?
A: In the absence of other symptoms, unfavorable temperatures are the most common culprit of tomato plants that flower but don’t set fruit. Daytime temperatures above 85°F or nighttime temperatures below 55°F may interfere with pollination. High winds can also dry flowers more rapidly, before they have time to pollinate. In Colorado, tomato plants may experience all of these conditions in a fairly short period of time. Striving plenty of consistent moisture in dry times and using windbreaks may alleviate the issue. Regardless, fruits should set normally once the weather has moderated.

Fore more information, see “Common Tomato Problems” by Planttalk Colorado™.

Q: When should I sow vegetable seeds indoors?

A: Starting seeds indoors depends on the type of vegetable you are planting. There are cool season crops, like peas, that can be planted outside well before the last frost date, so they don’t need to be started indoors. Cole crops (broccoli, brussel sprouts, cauliflower) take especially long to reach maturity, so they are often started inside just to get a head start on the season. For warm weather crops, time your indoor seeds to be ready to plant at the right time, noting that germination time and transplant readiness varies. Some crops, like squash and other vining plants, may be stunted by transplanting and should be sowed outdoors. Always check the information on the seed packet for each crop you intend to grow.

For more information, see Garden Notes 720: The Vegetable Planting Guide.

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Insects

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