How to Grow Phlox Seeds

Grow Guide #2305
Family: Polemoniaceae
Binomial name: Phlox sp.
Life Cycle: Annual or Perennial

This 'How to Grow' guide details everything a home gardener needs to know to plant, grow and care for Phlox (Phlox sp.).

(Phlox plants can be annual or perennial. Phlox drummondii, commonly called Bedding Phlox, is a half hardy annual. Phlox paniculata, commonly called Border Phlox or Garden Phlox, is an herbaceous perennial that should be planted in a permanent position in the garden. This guide applies to both annual and perennial phlox.)

When to Sow Phlox Seeds

Use the table below to identify the best time of year to sow phlox seeds in your climate.

Preparation

Perennial varieties only - Plants will live for several years. Choose a permanent position where plants can grow undisturbed by regular digging or disturbance.

Phlox plants are best grown in full sun or part shade. Choose a location that will receive at least 3 hours of full sun each day.

Phlox plants need a well drained soil enriched with plenty of organic matter. Prepare soil by weeding it thoroughly, digging it over to loosen it and adding aged animal manure or compost. Keep the area free of weeds until planting. Learn more about preparing soil for planting here.

Phlox plants can be grown in containers. If possible choose a variety that’s recommended for container growing. Use a good quality potting mix and make sure your container is large enough for mature plants; a minimum of 10 litres is recommended for phlox. During the growing season, keep in mind that container grown plants may need additional fertiliser to encourage healthy growth.

How to Sow Phlox Seeds

Phlox seeds do not require any treatment (eg soaking, stratification) before sowing.

Phlox seeds can be sown directly into the garden OR seedlings can be raised in trays or other containers and transplanted to the garden once established.

Sow Direct

1. Sow seeds directly in the garden 3mm deep and 15-25cm apart.
2. Keep soil moist but never wet or dry.
3. Seeds should germinate in around 14-21 days at a soil temperature of 18-20°C.
4. Young seedlings will need protection from pests, pets and weather until they are established.

Raise Seedlings

1. Fill trays, punnets or jiffy pots with a good quality seed-raising mix, or use soil starter pellets.
2. Sow seeds 3mm deep.
3. Keep soil moist but never wet or dry.
4. Seeds should germinate in around 14-21 days at a soil temperature of 18-20°C.
5. Transplant seedlings to the garden once they have their first true leaves and are large enough to handle (usually 5-10cm tall).
6. Plant out, spacing plants 15-25cm apart.

Phlox is a half hardy crop. Mature plants will survive light frosts but seedlings need protection until they are established. Do not transplant seedlings or sow seeds outside until all danger of frost has passed.

Optional: In cool climates phlox seeds can be sown indoors 6 weeks before the last expected frost. Grow them in a warm position with plenty of natural light.

How to Grow Phlox

Phlox plants need regular watering during the growing season. Do not let soil dry out; keep soil evenly moist but not waterlogged. Water deeply in the early morning or late afternoon. Avoid watering the leaves of plants to avoid fungal diseases. Learn more about watering here.

If soil was well prepared no extra fertiliser should be necessary. In poor soil or to give your plants an extra boost, application of a high-potassium fertiliser or one formulated for flowering plants can be beneficial:

• Apply slow release fertiliser at the recommended rate when transplanting or when seedlings are 5-10cm tall.
• Apply liquid fertiliser at the recommended rate and frequency while plants are fruiting or flowering.

Phlox plants should flower in approximately 70-90 days.

Deadhead phlox flowers regularly during the growing season. Using sharp secateurs or snips cut fading or dead flowers off just above a set of leaves. Removing old flowers regularly will encourage plants to produce more flowers. Learn more about deadheading flowering plants here.

Perennial varieties only - When plants have finished flowering prune them back to neaten them and encourage strong new growth. Using sharp secateurs or snips, cut individual stems just above a set of lower leaves.

If growing phlox for cut flowers, use sharp snips or secateurs to cut the longest stems possible, removing the lower leaves and placing the stems immediately in a clean bucket of water. Learn more about cutting and conditioning homegrown flowers here.

Common Problems when Growing Phlox

Like all plants, phlox is susceptible to some pests, diseases and other problems. Below is a list of the most common problems gardeners encounter when growing phlox plants:

• 

  Grey mould (Botrytis sp.) is a fungal disease that causes flowers to become mouldy and fruit to rot. Spores are transported by wind and can survive in soil or on green waste. The fungus spreads most in cool, damp weather. Prune off affected flowers and fruit, water plants at soil level (not on leaves) and if necessary spray with an appropriate fungicide or homemade spray.
• 

  Downy mildew is a fungal disease that causes yellow to grey-brown patches on leaves, especially the undersides. Water plants at soil level (not on the leaves), remove and destroy affected leaves and do not overcrowd plants to ensure adequate air flow. If problems persist, spray with a homemade milk spray or fungicide.
• 

  Powdery mildew is caused by fungal spores reproducing on the leaves of plants. First showing as white spots on leaves, affected areas can spread quickly to cover the entire leaf surface. While rarely fatal, powdery mildew can reduce yields. Water plants at soil level (not on leaves) to prevent spreading spores, allow good air flow between plants, remove affected leaves and if necessary spray with an appropriate fungicide or homemade spray. Read more here about powdery mildew here.
• 

  Slugs and snails are molluscs that feed on tender leaves and shoots, mostly at night, leaving slimy trails behind them. Control them by removing their hiding places, keeping free range poultry, collecting them by torchlight or by placing traps. Read more about slugs and snails here.